NASA's Chandra Shares a New View of Our Galactic Neighbor

Andromeda/M31

Credit X-ray: NASA/CXO/UMass/Z. Li & Q.D. Wang, ESA/XMM-Newton; Infrared: NASA/JPL-Caltech/WISE, Spitzer, NASA/JPL-Caltech/K. Gordon (U. Az), ESA/Herschel, ESA/Planck, NASA/IRAS, NASA/COBE; Radio: NSF/GBT/WSRT/IRAM/C. Clark (STScI); Ultraviolet: NASA/JPL-Caltech/GALEX; Optical: Andromeda, Unexpected © Marcel Drechsler, Xavier Andromeda/M31Strottner, Yann Sainty & J. Sahner, T. Kottary. Composite image processing: L. Frattare, K. Arcand, J.Major

JPEG (378.9 kb) - Large JPEG (17.3 MB) - Tiff (40.2 MB) - More Images

A Tour the Andromeda Galaxy - More Videos

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The Andromeda galaxy, also known as Messier 31 (M31), is the closest spiral galaxy to the Milky Way at a distance of about 2.5 million light-years. Astronomers use Andromeda to understand the structure and evolution of our own spiral, which is much harder to do since Earth is embedded inside the Milky Way.

The galaxy M31 has played an important role in many aspects of astrophysics, but particularly in the discovery of dark matter. In the 1960s, astronomer Vera Rubin and her colleagues studied M31 and determined that there was some unseen matter in the galaxy that was affecting how the galaxy and its spiral arms rotated. This unknown material was named “dark matter.” Its nature remains one of the biggest open questions in astrophysics today, one which NASA’s upcoming Nancy Grace Roman Space Telescope is designed to help answer.

This new composite image contains data of M31 taken by some of the world’s most powerful telescopes in different kinds of light. This image includes X-rays from NASA’s Chandra X-ray Observatory and ESA’s (European Space Agency’s) XMM-Newton (represented in red, green, and blue); ultraviolet data from NASA’s retired GALEX (blue); optical data from astrophotographers using ground based telescopes (Jakob Sahner and Tarun Kottary); infrared data from NASA’s retired Spitzer Space Telescope, the Infrared Astronomy Satellite, COBE, Planck, and Herschel (red, orange, and purple); and radio data from the Westerbork Synthesis Radio Telescope (red-orange).

The Andromeda Galaxy (M31) in Different Types of Light

Credit: X-ray: NASA/CXO/UMass/Z. Li & Q.D. Wang, ESA/XMM-Newton; Infrared: NASA/JPL-Caltech/WISE, Spitzer, NASA/JPL-Caltech/K. Gordon (U. Az), ESA/Herschel, ESA/Planck, NASA/IRAS, NASA/COBE; Radio: NSF/GBT/WSRT/IRAM/C. Clark (STScI); Ultraviolet: NASA/JPL-Caltech/GALEX; Optical: Andromeda, Unexpected © Marcel Drechsler, Xavier Strottner, Yann Sainty & J. Sahner, T. Kottary. Composite image processing: L. Frattare, K. Arcand, J.Major

Each type of light reveals new information about this close galactic relative to the Milky Way. For example, Chandra’s X-rays reveal the high-energy radiation around the supermassive black hole at the center of M31 as well as many other smaller compact and dense objects strewn across the galaxy. A recent paper about Chandra observations of M31 discusses the amount of X-rays produced by the supermassive black hole in the center of the galaxy over the last 15 years. One flare was observed in 2013, which appears to represent an amplification of the typical X-rays seen from the black hole.

These multi-wavelength datasets are also being released as a sonification, which includes the same wavelengths of data in the new composite. In the sonification, the layer from each telescope has been separated out and rotated so that they stack on top of each other horizontally, beginning with X-rays at the top and then moving through ultraviolet, optical, infrared, and radio at the bottom. As the scan moves from left to right in the sonification, each type of light is mapped to a different range of notes, from lower-energy radio waves up through the high energy of X-rays. Meanwhile, the brightness of each source controls volume, and the vertical location dictates the pitch.

Andromeda Galaxy (M31) Sonification. Sonification
Credit: NASA/CXC/SAO/K.Arcand, SYSTEM Sounds (M. Russo, A. Santaguida)

This new image of M31 is released in tribute to the groundbreaking legacy of Dr. Vera Rubin, whose observations transformed our understanding of the universe. Rubin’s meticulous measurements of Andromeda’s rotation curve provided some of the earliest and most convincing evidence that galaxies are embedded in massive halos of invisible material — what we now call dark matter. Her work challenged long-held assumptions and catalyzed a new era of research into the composition and dynamics of the cosmos. In recognition of her profound scientific contributions, the United States Mint has recently released a quarter in 2025 featuring Rubin as part of its American Women Quarters Program — making her the first astronomer honored in the series.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

Quick Look: NASA's Chandra Shares a New View of Our Galactic Neighbor

Source: NASA’s Chandra X-ray Observatory

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Visual Description:

This release features several images and a sonification video examining the Andromeda galaxy, our closest spiral galaxy neighbor. This collection helps astronomers understand the evolution of the Milky Way, our own spiral galaxy, and provides a fascinating insight into astronomical data gathering and presentation.

Like all spiral galaxies viewed at this distance and angle, Andromeda appears relatively flat. Its spiraling arms circle around a bright core, creating a disk shape, like a large dinner plate. In most of the images in this collection, Andromeda's flat surface is tilted to face our upper left.

This collection features data from some of the world's most powerful telescopes, each capturing light in a different spectrum. In each single-spectrum image, Andromeda has a similar shape and orientation, but the colors and details are dramatically different.

In radio waves, the spiraling arms appear red and orange, like a burning, loosely coiled rope. The center appears black, with no core discernible. In infrare,d light, the outer arms are similarly fiery. Here, a white spiraling ring encircles a blue center with a small golden core. The optical image is hazy and grey, with spiraling arms like faded smoke rings. Here, the blackness of space is dotted with specks of light, and a small bright dot glows at the core of the galaxy. In ultraviolet light the spiraling arms are icy blue and white, with a hazy white ball at the core. No spiral arms are present in the X-ray image, making the bright golden core and nearby stars clear and easy to study.

In this release, the single-spectrum images are presented side by side for easy comparison. They are also combined into a composite image. In the composite, Andromeda's spiraling arms are the color of red wine near the outer edges, and lavender near the center. The core is large and bright, surrounded by a cluster of bright blue and green specks. Other small flecks in a variety of colors dot the galaxy, and the blackness of space surrounding it.

This release also features a thirty second video, which sonifies the collected data. In the video, the single-spectrum images are stacked vertically, one atop the other. As the video plays, an activation line sweeps across the stacked images from left to right. Musical notes ring out when the line encounters light. The lower the wavelength energy, the lower the pitches of the notes. The brighter the source, the louder the volume.

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Fast Facts for M31

Scale: Image; is about 192 arcmin (150,000 light-years) across.
Category: Normal Galaxies & Starburst Galaxies
Coordinates (J2000): RA 0h 42m 44s | Dec +41° 16´ 09"
Constellation: Andromeda
Observation Dates: 152 pointings between 1999 and 2012
Observation Time: 55 hours 30 minutes (2 days 7 hours 30 minutes)
Instrument: ACIS/HRC
Also Known As: Andromeda

TReferences: DiKerby, S., Zhang, S., and Irwin, J., 2025, ApJ, 982, 50; DOI:10.3847/1538-4357/adb1d5
Color Code: X-ray: red, green, and blue; UV: red, green, and purple; Optical: red, green, and teal; Infrared: red, orange, and purple; Radio: red and orange
Distance Estimate: About 2.5 million light-years

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NASA's Hubble Traces Hidden History of Andromeda Galaxy

Hubble M31 PHAT+PHAST Mosaic
Science: NASA, ESA, Benjamin F. Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern)
Image Processing: Joseph DePasquale (STScI)

Compass and Scale Image of M31 PHAT+PHAST Mosaic
Science: NASA, ESA, Benjamin F. Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern)
Image Processing: Joseph DePasquale (STScI)

Andromeda M31 PHAST Mosaic Video
Credits/Visualization: NASA, ESA, Greg Bacon (STScI)
Science: Benjamin F. Williams (UWashington)

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In the years following the launch of NASA's Hubble Space Telescope, astronomers have tallied over 1 trillion galaxies in the universe. But only one galaxy stands out as the most important nearby stellar island to our Milky Way — the magnificent Andromeda galaxy (Messier 31). It can be seen with the naked eye on a very clear autumn night as a faint cigar-shaped object roughly the apparent angular diameter of our Moon.

A century ago, Edwin Hubble first established that this so-called "spiral nebula" was actually very far outside our own Milky Way galaxy —at a distance of approximately 2.5 million light-years or roughly 25 Milky Way diameters. Prior to that, astronomers had long thought that the Milky way encompassed the entire universe. Overnight, Hubble's discovery turned cosmology upside down by unveiling an infinitely grander universe.

Now, a century later, the space telescope named for Hubble has accomplished the most comprehensive survey of this enticing empire of stars. The Hubble telescope is yielding new clues to the evolutionary history of Andromeda, and it looks markedly different from the Milky Way's history.

Without Andromeda as a proxy for spiral galaxies in the universe at large, astronomers would know much less about the structure and evolution of our own Milky Way. That's because we are embedded inside the Milky Way. This is like trying to understand the layout of New York City by standing in the middle of Central Park.

"With Hubble we can get into enormous detail about what's happening on a holistic scale across the entire disk of the galaxy. You can't do that with any other large galaxy," said principal investigator Ben Williams of the University of Washington. Hubble's sharp imaging capabilities can resolve more than 200 million stars in the Andromeda galaxy, detecting only stars brighter than our Sun. They look like grains of sand across the beach. But that's just the tip of the iceberg. Andromeda's total population is estimated to be 1 trillion stars, with many less massive stars falling below Hubble's sensitivity limit.

Photographing Andromeda was a herculean task because the galaxy is a much bigger target on the sky than the galaxies Hubble routinely observes, which are often billions of light-years away. The full mosaic was carried out under two Hubble programs. In total it required over 1,000 Hubble orbits, spanning more than a decade.

This panorama started with the Panchromatic Hubble Andromeda Treasury (PHAT) program about a decade ago. Images were obtained at near-ultraviolet, visible, and near-infrared wavelengths using the Advanced Camera for Surveys and the Wide Field Camera 3 aboard Hubble to photograph the northern half of Andromeda.

This program was followed up by the Panchromatic Hubble Andromeda Southern Treasury (PHAST), recently published in The Astrophysical Journal and led by Zhuo Chen at the University of Washington, which added images of approximately 100 million stars in the southern half of Andromeda. This region is structurally unique and more sensitive to the galaxy's merger history than the northern disk mapped by the PHAT survey.

The combined programs collectively cover the entire disk of Andromeda, which is seen almost edge-on — tilted by 77 degrees relative to Earth's view. The galaxy is so large that the mosaic is assembled from approximately 600 separate fields of view. The mosaic image is made up of at least 2.5 billion pixels.

The complementary Hubble survey programs provide information about the age, heavy-element abundance and stellar masses inside Andromeda. This will allow astronomers to distinguish between competing scenarios where Andromeda merged with one or more galaxies. Hubble's detailed measurements constrain models of Andromeda's merger history and disk evolution.

A Galactic 'Train Wreck'

Though the Milky Way and Andromeda formed presumably around the same time many billions of years ago, observational evidence shows that they have very different evolutionary histories, despite growing up in the same cosmological neighborhood. Andromeda seems to be more highly populated with younger stars and unusual features like coherent streams of stars, say researchers. This implies it has a more active recent star-formation and interaction history than the Milky Way.

"Andromeda's a train wreck. It looks like it has been through some kind of event that caused it to form a lot of stars and then just shut down," said Daniel Weisz at the University of California, Berkeley. "This was probably due to a collision with another galaxy in the neighborhood."

A possible culprit is the compact satellite galaxy Messier 32, which resembles the stripped-down core of a once-spiral galaxy that may have interacted with Andromeda in the past. Computer simulations suggest that when a close encounter with another galaxy uses up all the available interstellar gas, star formation subsides.

"Andromeda looks like a transitional type of galaxy that's between a star-forming spiral and a sort of elliptical galaxy dominated by aging red stars," said Weisz. "We can tell it's got this big central bulge of older stars and a star-forming disk that's not as active as you might expect given the galaxy's mass."

"This detailed look at the resolved stars will help us to piece together the galaxy's past merger and interaction history," added Williams.

Hubble's new findings will support future observations by NASA's James Webb Space Telescope and the upcoming Nancy Grace Roman Space Telescope. Essentially a wide-angle version of Hubble (with the same sized mirror), Roman will capture the equivalent of at least 100 high-resolution Hubble images in a single exposure. These observations will complement and extend Hubble's huge dataset.
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The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

Source: Space Telescope Science Institute (STScI)/News

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About This Release

Credits:

Media Contact:

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland

Science Contact:

Benjamin F. Williams
University of Washington, Seattle, Washington

Zhuo Chen
University of Washington, Seattle, Washington

Permissions: Content Use Policy

Related Links and Documents

• Science Paper: The science paper by Zhuo Chen et al. (ApJ 979 35), PDF (19.78 MB)
• NASA's Hubble Portal
• Hubble's High-Definition Panoramic View of the Andromeda Galaxy (STScI-PR2015-02)
• ESA/Hubble's Release
• NASA Goddard's Video (on YouTube)

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NASA Celebrates Edwin Hubble's Discovery of a New Universe

M31 Cepheid Variable Star V1
Credits/Image: NASA, ESA, Hubble Heritage Project (STScI, AURA)
Acknowledgment: Robert Gendler

Compass Scale Image of V1 in M31
Credits/Image: NASA, ESA, Hubble Heritage Project (STScI, AURA)

Cepheid Variable Star V1 in Andromeda Galaxy
Credits/Image: NASA, ESA, Hubble Heritage Project (STScI, AURA)
Acknowledgment: Robert Gendler

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For humans, the most important star in the universe is our Sun. The second-most important star is nestled inside the Andromeda galaxy. Don't go looking for it — the flickering star is 2.2 million light-years away, and is 1/100,000th the brightness of the faintest star visible to the human eye.

Yet, a century ago, its discovery by Edwin Hubble, then an astronomer at Carnegie Observatories, opened humanity's eyes as to how large the universe really is, and revealed that our Milky Way galaxy is just one of hundreds of billions of galaxies in the universe ushered in the coming-of-age for humans as a curious species that could scientifically ponder our own creation through the message of starlight. Carnegie Science and NASA are celebrating this centennial at the 245th meeting of the American Astronomical Society in Washington, D.C.

The seemingly inauspicious star, simply named V1, flung open a Pandora's box full of mysteries about time and space that are still challenging astronomers today. Using the largest telescope in the world at that time, the Carnegie-funded 100-inch Hooker Telescope at Mount Wilson Observatory in California, Hubble discovered the demure star in 1923. This rare type of pulsating star, called a Cepheid variable, is used as milepost markers for distant celestial objects. There are no tape-measures in space, but by the early 20th century Henrietta Swan Leavitt had discovered that the pulsation period of Cepheid variables is directly tied to their luminosity.

Many astronomers long believed that the edge of the Milky Way marked the edge of the entire universe. But Hubble determined that V1, located inside the Andromeda "nebula," was at a distance that far exceeded anything in our own Milky Way galaxy. This led Hubble to the jaw-dropping realization that the universe extends far beyond our own galaxy.

In fact Hubble had suspected there was a larger universe out there, but here was the proof in the pudding. He was so amazed he scribbled an exclamation mark on the photographic plate of Andromeda that pinpointed the variable star.

As a result, the science of cosmology exploded almost overnight. Hubble's contemporary, the distinguished Harvard astronomer Harlow Shapley, upon Hubble notifying him of the discovery, was devastated. "Here is the letter that destroyed my universe," he lamented to fellow astronomer Cecilia Payne-Gaposchkin, who was in his office when he opened Hubble's message.

Just three years earlier, Shapley had presented his observational interpretation of a much smaller universe in a debate one evening at the Smithsonian Museum of Natural History in Washington. He maintained that the Milky Way galaxy was so huge, it must encompass the entirety of the universe. Shapley insisted that the mysteriously fuzzy "spiral nebulae," such as Andromeda, were simply stars forming on the periphery of our Milky Way, and inconsequential.

Little could Hubble have imagined that 70 years later, an extraordinary telescope named after him, lofted hundreds of miles above the Earth, would continue his legacy. The marvelous telescope made "Hubble" a household word, synonymous with wonderous astronomy.

Today, NASA's Hubble Space Telescope pushes the frontiers of knowledge over 10 times farther than Edwin Hubble could ever see. The space telescope has lifted the curtain on a compulsive universe full of active stars, colliding galaxies, and runaway black holes, among the celestial fireworks of the interplay between matter and energy.

Edwin Hubble was the first astronomer to take the initial steps that would ultimately lead to the Hubble Space Telescope, revealing a seemingly infinite ocean of galaxies. He thought that, despite their abundance, galaxies came in just a few specific shapes: pinwheel spirals, football-shaped ellipticals, and oddball irregular galaxies. He thought these might be clues to galaxy evolution – but the answer had to wait for the Hubble Space Telescope's legendary Hubble Deep Field in 1994.

The most impactful finding that Edwin Hubble's analysis showed was that the farther the galaxy is, the faster it appears to be receding from Earth. The universe looked like it was expanding like a balloon. This was based on Hubble tying galaxy distances to the reddening of light — the redshift – that proportionally increased the father away the galaxies are.

The redshift data were first collected by Lowell Observatory astronomer Vesto Slipher, who spectroscopically studied the "spiral nebulae" a decade before Hubble. Slipher did not know they were extragalactic, but Hubble made the connection. Slipher first interpreted his redshift data an example of the Doppler effect. This phenomenon is caused by light being stretched to longer, redder wavelengths if a source is moving away from us. To Slipher, it was curious that all the spiral nebulae appeared to be moving away from Earth.

Two years prior to Hubble publishing his findings, the Belgian physicist and Jesuit priest Georges Lemaître analyzed the Hubble and Slifer observations and first came to the conclusion of an expanding universe. This proportionality between galaxies' distances and redshifts is today termed Hubble–Lemaître's law. Because the universe appeared to be uniformly expanding, Lemaître further realized that the expansion rate could be run back into time – like rewinding a movie – until the universe was unimaginably small, hot and dense. It wasn't until 1949 that the term "big bang" came into fashion.

This was a relief to Edwin Hubble's contemporary, Albert Einstein, who deduced the universe could not remain stationary without imploding under gravity's pull. The rate of cosmic expansion is now known as the Hubble Constant

Ironically, Hubble himself never fully accepted the runaway universe as an interpretation of the redshift data. He suspected that some unknown physics phenomenon was giving the illusion that the galaxies were flying away from each other. He was partly right in that Einstein's theory of special relativity explained redshift as an effect of time-dilation that is proportional to the stretching of expanding space. The galaxies only appear to be zooming through the universe. Space is expanding instead.

After decades of precise measurements, the Hubble telescope came along to nail down the expansion rate precisely, giving the universe an age of 13.8 billion years. This required establishing the first rung of what astronomers call the "cosmic distance ladder" needed to build a yardstick to far-flung galaxies. They are cousins to V1, Cepheid variable stars that the Hubble telescope can detect out to over 100 times farther from Earth than the star Edwin Hubble first found.

Astrophysics was turned on its head again in 1998 when the Hubble telescope and other observatories discovered that the universe was expanding at an ever-faster rate, through a phenomenon dubbed "dark energy." Einstein first toyed with this idea of a repulsive form of gravity in space, calling it the cosmological constant

Even more mysteriously, the current expansion rate appears to be different than what modern cosmological models of the developing universe would predict, further confounding theoreticians. Today astronomers are wrestling with the idea that whatever is accelerating the universe may be changing over time. NASA's Roman Space Telescope, with the ability to do large cosmic surveys, should lead to new insights into the behavior of dark matter and dark energy. Roman will likely measure the Hubble constant via lensed supernovae.

This grand century-long adventure, plumbing depths of the unknown, began with Hubble photographing a large smudge of light, the Andromeda galaxy, at the Mount Wilson Observatory high above Los Angeles.

In short, Edwin Hubble is the man who wiped away the ancient universe and discovered a new universe that would shrink humanity's self-perception into being an insignificant speck in the cosmos.

The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

Source: HubbleSite/News

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About This Release

Credits: Media Contact:

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland

Permissions: Content Use Policy

Contact Us: Direct inquiries to the News Team.

Related Links and Documents

• NASA's Hubble Portal
• Hubble Views the Star that Changed the Universe (STScI-PR2011-15)
• Carnegie Science's Release

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When one plus one (eventually) equals one

Arp 122, NGC 6040, NGC 6041

Two spiral galaxies are merging together at the right side of the image. One is seen face-on and is circular in shape. The other seems to lie in front of the first one. This galaxy is seen as a disc tilted away from the viewer and it is partially warped. In the lower-left corner, cut off by the frame, a large elliptical galaxy appears as light radiating from a point. Various small galaxies cover the background. Credit: ESA/Hubble & NASA, J. Dalcanton, Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA. Acknowledgement: L. Shatz

This Hubble Picture of the Week features Arp 122, a peculiar galaxy that in fact comprises two galaxies — NGC 6040, the tilted, warped spiral galaxy and LEDA 59642, the round, face-on spiral — that are in the midst of a collision. This dramatic cosmic encounter is located at the very safe distance of roughly 570 million light-years from Earth. Peeking in at the corner is the elliptical galaxy NGC 6041, a central member of the galaxy cluster that Arp 122 resides in, but otherwise not participating in this monster merger.

Galactic collisions and mergers are monumentally energetic and dramatic events, but they take place on a very slow timescale. For example, the Milky Way is on track to collide with its nearest galactic neighbour, the Andromeda Galaxy (M31), but these two galaxies have a good four billion years to go before they actually meet. The process of colliding and merging will not be a quick one either: it might take hundreds of millions of years to unfold. These collisions take so long because of the truly massive distances involved.

Galaxies are composed of stars and their solar systems, dust and gas. In galactic collisions, therefore, these constituent components may experience enormous changes in the gravitational forces acting on them. In time, this completely changes the structure of the two (or more) colliding galaxies, and sometimes ultimately results in a single, merged galaxy. That may well be what results from the collision pictured in this image. Galaxies that result from mergers are thought to have a regular or elliptical structure, as the merging process disrupts more complex structures (such as those observed in spiral galaxies). It would be fascinating to know what Arp 122 will look like once this collision is complete . . . but that will not happen for a long, long time.

Source: ESA/Hubble/potw

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A New Look at Gamma Rays from Our Galaxy’s Next-Door Neighbour

An ultraviolet image of the Andromeda Galaxy from NASA's Galaxy Evolution Explorer.
Credit: NASA/JPL-Caltech

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Title: On the Gamma-Ray Emission of the Andromeda Galaxy M31
Authors: Yi Xing et al.
First Author’s Institution: Shanghai Astronomical Observatory, Chinese Academy of Sciences
Status: Published in ApJL

Gamma rays are the highest-energy photons in our universe. Naturally, they come from some of the most extreme environments in the universe, such as pulsars, active galactic nuclei, supernovae, and potentially even dark matter. Though many gamma-ray sources have been detected both in the Milky Way and extragalactically, the nature of gamma-ray emission from our closest neighbouring galaxy, Andromeda (or Messier 31), remains somewhat of a mystery.

The Fermi Large Area Telescope (Fermi-LAT) is an instrument on the Fermi Gamma-ray Space Telescope that has been surveying the sky for high-energy gamma rays since 2008, with ample data taken on Andromeda throughout its flight. Many groups have analyzed these data, with more data giving more insight into what’s making these gamma rays.

Figure 1: Significance maps of Andromeda at energies from 0.1 to 500 gigaelectronvolts (left) and 2 to 500 gigaelectronvolts (right). The region of optical emission is represented by the white contour. The colorbar corresponds to test statistic, which is similar to significance. A test statistic of 25 corresponds to a detection. Green markers correspond to nearby sources found in the SIMBAD database. The left figure shows a hint of additional structure in the southeast region of Andromeda, but both point sources emerge out of the seemingly extended region only with the lowest energies cut out. Credit: Xing et al. 2023

To Extend or Not to Extend?

Up until today’s article, it looked like gamma rays from Andromeda were coming from a blob-like shape (i.e., extended emission) surrounding the centre of the galaxy (similar to Figure 1, left). This was particularly exciting, since extended structure in gamma-ray emission often suggests either a distribution of cosmic rays or the presence of a massive dark matter halo.

Cosmic rays are charged particles that travel at relativistic speeds through the universe but get easily diverted by magnetic fields, making it very difficult to trace their origin from Earth. Luckily, since there are processes that produce gamma rays from charged particles (hadronic processes), identifying regions of extended gamma rays can trace regions where populations of cosmic rays are interacting with their environments. On the other hand, clumps of massive dark matter located in the centre of Andromeda could decay or annihilate, producing gamma rays in the process.

Figure 2: A spectral energy distribution showing flux (quantity of gamma rays received) plotted against energy of Andromeda’s centre (black) and southeast (red) emission regions, along with the Milky Way’s galactic centre (blue). It is apparent that both sources are not only similar in brightness but are also producing significantly more gamma rays than our galactic centre. Click to enlarge. Credit: Xing et al. 2023

 
Where are the Gamma Rays Coming From?

A reanalysis of 14 years of Fermi-LAT data by the authors reveals that the emission of gamma rays isn’t extended after all. In fact, it seems that it’s constrained to two point sources: one located right at the centre of the galaxy and another ~20,000 light-years to the southeast (see Figure 1). This only became apparent when the authors cut out the lowest-energy gamma rays, which still make the data appear more or less extended when they’re included. Even more curiously, the authors found that both of these regions are significantly brighter than expected when compared to the gamma-ray emission of our own galactic centre (see Figure 2).

This new picture of Andromeda’s gamma rays changes a lot about our understanding of the galaxy. It’s no longer likely that Andromeda’s central gamma-ray hotspot is coming from a dark matter halo or cosmic ray distribution, so the authors looked to the Milky Way’s galactic centre to figure out what sorts of objects could be responsible for the gamma rays. One of the leading theories for our own galactic centre gamma rays is a population of old, unresolved objects, such as millisecond pulsars. However, in the case of Andromeda, at least 15,000 millisecond pulsars would be needed to account for the especially bright gamma-ray emission. While it’s still uncertain whether or not the centre of Andromeda can host this huge number of pulsars, we’ve only detected around 200 in the Milky Way’s centre, so this explanation seems unlikely.

The authors also investigate the southeast source that appeared in their new analysis. Since galaxies are pretty far apart from one another, the chance of finding two or more galaxies by coincidence in a circle drawn around both the central and southeast sources is only ~0.4%. This means that the emission is most likely coming from within Andromeda. As seen in Figure 2, the off-centre source is almost exactly the same brightness as Andromeda’s centre source (which is peculiar in its own right!), leading to the same problem of identifying sources capable of emitting such bright emission. After looking through X-ray and optical surveys, the authors determined that there weren’t any good counterparts for this region in other wavelengths either. Even considering the low probability of this being an extragalactic source behind Andromeda, there aren’t any known counterparts in the region of the sky where this hotspot is located.

The results are certainly unexpected and open up a whole new can of worms when it comes to figuring out the origin of the gamma rays in our neighbouring galaxy. Even though there are still a lot of unknowns, future observations and analyses of these newly constrained regions will help us understand how bright gamma rays are produced near the centres of galaxies and may even help us better understand our own galactic centre.

Original astrobite edited by Ivey Davis and Katya Gozman.

Source: American Astronomical Society - AAS Nova

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Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

 

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About the author, Samantha Wong:

I’m a graduate student at McGill University, where I study high-energy astrophysics. This includes studying all sorts of extreme environments in the universe like active galactic nuclei, pulsars, and supernova remnants with the VERITAS gamma-ray telescope.

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New Images Using Data From Retired Telescopes Reveal Hidden Features

Infrared-Radio Image of the Large Magellanic Cloud
Credits: Image: ESA, NASA, NASA-JPL, Caltech, Christopher Clark (STScI), S. Kim (Sejong University), T. Wong (UIUC)

Infrared-Radio Image of the Large Magellanic Cloud
Credits: Image: ESA, NASA, NASA-JPL, Caltech, Christopher Clark (STScI), S. Kim (Sejong University), T. Wong (UIUC)

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nfrared-Radio Image of the Andromeda Galaxy (M31)

Credits: Image: ESA, NASA, NASA-JPL, Caltech, Christopher Clark (STScI), R. Braun (SKA Observatory), C. Nieten (MPI Radioastronomie), Matt Smith (Cardiff University)

Infrared-Radio Image of the Triangulum Galaxy (M33)

Credits: Image: ESA, NASA, NASA-JPL, Caltech, Christopher Clark (STScI), E. Koch (University of Alberta), C. Druard (University of Bordeaux)

Release Images

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New images using data from European Space Agency (ESA) and NASA missions showcase the gas and dust that fill the space between stars in four of the galaxies closest to our own Milky Way. More than striking, the snapshots are also a scientific trove, lending insight into how dramatically the density of dust clouds can vary within a galaxy.

With a consistency similar to smoke, dust is created by dying stars and is one of the materials that forms new stars. The dust clouds observed by space telescopes are constantly shaped and molded by exploding stars, stellar winds, and the effects of gravity. Almost half of all the starlight in the universe is absorbed by dust. Many of the heavy chemical elements essential to forming planets like Earth are locked up in dust grains in interstellar space. Understanding dust is an essential part of understanding our universe.

The observations were made possible through the work of ESA’s Herschel Space Observatory, which operated from 2009 to 2013. Herschel’s super-cold instruments were able to detect the thermal glow of dust, which is emitted as far-infrared light, a range of wavelengths longer than what human eyes can detect.

Herschel’s images of interstellar dust provide high-resolution views of fine details in these clouds, revealing intricate substructures. But the way the space telescope was designed meant that it often couldn’t detect light from clouds that are more spread out and diffuse, especially in the outer regions of galaxies, where the gas and dust become sparse and thus fainter. For some nearby galaxies, that meant Herschel missed up to 30% of all the light given off by dust. With such a significant gap, astronomers struggled to use the Herschel data to understand how dust and gas behaved in these environments. To fill out the Herschel dust maps, the new images combine data from three other missions: ESA’s retired Planck observatory, along with two retired NASA missions, the Infrared Astronomical Satellite (IRAS) and Cosmic Background Explorer (COBE).

The images show the Andromeda galaxy, also known as M31; the Triangulum galaxy, or M33; and the Large and Small Magellanic Clouds – dwarf galaxies orbiting the Milky Way that do not have the spiral structure of the Andromeda and Triangulum galaxies. All four are within 3 million light-years of Earth.

In the images, red indicates hydrogen gas, the most common element in the universe. The image of the Large Magellanic Cloud shows a red tail coming off the bottom left of the galaxy that was likely created when it collided with the Small Magellanic Cloud about 100 million years ago. Bubbles of empty space indicate regions where stars have recently formed, because intense winds from the newborn stars blow away the surrounding dust and gas. The green light around the edges of those bubbles indicates the presence of cold dust that has piled up as a result of those winds. Warmer dust, shown in blue, indicates where stars are forming or other processes have heated the dust.

Many heavy elements in nature – like carbon, oxygen, and iron – can get stuck to dust grains, and the presence of different elements changes the way dust absorbs starlight. This in turn affects the view astronomers get of events like star formation. In the densest dust clouds, almost all the heavy elements can get locked up in dust grains, which increases the dust-to-gas ratio. But in less dense regions, the destructive radiation from newborn stars or shockwaves from exploding stars will smash the dust grains and return some of those locked-up heavy elements back into the gas, changing the ratio once again. Scientists who study interstellar space and star formation want to better understand this ongoing cycle. The Herschel images show that the dust-to-gas ratio can vary within a single galaxy by up to a factor of 20, far more than previously estimated.

“These improved Herschel images show us that the dust ‘ecosystems’ in these galaxies are very dynamic,” said Christopher Clark, an astronomer at the Space Science Telescope Institute in Baltimore, Maryland, who led the work to create the new images.

These results were featured in a press conference at the summer meeting of the American Astronomical Society.

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Astronomers Release New All-Sky Map of Milky Way's Outer Reaches

Image of the Milky Way and the Large Magellanic Cloud (LMC) are overlaid on a map of the surrounding galactic halo. The smaller structure is a wake created by the LMC’s motion through this region. The larger light-blue feature corresponds to a high density of stars observed in the northern hemisphere of our galaxy. Credit: NASA/ESA/JPL-Caltech/Conroy et. al. 2021

The highlight of the new chart is a wake of stars, stirred up by a small galaxy set to collide with the Milky Way. The map could also offer a new test of dark matter theories. 

Astronomers using data from NASA and ESA (European Space Agency) telescopes have released a new all-sky map of the outermost region of our galaxy. Known as the galactic halo, this area lies outside the swirling spiral arms that form the Milky Way’s recognizable central disk and is sparsely populated with stars. Though the halo may appear mostly empty, it is also predicted to contain a massive reservoir of dark matter, a mysterious and invisible substance thought to make up the bulk of all the mass in the universe.

The data for the new map comes from ESA’s Gaia mission and NASA’s Near Earth Object Wide Field Infrared Survey Explorer, or NEOWISE, which operated from 2009 to 2013 under the moniker WISE. The study makes use of data collected by the spacecraft between 2009 and 2018.

The new map reveals how a small galaxy called the Large Magellanic Cloud (LMC) – so named because it is the larger of two dwarf galaxies orbiting the Milky Way – has sailed through the Milky Way’s galactic halo like a ship through water, its gravity creating a wake in the stars behind it. The LMC is located about 160,000 light-years from Earth and is less than one-quarter the mass of the Milky Way.

Simulation of Dark Matter in the Milky Way Halo

A simulation of dark matter surrounding the Milky Way galaxy (small ring at center) and the Large Magellanic Cloud (LMC) reveals two areas of high density: the smaller of the two light blue areas is a wake created by the LMC’s motion through this region. The larger corresponds to an excess of stars in the Milky Way’s northern hemisphere. Credit: NASA/JPL-Caltech/NSF/R. Hurt/N. Garavito-Camargo & G. Besla

Though the inner portions of the halo have been mapped with a high level of accuracy, this is the first map to provide a similar picture of the halo’s outer regions, where the wake is found – about 200,000 light-years to 325,000 light-years from the galactic center. Previous studies have hinted at the wake’s existence, but the all-sky map confirms its presence and offers a detailed view of its shape, size, and location.

This disturbance in the halo also provides astronomers with an opportunity to study something they can’t observe directly: dark matter. While it doesn’t emit, reflect, or absorb light, the gravitational influence of dark matter has been observed across the universe. It is thought to create a scaffolding on which galaxies are built, such that without it, galaxies would fly apart as they spin. Dark matter is estimated to be five times more common in the universe than all the matter that emits and/or interacts with light, from stars to planets to gas clouds.

Although there are multiple theories about the nature of dark matter, all of them indicate that it should be present in the Milky Way’s halo. If that’s the case, then as the LMC sails through this region, it should leave a wake in the dark matter as well. The wake observed in the new star map is thought to be the outline of this dark matter wake; the stars are like leaves on the surface of this invisible ocean, their position shifting with the dark matter.

The interaction between the dark matter and the Large Magellanic Cloud has big implications for our galaxy. As the LMC orbits the Milky Way, the dark matter’s gravity drags on the LMC and slows it down. This will cause the dwarf galaxy’s orbit to get smaller and smaller, until the galaxy finally collides with the Milky Way in about 2 billion years. These types of mergers might be a key driver in the growth of massive galaxies across the universe. In fact, astronomers think the Milky Way merged with another small galaxy about 10 billion years ago.

“This robbing of a smaller galaxy’s energy is not only why the LMC is merging with the Milky Way, but also why all galaxy mergers happen,” said Rohan Naidu, a doctoral student in astronomy at Harvard University and a co-author of the new paper. “The wake in our map is a really neat confirmation that our basic picture for how galaxies merge is on point!”

 A Rare Opportunity

The authors of the paper also think the new map – along with additional data and theoretical analyses – may provide a test for different theories about the nature of dark matter, such as whether it consists of particles, like regular matter, and what the properties of those particles are.

“You can imagine that the wake behind a boat will be different if the boat is sailing through water or through honey,” said Charlie Conroy, a professor at Harvard University and an astronomer at the Center for Astrophysics | Harvard & Smithsonian, who coauthored the study. “In this case, the properties of the wake are determined by which dark matter theory we apply.”

Conroy led the team that mapped the positions of over 1,300 stars in the halo. The challenge arose in trying to measure the exact distance from Earth to a large portion of those stars: It’s often impossible to figure out whether a star is faint and close by or bright and far away. The team used data from ESA’s Gaia mission, which provides the location of many stars in the sky but cannot measure distances to the stars in the Milky Way’s outer regions.

After identifying stars most likely located in the halo (because they were not obviously inside our galaxy or the LMC), the team looked for stars belonging to a class of giant stars with a specific light “signature” detectable by NEOWISE. Knowing the basic properties of the selected stars enabled the team to figure out their distance from Earth and create the new map. It charts a region starting about 200,000 light-years from the Milky Way’s center, or about where the LMC’s wake was predicted to begin, and extends about 125,000 light-years beyond that.

Conroy and his colleagues were inspired to hunt for LMC’s wake after learning about a team of astrophysicists at the University of Arizona in Tucson that makes computer models predicting what dark matter in the galactic halo should look like. The two groups worked together on the new study.

One model by the Arizona team, included in the new study, predicted the general structure and specific location of the star wake revealed in the new map. Once the data had confirmed that the model was correct, the team could confirm what other investigations have also hinted at: that the LMC is likely on its first orbit around the Milky Way. If the smaller galaxy had already made multiple orbits, the shape and location of the wake would be significantly different from what has been observed. Astronomers think the LMC formed in the same environment as the Milky Way and another nearby galaxy, M31, and that it is close to completing a long first orbit around our galaxy (about 13 billion years). Its next orbit will be much shorter due to its interaction with the Milky Way.

“Confirming our theoretical prediction with observational data tells us that our understanding of the interaction between these two galaxies, including the dark matter, is on the right track,” said University of Arizona doctoral student in astronomy Nicolás Garavito-Camargo, who led work on the model used in the paper.

The new map also provides astronomers with a rare opportunity to test the properties of the dark matter (the notional water or honey) in our own galaxy. In the new study, Garavito-Camargo and colleagues used a popular dark matter theory called cold dark matter that fits the observed star map relatively well. Now the University of Arizona team is running simulations that use different dark matter theories to see which one best matches the wake observed in the stars.

“It’s a really special set of circumstances that came together to create this scenario that lets us test our dark matter theories,” said Gurtina Besla, a co-author of the study and an associate professor at the University of Arizona. “But we can only realize that test with the combination of this new map and the dark matter simulations that we built.”

Launched in 2009, the WISE spacecraft was placed into hibernation in 2011 after completing its primary mission. In September 2013, NASA reactivated the spacecraft with the primary goal of scanning for near-Earth objects, or NEOs, and the mission and spacecraft were renamed NEOWISE. NASA’s Jet Propulsion Laboratory in Southern California managed and operated WISE for NASA’s Science Mission Directorate. The mission was selected competitively under NASA’s Explorers Program managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland. NEOWISE is a project of JPL, a division of Caltech, and the University of Arizona, supported by NASA’s Planetary Defense Coordination Office.

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How galaxies have produced their stars: ASPECS survey provides key chapter of cosmic history

The Hubble Ultra-Deep Field (UDF, left) is one of the best-studied regions of the sky. Using the Hubble Space Telescope, astronomers have identified hundreds of galaxies in the UDF. The light of the most distant of those galaxies has travelled more than 13 billion years to reach us. The right-hand image shows the same region on the sky, observed as part of the ASPECS ALMA Large Program. That image shows millimeter waves emitted by the dust of the UDF galaxies. It provides the deepest view of the distant dusty universe to date. Credit: Space Telescope Science Institute and ASPECS team

Astronomers have used the ALMA observatory to trace the fuel for star formation – molecular hydrogen gas – in the iconic Hubble Ultra-Deep Field, one of the best-studied regions of the sky. The observations allowed a group led by Fabian Walter of the Max Planck Institute for Astronomy to track how the universe’s inventories of gas and dust have changed over time from just two billion years after the big bang to the present. Comparing their own observations with additional observational data and modern simulations, the astronomers were able to characterize and quantify the gas flows that are necessary prerequisites for the formation of stars within galaxies. The result is a broad-brush history of cosmic star formation that includes all the important pieces: the history of star production itself as well as information about the supply chain that enables stars to be produced in the first place.

Tracing the origin of a common household item, like an appliance, amounts to reconstructing a supply chain: the raw materials transformed into more elaborate components, and those components assembled into a finished product. If supplies are missing, production will slow down, or might even grind to a halt. Documenting the factory's inventory of the necessary components or raw material is a useful way of learning about the production history. 

When galaxies form stars, there is of course no planning behind it, economic or otherwise. Stars form whenever the conditions are right for them to form, whenever the right material is available. In order to produce stars, we need cool gas made of hydrogen molecules. Such cool gas is produced when a sufficiently dense cloud of warmer gas made of hydrogen atoms cools down – under the right conditions, the hydrogen atoms pair off, each pair forming a hydrogen molecule H₂. 

The atomic hydrogen inventory can be replenished as well. There is a huge reservoir of ionized hydrogen in the vast spaces between galaxies, warm intergalactic plasma that contains more than 90% of all hydrogen in the universe. Keep track of how those inventories change over time, reconstruct the supply chain, and you can learn about the production history of stars. Keeping track of change is possible because astronomers always look into the past.

A deep look into cosmic history

If we point our telescopes at one of our nearest neighbors, the Andromeda galaxy M31, we see that galaxy as it was 2.5 million years ago, because it took the light we receive now 2.5 million years to travel from Andromeda to us. We cannot observe our own past that way, but we can do the next best thing: All our current knowledge points towards the fact that, on average, the universe is the same everywhere. Regardless of where in the cosmos we are: If we consider a suitably large region, at the present time, we will always find about the same number of larger galaxies, the same number of smaller galaxies, roughly the same number of stars, and the same amount of molecular gas.

That allows astronomers to reconstruct a cross-section of cosmic history. If you want to know what the average properties of the universe were, say, a billion years ago, look at objects so distant that their light takes a billion years to reach us! Repeat the process for different distances, corresponding to different cosmic epochs, and you will obtain at least an average history of the cosmos. The details will vary, but the big picture of cosmic evolution obtained in this way should be valid universally, providing clues about our own cosmic history over the past billions of years.

The history of stellar production rates

Over the past two decades, deep sky surveys using visible light and infrared radiation have given us a fairly complete picture of how many stars there were in galaxies in each cosmic epoch, from the first billion years after the big bang to the present. Particularly important was the Hubble Ultra-Deep Field (UDF): a small region in the sky, about one tenth the apparent diameter of the full moon, where the Hubble Space Telescope captured hundreds of images between 2003 and 2004, with a total of nearly 16 days exposure time, which were then combined into a single image. 

The UDF and other surveys lead to a consistent picture of star formation history, with star production ramping up to a veritable boom some 10 billion years ago, followed by a continuous decline in production rates. Half the stars in the universe had already been produced by the time the universe was 4.5 billion years old, a third of its current age. But why the increase and decline? To answer that, it makes sense to see how much raw material, molecular hydrogen, was available at different times.

The ASPECS observations revealed a three-dimensional view of distant galaxies in the Hubble Ultra-Deep Field (UDF). The third dimension, depth, comes into play because of the cosmological redshift. ALMA observes molecular gas using spectral lines of carbon monoxide. For more distant galaxies, those lines are shifted towards lower frequencies due to the expansion of the universe. ALMA allows astronomers to determine the frequencies of dust emissions in the UDF. Thanks to cosmic expansion, that third dimension of the observations, frequency, is equivalent to line-of-sight distance, resulting in a three-dimensional overall image. The figure shows a rendering of the ALMA data in which the ‘islands’ in the volume correspond to molecular gas emission lines of distant galaxies. Credit:Space Telescope Science Institute and ASPECS team

Molecular gas: the missing piece of the puzzle

This is where ASPECS comes in, the ALMA Spectroscopic Survey in the Hubble Ultra-Deep Field, organised by Fabian Walter (MPIA) and his colleagues. The astronomers used the ALMA observatory in Chile, fully operational since 2013, which can combine up to 50 large (sub)millimeter telescopes in what is called interferometry: a technique that combines telescopes in a way that allows the imaging of fine details that would only be accessible to a much larger single telescope. 

For studying molecular gas in distant galaxies, facilities like ALMA are ideal. Detecting cosmic molecules requires measuring light at specific wavelengths. Because our universe is expanding, there is what is known as the cosmological redshift: The more distant a galaxy is, the farther its light is shifted towards longer wavelengths. For distant galaxies, the wavelengths needed to deduce the presence of hydrogen molecules fall into the millimeter region of the electromagnetic spectrum, corresponding to short radio waves – which is exactly what ALMA was designed to observe. 

The overall collecting area of ALMA is much larger than for any previous millimetre/submillimetre telescopes, so the observatory is very sensitive. That is necessary, as the light reaching us from galaxies billions of light-years away is exceedingly faint. Before ALMA, a survey with the sensitivity of ASPECS would not have been possible. Even with ALMA, ASPECS needed a total of almost 200 hours of observation time, which makes it one of ALMA's so-called large programs – the first such program specifically searching for molecular gas in the distant universe.

An unbiased view of Hubble Ultra-Deep-Field

In order to yield information that can be generalized to the universe as a whole, a survey such as ASPECS needs to be unbiased. (Consider the analogous situation of an opinion poll: In order to reconstruct public opinion, you will need a representative sample of respondents.) To that end, ASPECS chose the best-studied region of the sky, at least when it comes to distant galaxies: the Hubble Ultra-Deep Field (UDF). The combined image Hubble Ultra-Deep Field contains around 10,000 identifiable galaxies. Light from the most distant galaxy took 13 billion years to reach us. (For comparison: The big bang happened 13.8 billion years ago.) ASPECS scanned the Hubble Ultra-Deep Field at wavelengths around 1.3 mm and 3 mm. In their survey, the researchers followed an observational approach that had been shown to work well through a number of pilot programs, both with the IRAM Plateau de Bure Interferometer and with earlier ALMA observations. At those specific wavelengths, the Earth's atmosphere is virtually transparent, in particular at high-elevation locations such as the Chajnantor plateau in Chile where ALMA is located, at an elevation of 5000 meters.

More specifically, at each location within the Hubble Ultra-Deep Field, the astronomers took two spectra, carefully mapping the intensity of light received at different wavelengths between 1.1 and 1.4 mm, and also between 2.6 and 3.6 mm. In such spectra, molecules reveal themselves via so-called emission lines – narrow wavelength regions where there is a sharp maximum of intensity. While molecular hydrogen has no detectable emission lines, a molecule that is typically found in its company does: Carbon monoxide CO has a number of clearly detectable lines. 

From the nearby cosmos, we know that in a typical interstellar gas cloud, for each CO molecule, you will find on the order of 10,000 hydrogen molecules. As hydrogen molecules bump into CO molecules, the CO molecules gain energy – which they then emit in the form of electromagnetic radiation, at the wavelengths corresponding to their emission line. Measure the intensity of those CO lines, and you can deduce the amount of molecular hydrogen that is around in that specific region, occasionally bumping into CO. By taking into account the redshift observed for a particular set of lines, it is possible to reconstruct the distance of the gas in question: in an expanding universe like ours, the (cosmological) redshift is directly related to an object's distance from us. In this way, ASPECS was able to probe the cosmological volume of the Ultra-Deep Field, mapping gas-cloud positions in three dimensions.

Keeping track of galaxies-ant their molecular gas

The estimate can be made more precise by combining it with another method. Because cosmic dust acts as a catalyst in the formation of molecular hydrogen, there is a correlation between the amount of dust and molecular hydrogen present. ALMA can measure the thermal radiation from that dust in parallel to the CO, allowing for a cross-check. 

In the end, the ASPECS data provided the deepest view of the dusty universe to date, and was able to pinpoint which of the many galaxies visible in the Hubble Space Telescope observations are rich in molecular gas and dust: the material that is essential for star formation to proceed. These galaxies showed a wide range of physical properties: many of them are "normal galaxies" (with average stellar masses and star formation rates), but others are classified as starbursts (with unusually high star formation activity) or quiescent galaxies (unusually low activity).

Reconstructiin the star-production supply chain

Once they had made their observations, Fabian Walter and his colleagues were ready to reconstruct the history of molecular hydrogen supplies throughout cosmic history – more specifically: from about 2 billion years after the big bang (nearly 12 billion years ago) to the present. To this end, they drew together the data from previous studies, namely data about atomic hydrogen and about the total mass of all stars in a given epoch. They also compared their findings with large-scale simulations of cosmic history from the big bang to the present.

If you are not an astronomer, the resulting history might not sound all that exciting, compared to the human history you know and can relate to. But for astronomers, it captures deep truths about how our cosmos has changed over time. In that history, the amount of molecular hydrogen steadily increased until about 10 billion years ago, about 4 billion years after the big bang (at about cosmic redshift z=1.5, to use the astronomers' preferred way of denoting a cosmic epoch), with the inventory almost doubling within 3 billion years. This evolution had already been suggested by previous studies. But it is only now that the observations were sufficiently accurate for the firm conclusion that cosmic gas density rises and falls over cosmic time. That rise, then, corresponds to the Golden Age of star formation: With plenty of raw material just waiting to be turned into blazing suns, and with half of the stars that ever existed coming into being in that first third of cosmic history. At the high point, there was about as much molecular hydrogen as there was atomic hydrogen.

What is behind the history of star formation?

In comparing their data with simulations, the astronomers found that behind those boom times was a combination of factors. Galaxies are only the visible tip of the iceberg – their backbone, so to speak, are accumulations of dark matter, matter that does not interact with electromagnetic radiation and thus remains invisible to direct observations. Dark matter accounts for about 80% of all mass in the universe. Just like all other matter, dark matter started out distributed almost perfectly homogeneously through the cosmos shortly after the big bang, but has clumped, and thus become increasingly inhomogeneous, owing to mutual gravitational attraction. In the present-day universe, on a scale of hundreds of millions of light-years, dark matter forms a sponge-like network of filaments, sprinkled with particularly dense regions known as halos. 

Galaxies formed as ordinary matter, mostly hydrogen gas, was drawn into those halos, following their gravitational attraction: First, plasma falls onto halos from the huge reservoir in intergalactic space, cooling down to form atoms. This process replenishes the supply of atomic hydrogen within galaxies. Then, the atomic hydrogen is drawn towards the centers of galaxies, cooling down further until it forms molecular hydrogen, and eventually stars. Through the ASPECS observations, Walter and his colleagues were able to quantify these gas flows as a function of cosmic time.

Looking towards the future, as halo growth slows down and less hydrogen plasma is drawn onto galaxies, star production becomes less and less effective. At the present time, galaxies form stars at a mere tenth of the production rate of the Golden Age. Production rates have been in sharp decline for the past 9 billion years. Based on their observations, Walter and his colleagues predict a continuing trend: Over the next 5 billion years, the molecular gas reservoirs will shrink by an additional factor of 2, while the total mass of stars in the universe increases by a mere 10%. In this picture, star production would eventually cease altogether.

Next Steps

The ASPECS observations were designed to be very sensitive, by summing up the light from a larger region in each image pixel. But that automatically meant they could not distinguish smaller details – such as mapping the molecular hydrogen within each galaxy. But now that the combination and ASPECS and Ultra-Deep Field images has enabled astronomers to pinpoint its gas-rich and dust-rich galaxies, the next step will be to take a closer look at those galaxies individually. ALMA has a high-resolution mode that is ideal for that kind of close scrutiny.

This would allow Walter and his colleagues to compare the structure of the molecular gas and dust in those galaxies to the distribution of stars – are the two directly related? Do we indeed find molecular gas and dust in the same region where we find young stars? The more detailed measurements would also yield information about key parameters such as the kinematics, temperature and density of the gas.

With that new ALMA data, plus complementary results from observing campaigns of the Ultra-Deep Field planned for the upcoming James Webb Space Telescope (JWST), the astronomers hope to reconstruct the cosmic history of star formation in even more detail.

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 Background information 

The results described here have been accepted for publication as F. Walter et al., " The Evolution of the Baryons Associated with Galaxies Averaged over Cosmic Time and Space" in The Astrophysical Journal.

Original article for this press release:

• Walter et al. 2020 (1.29 MB)

The ASPECS collaboration is presenting their results on a new website, that will be open to the public from 24 September 2020 onwards. The website also features images, videos and an interactive presentation of the ASPECS results: 

• ASPECS website
• All published articles of the ASPECS project

The research was carried out by MPIA's Fabian Walter, Marcel Neeleman and Hans-Walter Rix in collaboration with Manuel Aravena (Universidad Diego Portales, Chile), Chris Carilli (NRAO, Socorro, USA) and Roberto Decarli (INAF, Bologna , Italy).

The research is part of the of the project Cosmic_Gas that has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant agreement No. 740246). 

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA. 

 

Source:  Max Planck Institute for Astronomy

 

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Hubble Maps a Giant Halo Around the Andromeda Galaxy

This illustration shows the location of the 43 quasars scientists used to probe Andromeda’s gaseous halo. These quasars—the very distant, brilliant cores of active galaxies powered by black holes—are scattered far behind the halo, allowing scientists to probe multiple regions. Looking through the immense halo at the quasars’ light, the team observed how this light is absorbed by the halo and how that absorption changes in different regions. By tracing the absorption of light coming from the background quasars, scientists are able to probe the halo’s material. Credits:NASA,ESA, and E. Wheatley (STScI)

At a distance of 2.5 million light-years, the majestic spiral Andromeda galaxy it is so close to us that it appears as a cigar-shaped smudge of light high in the autumn sky. If its gaseous halo could be seen with the naked eye, it would be about three times the width of the Big Dipper—easily the biggest feature on the nighttime sky. Credits: NASA, ESA, J. DePasquale and E. Wheatley (STScI) and Z. Levay 

This diagram shows the light from a background quasar passing through the vast, gaseous halo around the neighboring Andromeda galaxy (c, as spectroscopically measured by the Hubble Space Telescope. The colored regions show absorption from two components that make up the halo. For ionized silicon, a significant absorption is shown in both plots. The more highly ionized carbon is absorbed by only one component. Astronomers can tell the two components apart because their line-of-sight motions, known as radial velocity, cause a Doppler shift that changes the wavelength of light being absorbed. Credits: NASA, ESA, and E. Wheatley (STScI) 

In a landmark study, scientists using NASA’s Hubble Space Telescope have mapped the immense envelope of gas, called a halo, surrounding the Andromeda galaxy, our nearest large galactic neighbor. Scientists were surprised to find that this tenuous, nearly invisible halo of diffuse plasma extends 1.3 million light-years from the galaxy—about halfway to our Milky Way—and as far as 2 million light-years in some directions. This means that Andromeda’s halo is already bumping into the halo of our own galaxy.

They also found that the halo has a layered structure, with two main nested and distinct shells of gas. This is the most comprehensive study of a halo surrounding a galaxy.

“Understanding the huge halos of gas surrounding galaxies is immensely important,” explained co-investigator Samantha Berek of Yale University in New Haven, Connecticut. “This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae. It’s full of clues regarding the past and future evolution of the galaxy, and we’re finally able to study it in great detail in our closest galactic neighbor.”

“We find the inner shell that extends to about a half million light-years is far more complex and dynamic,” explained study leader Nicolas Lehner of the University of Notre Dame in Indiana. “The outer shell is smoother and hotter. This difference is a likely result from the impact of supernova activity in the galaxy’s disk more directly affecting the inner halo.”

A signature of this activity is the team’s discovery of a large amount of heavy elements in the gaseous halo of Andromeda. Heavier elements are cooked up in the interiors of stars and then ejected into space—sometimes violently as a star dies. The halo is then contaminated with this material from stellar explosions.

The Andromeda galaxy, also known as M31, is a majestic spiral of perhaps as many as 1 trillion stars and comparable in size to our Milky Way. At a distance of 2.5 million light-years, it is so close to us that the galaxy appears as a cigar-shaped smudge of light high in the autumn sky. If its gaseous halo could be viewed with the naked eye, it would be about three times the width of the Big Dipper. This would easily be the biggest feature on the nighttime sky.

Through a program called Project AMIGA (Absorption Map of Ionized Gas in Andromeda), the study examined the light from 43 quasars—the very distant, brilliant cores of active galaxies powered by black holes—located far beyond Andromeda. The quasars are scattered behind the halo, allowing scientists to probe multiple regions. Looking through the halo at the quasars’ light, the team observed how this light is absorbed by the Andromeda halo and how that absorption changes in different regions. The immense Andromeda halo is made of very rarified and ionized gas that doesn’t emit radiation that is easily detectable. Therefore, tracing the absorption of light coming from a background source is a better way to probe this material.

The researchers used the unique capability of Hubble’s Cosmic Origins Spectrograph (COS) to study the ultraviolet light from the quasars. Ultraviolet light is absorbed by Earth’s atmosphere, which makes it impossible to observe with ground-based telescopes. The team used COS to detect ionized gas from carbon, silicon and oxygen. An atom becomes ionized when radiation strips one or more electrons from it.

Andromeda’s halo has been probed before by Lehner’s team. In 2015, they discovered that the Andromeda halo is large and massive. But there was little hint of its complexity; now, it’s mapped out in more detail, leading to its size and mass being far more accurately determined.

“Previously, there was very little information—only six quasars—within 1 million light-years of the galaxy. This new program provides much more information on this inner region of Andromeda’s halo,” explained co-investigator J. Christopher Howk, also of Notre Dame. “Probing gas within this radius is important, as it represents something of a gravitational sphere of influence for Andromeda.”

Because we live inside the Milky Way, scientists cannot easily interpret the signature of our own galaxy’s halo. However, they believe the halos of Andromeda and the Milky Way must be very similar since these two galaxies are quite similar. The two galaxies are on a collision course, and will merge to form a giant elliptical galaxy beginning about 4 billion years from now.

Scientists have studied gaseous halos of more distant galaxies, but those galaxies are much smaller on the sky, meaning the number of bright enough background quasars to probe their halo is usually only one per galaxy. Spatial information is therefore essentially lost. With its close proximity to Earth, the gaseous halo of Andromeda looms large on the sky, allowing for a far more extensive sampling.

“This is truly a unique experiment because only with Andromeda do we have information on its halo along not only one or two sightlines, but over 40,” explained Lehner. “This is groundbreaking for capturing the complexity of a galaxy halo beyond our own Milky Way.”

In fact, Andromeda is the only galaxy in the universe for which this experiment can be done now, and only with Hubble. Only with an ultraviolet-sensitive future space telescope will scientists be able to routinely undertake this type of experiment beyond the approximately 30 galaxies comprising the Local Group.

“So Project AMIGA has also given us a glimpse of the future,” said Lehner.

The team’s findings appear in the August 27 edition of The Astrophysical Journal.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.

Source: HubbleSite/News

Release Images

Contact

Ann Jenkins / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4514
jenkins@stsci.edu / villard@stsci.edu 

Nicolas Lehner
University of Notre Dame, Notre Dame, Indiana
574-631-5755
nlehner@nd.edu

Relatec Links

The science paper by N. Lehner et al.  

NASA's Hubble Portal

Hubble Finds Giant Halo Around the Andromeda Galaxy (STScI's News Release - May 7, 2015) 

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