NASA's Hubble Provides Bird's-Eye View of Andromeda Galaxy's Ecosystem

Survey of Andromeda's Satellite Galaxies
Credits/Image: NASA, ESA, Alessandro Savino (UC Berkeley), Joseph DePasquale (STScI), Akira Fujii DSS2.

The Distribution of Satellite Galaxies around M31
Credits/Visualization: NASA, ESA, Christian Nieves (STScI)
Science: Alessandro Savino (UC Berkeley)
Acknowledgment:Joseph DePasquale (STScI), Frank Summers (STScI), Robert Gendler

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Located 2.5 million light-years away, the majestic Andromeda galaxy appears to the naked eye as a faint, spindle-shaped object roughly the angular size of the full Moon. What backyard observers don't see is a swarm of nearly three dozen small satellite galaxies circling the Andromeda galaxy, like bees around a hive.

These satellite galaxies represent a rambunctious galactic "ecosystem" that NASA's Hubble Space Telescope is studying in unprecedented detail. This ambitious Hubble Treasury Program used observations from more than a whopping 1,000 Hubble orbits. Hubble's optical stability, clarity, and efficiency made this ambitious survey possible. This work included building a precise 3D mapping of all the dwarf galaxies buzzing around Andromeda and reconstructing how efficiently they formed new stars over the nearly 14 billion years of the universe's lifetime.

In the study published in The Astrophysical Journal, Hubble reveals a markedly different ecosystem from the smaller number of satellite galaxies that circle our Milky Way. This offers forensic clues as to how our Milky Way galaxy and Andromeda have evolved differently over billions of years. Our Milky Way has been relatively placid. But it looks like Andromeda has had a more dynamic history, which was probably affected by a major merger with another big galaxy a few billion years ago. This encounter, and the fact that Andromeda is as much as twice as massive as our Milky Way, could explain its plentiful and diverse dwarf galaxy population.

Surveying the Milky Way's entire satellite system in such a comprehensive way is very challenging because we are embedded inside our galaxy. Nor can it be accomplished for other large galaxies because they are too far away to study the small satellite galaxies in much detail. The nearest galaxy of comparable mass to the Milky Way beyond Andromeda is M81, at nearly 12 million light-years.

This bird's-eye view of Andromeda's satellite system allows us to decipher what drives the evolution of these small galaxies. "We see that the duration for which the satellites can continue forming new stars really depends on how massive they are and on how close they are to the Andromeda galaxy," said lead author Alessandro Savino of the University of California at Berkeley. "It is a clear indication of how small-galaxy growth is disturbed by the influence of a massive galaxy like Andromeda."

"Everything scattered in the Andromeda system is very asymmetric and perturbed. It does appear that something significant happened not too long ago," said principal investigator Daniel Weisz of the University of California at Berkeley. "There's always a tendency to use what we understand in our own galaxy to extrapolate more generally to the other galaxies in the universe. There's always been concerns about whether what we are learning in the Milky Way applies more broadly to other galaxies. Or is there more diversity among external galaxies? Do they have similar properties? Our work has shown that low-mass galaxies in other ecosystems have followed different evolutionary paths than what we know from the Milky Way satellite galaxies."

For example, half of the Andromeda satellite galaxies all seem to be confined to a plane, all orbiting in the same direction. "That's weird. It was actually a total surprise to find the satellites in that configuration and we still don't fully understand why they appear that way," said Weisz.

The brightest companion galaxy to Andromeda is Messier 32 (M32). This is a compact ellipsoidal galaxy that might just be the remnant core of a larger galaxy that collided with Andromeda a few billion years ago. After being gravitationally stripped of gas and some stars, it continued along its orbit. Galaxy M32 contains older stars, but there is evidence it had a flurry of star formation a few billion years ago. In addition to M32, there seems to be a unique population of dwarf galaxies in Andromeda not seen in the Milky Way. They formed most of their stars very early on, but then they didn't stop. They kept forming stars out of a reservoir of gas at a very low rate for a much longer time.

"Star formation really continued to much later times, which is not at all what you would expect for these dwarf galaxies," continued Savino. "This doesn't appear in computer simulations. No one knows what to make of that so far."

"We do find that there is a lot of diversity that needs to be explained in the Andromeda satellite system," added Weisz. "The way things come together matters a lot in understanding this galaxy's history."

Hubble is providing the first set of imaging where astronomers measure the motions of the dwarf galaxies. In another five years Hubble or NASA's James Webb Space Telescope will be able to get the second set of observations, allowing astronomers to do a dynamical reconstruction for all 36 of the dwarf galaxies, which will help astronomers to rewind the motions of the entire Andromeda ecosystem billions of years into the past.

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

Science Contact:

Alessandro Savino
University of California, Berkeley, California

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Contact Us: Direct inquiries to the News Team.

Related Links and Documents

• Science Paper: The science paper by Alessandro Savino et al. (on ApJ), PDF (5.77 MB)
• NASA's Hubble Portal

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Minor Merger Kicks Supermassive Black Hole into High Gear

Figure 1:  The deep image of Messier 77 taken with the Hyper Suprime-Cam (HSC) mounted at the Subaru Telescope. The picture is created by adding the color information from the Sloan Digital Sky Survey (Note 1) to the monochromatic image acquired by the HSC. (Credit: NAOJ/SDSS/David Hogg/Michael Blanton. Image Processing: Ichi Tanaka)

The galaxy Messier 77 (M77) is famous for its super-active nucleus that releases enormous energy across the electromagnetic spectrum, ranging from x-ray to radio wavelengths. Yet, despite its highly active core, the galaxy looks like any normal quiet spiral. There's no visual sign of what is causing its central region to radiate so extensively. It has long been a mystery why only the center of M77 is so active. Astronomers suspect a long-ago event involving a sinking black hole, which could have kicked the core into high gear.

To test their ideas about why the central region of M77 beams massive amounts of radiation, a team of researchers at the National Astronomical Observatory of Japan and the Open University of Japan used the Subaru Telescope to study M77. The unprecedented deep image of the galaxy reveals evidence of a hidden minor merger billions of years ago. The discovery gives crucial evidence for the minor merger origin of active galactic nuclei.

The Mystery of Seyfert Galaxies

The galaxy Messier 77 (NGC 1068) is famous for harboring an active nucleus at its core that releases an enormous amount of energy. The existence of such active galaxies in the nearby universe was first noted by the American astronomer Carl Seyfert more than 70 years ago. Nowadays they are called the Seyfert galaxies (Note 2). Astronomers think that the source of such powerful activity is the gravitational energy released from superheated matter falling onto a supermassive black hole (SMBH) that resides in the center of the host galaxy. The estimated mass of such a SMBH for M77 is about 10 million times that of the Sun.

It takes a massive amount of gas dumped on the galaxy's central black hole to create such strong energies. That may sound like an easy task, but it's actually very difficult. The gas in the galactic disk will circulate faster and faster as it spirals into the vicinity of the SMBH. Then, at some point the "centrifugal force" balances with the gravitational pull of the SMBH. That actually prevents the gas from falling into the center. The situation is similar to water draining out of a bathtub. Due to the centrifugal force, the rapidly rotating water will not drain out rapidly. So, how can the angular momentum be removed from the gas circling near an active galactic nucleus? Finding the answer to that question is one of the big challenges for researchers today.

A Prediction Posed 18 Years Ago

In 1999, Professor Yoshiaki Taniguchi (currently at the Open University of Japan), the team leader of the current Subaru study, published a paper about the driving mechanism of the active nucleus of Seyfert galaxies such as M 77. He pointed out that a past event – a "minor merger" where the host galaxy ate up its "satellite" galaxy (a small low-mass galaxy orbiting it) – would be the key to activating the Seyfert nucleus (Note 3).

Usually, a minor merger event simply breaks up a low-mass satellite galaxy. The resulting debris is absorbed into the disk of the more massive host galaxy before it approaches the center. Therefore, it was not considered as the main driver of the nuclear activity. "However, the situation could be totally different if the satellite galaxy has a (smaller) SMBH in its center (Note 4)," Professor Taniguchi suggests, "because the black hole can never be broken apart. If it exists, it should eventually sink into the center of the host galaxy."

The sinking SMBH from the satellite galaxy would eventually create a disturbance in the rotating gas disk around the main galaxy's SMBH. Then, the disturbed gas would eventually rush into the central SMBH while releasing enormous gravitational energy. "This must be the main ignition mechanism of the active Seyfert nuclei," Taniguchi argued. "The idea can naturally explain the mystery about the morphology of the Seyfert galaxies," said Professor Taniguchi, pointing out the advantage of the model of normal-looking galaxies also being very active at their cores. (Note 5).

Probing the Theory Using the Subaru Telescope

Recent advances in observational technique allow the detection of the extremely faint structure around galaxies, such as loops or debris that are likely made by dynamical interactions with satellite galaxies.. The outermost parts of galaxies are often considered as relatively "quiet" with a longer dynamical timescale than anywhere inside. Simulations show that the faint signature of a past minor merger can remain several billion years after the event. "Such a signature can be a key test for our minor merger hypothesis for Seyfert galaxies. Now it is time to revisit M77," said Taniguchi.

The team's choice to look for 'the past case' was, of course, the Subaru Telescope and its powerful imaging camera, Hyper Suprime-Cam. The observing proposal was accepted and executed on Christmas night 2016. "The data was just amazing," said Dr. Ichi Tanaka, the primary investigator of the project. "Luckily, we could also retrieve the other data that was taken in the past and just released from the Subaru Telescope's data archive. Thus, the combined data we got finally is unprecedentedly deep."

Figure 2: (Left) The newly-discovered, extremely　diffuse structures around M77. The innermost color part of the picture shows the bright part of the galaxy (from SDSS: see the center of Figure 1). The middle part in red-brown is the contrast-enhanced expression of the faint one-arm structure (labeled as "Banana") to the right, as well as the ripple structure (labeled as "Ripple") to the left. All the fore/background objects unrelated to M77 are removed during the process. The outermost monochrome part shows the faint ultra-diffuse structures in yellow circles (labelled as "UDO-SE", "UDO-NE", "UDO-SW"). A deep look at them indicates the latter two ("UDO-NE", "UDO-SW") constitute a part of the large loop-like structure. (Credit: NAOJ)

(Right) Artist's impression of M77. The illustration in the right is created and copyrighted by Mr. Akihiro Ikeshita. (Credit: Akihiro Ikeshita

Subaru's great photon-collecting power and the superb performance of the Hyper Suprime-Cam were crucial in the discovery of the extremely faint structures in M77. Their discovery reveals the normal-looking galaxy's hidden violent past.. "Though people may sometimes make a lie, galaxies never do. The important thing is to listen to their small voices to understand the galaxies," said Professor Taniguchi.

The team will expand its study to more Seyfert galaxies using the Subaru Telescope. Dr. Masafumi Yagi, who leads the next phase of the project said, "We will discover more and more evidences of the satellite merger around Seyfert host galaxies. We expect that the project can provide a critical piece for the unified picture for the triggering mechanism for active galactic nuclei."

The result is going to be published in the Volume 69 Issue 6 of the Publications of the Astronomical Society of Japan (I. Tanaka, M.Yagi & Y. Taniguchi 2017, "Morphological evidence for a past minor merger in the Seyfert galaxy NGC 1068"). The research is financially supported by the Basic Research A grant JP16H02166 by the Grant-in-Aid for Scientific Research progrram.

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Notes

Note1: The color image by the Sloan Digital Sky Survey used for Figure 1 is under the copyright of David W. Hogg and Michael R. Blanton.

Note 2: Seyfert galaxies are actually a subclass of the active galactic nuclei. There are even more powerful active galactic nuclei called quasar in the universe. Usually quasars are found much farther away than M77.

Note 3: Satellite galaxies are common for large galaxies. For example, there are two bright satellite galaxies called Large and Small Magellanic Clouds associated with our Milky Way. The Andromeda galaxy, our nearest neighbor, also has two bright satellites called Messier 32 and NGC 205.

Note 4: Astronomers believe that most galaxies have an SMBH in their central regions, with its mass mysteriously scaled to the mass of the host galaxy. It is also known that some satellite galaxies also have smaller SMBH. For example, Messier 32 (satellite of the Andromeda galaxy) is likely to have a SMBH much heavier than a million times the mass of our Sun. It is however not easy to directly prove the existence of the SMBH for satellite galaxies due to its light weight.

Note 5: Y. Taniguchi 1999, ApJ, 524, 65, for the reference.

The research team:

• Ichi Tanaka: Subaru Telescope, National Astronomical Observatory of Japan
• Masafumi Yagi: National Astronomical Observatory of Japan
• Yoshiaki Taniguchi: The Open University of Japan

Source: Subaru Telescope

SOFIA Opens New Window on Star Formation in Orion

SOFIA’s mid-infrared image of Messier 42 (right) with comparison images of the same region made at other wavelengths by the Hubble Space Telescope (left) and European Southern Observatory (middle). (Credits: Visible-light image: NASA/ESA/HST/AURA/STScI/O’Dell & Wong; Near-IR image: ESO/McCaughrean et al.; Mid-IR image: NASA/DLR/SOFIA/USRA/DSI/FORCAST Team. View Larger Image

SOFIA mid-infrared mosaic image of the Messier 42 region from the Short Science 1 observing program, December 2010. (19 microns in green and 37 microns in red) (NASA/DLR/SOFIA/USRA/DSI/FORCAST Team. View Larger Image

A mid-infrared mosaic image from the Stratospheric Observatory for Infrared Astronomy, or SOFIA, offers new information about processes of star formation in and around the nebula Messier 42 in the constellation Orion. The image data were acquired using the Faint Object Infrared Camera for the SOFIA Telescope, or FORCAST, by principal investigator Terry Herter, of Cornell University during SOFIA’s Short Science 1 observing program in December 2010.

SOFIA’s mid-infrared image of Messier 42, plus comparison images of the same region made at other wavelengths by other observatories, are available at: http://www.nasa.gov/sofia.

SOFIA's view combines images at mid-infrared wavelengths of 19.7 microns (green) and 37.1 microns (red). The latter wavelength cannot be accessed by any telescope on the ground or currently in space. Detailed structures in the clouds of star construction material can be seen, as well as warm clouds of dust and gas surrounding, and partly obscuring, a cluster of luminous newborn stars at upper right.

The left and center panels of the three-image comparison have the same scale and orientation as the SOFIA image.

The image in the left panel, made at wavelengths visible to the human eye, shows dense clouds of interstellar dust blocking our view into parts of the star forming region, plus the rosy glow of hydrogen gas excited by radiation from the young stars just above the center.

In the center panel, the near-infrared image penetrates some of the dust and reveals numerous stars at various stages of formation, embedded inside the clouds.

SOFIA’s observations reveal distinctly different aspects of the M42 star formation complex than the other images. For example, the dense dust cloud at upper left is completely opaque in the visible-light image, partly transparent in the near-infrared image, and is seen shining with its own heat radiation in the SOFIA mid-infrared image. The hot stars of the Trapezium cluster are seen just above the centers of the visible-light and near-infrared images, but they are almost undetectable in the SOFIA image. At upper right, the dust-embedded cluster of high-luminosity stars that is the most prominent feature in the SOFIA mid-infrared image is less apparent in the near-infrared image and is completely hidden in the visible-light image.

For more information about SOFIA, visit:
http://www.nasa.gov/sofia
http://www.dlr.de/en/sofia

For information about SOFIA's science mission, visit:
http://www.sofia.usra.edu
http://www.dsi.uni-stuttgart.de/index.en.html

Contact
Cathy Weselby/Nicholas A. Veronico
NASA Ames Research Center, Moffett Field, Calif.
650-604-2791/650-604-4589
cathy.weselby@nasa.gov, nveronico@sofia.usra.edu

Beth Hagenauer
NASA Dryden Flight Research Center, Edwards, Calif.
661-276-7960
beth.hagenauer-1@nasa.gov