A superbubble scene

LHA 120-N44

TA dense field of stars fills the image, surrounded by wispy clouds of gas and dust. Pale blue and grey nebulosity forms an intricate web across the scene, with darker clouds of dust concentrated towards the lower right. Numerous bright stars appear in shades of blue, white and orange. Credit: ESA/Hubble & NASA, D. Gouliermis.

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This sprawling cosmic vista and subject of today’s ESA/Hubble Picture of the Month comes from the Large Magellanic Cloud, or LMC. The LMC is the largest of the small galaxies that orbit our Milky Way galaxy. At just 160 000 light-years away, the LMC offers a close look at highly active stellar birthsites like the nebula shown here. This nebula is named LHA 120-N44, or N44 for short, and it’s located in the constellation Dorado.

The appearance of this photogenic nebula is dominated by two features: a vast central void and a shell of dense, dusty gas. The central void is a ‘superbubble’ spanning roughly 210 by 140 light-years. The glittering stars at the centre of the void are responsible for its creation; through their powerful stellar winds and explosive supernovae, these stars have expelled much of the gas from which they were born.

When the stars of N44’s central star cluster swept away this gas, the expelled gas formed a shell around the superbubble. New stars are forming in this compressed gas shell, making N44 an interesting target for researchers studying the process of star formation. In particular, astronomers have turned to this nebula as an ideal place to time this process from start to finish. Their goal is to understand how long it takes from the collapse of cold gas clouds into dense knots to the moment nuclear fusion ignites in the heart of a newborn star.

Researchers used Hubble to survey N44 and take a census of its stars, cataloguing nearly half a million stars within the cluster as well as interlopers drifting in front of it. Of the stars surveyed, nearly 30 000 are what astronomers call pre-main-sequence stars, which have yet to begin fusing hydrogen into helium in their cores. This treasure trove of baby stars was discoverable thanks to the high sensitivity and fine spatial resolution of Hubble’s instruments that can pick out faint objects in crowded clusters.

The gas shell surrounding the superbubble is energised by ultraviolet radiation from massive stars, causing it to glow and highlighting several distinct features. Each feature within the broader N44 star-forming complex was catalogued by astronomer Karl Henize in the 1950s. One feature is a smaller bubble, catalogued as N44F, that is located near the upper-right corner of this image. N44F is an interstellar bubble blown by the intense stellar winds of a single hot and massive star. As this previously released Hubble closeup shows, the star’s furious winds and radiation have sculpted the surrounding bubble and created pillars of dusty gas.

The data used to create this image come from an observing programme (#14689; PI: Gouliermis) that aimed to probe stars in the N44 complex that have not yet begun fusing hydrogen into heavier elements in their cores. These data help to determine how long the process of star formation takes, as well as what masses newborn stars typically have. Hubble's sensitive observations of the lowest-mass stars in this region open a new window onto star formation in regions that, like the LMC or the galaxies of the early Universe, are poor in elements heavier than helium.

Source: ESA/Hubble/potm

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Links

• Image on ESA website
• Video: Space Sparks episode 28
• Video: Pan of LHA 120-N44
• Video: Zoom into LHA 120-N44

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An inside-out view of active supermassive black holes and their host galaxies

Different telescopes reveal different signatures of active black holes. LOFAR (left) detects radio jets; WISE (centre) detects hot dust; SDSS/MaNGA (right) provides spatially resolved optical spectra of stars and gas. Credits: ASTRON, NASA/JPL-Caltech, SDSS, MPA

Active supermassive black holes do not all look the same. Some are obscured by hot dust, some are surrounded by fast-moving ionised gas, and some can eject giant radio jets extending over millions of light-years. By mapping about two thousand such objects from their galactic centres outward, researchers at the Max Planck Institute for Astrophysics found that these variations are related to the properties of their host galaxies. Infrared and optical AGNs are found in star-forming galaxies with young centres and ionised winds, whereas radio AGNs are mostly found in older, quieter systems. A small group showing both kinds of activity sits in between.

Seeing Active Black Holes in a Different Light

An active galactic nucleus (AGN) can appear in several forms, such as a hot, dusty torus that glows in infrared light; broad emission lines from gas moving close to the black hole; narrow lines from gas farther out; or a radio jet blasting away from the centre. Some of this diversity is simply due to our viewing angle. For example, an AGN seen edge-on through its dusty torus looks different from one seen face-on. However, orientation alone cannot explain everything. A long-standing question is whether these different 'faces' of AGN activity are also connected to what is happening in the surrounding galaxy, and if so, how

Host-galaxy properties change systematically from the nucleus outward for different AGN types. Each column shows one AGN class, with radial profiles of star formation (top), stellar age (middle), and gas ionisation (bottom); grey lines show reference profiles for normal galaxies. Infrared and optical AGN hosts show enhanced central star formation and younger stellar populations, while radio AGN hosts resemble quiescent galaxies at all radii. Credit: SDSS, MPA/G. Jin

From the Nucleus to the Outskirts

To find out, the team combined data on around ten thousand nearby galaxies, including nearly two thousand identified AGNs, taken from three surveys, each of which contributes a different piece of the picture. WISE, an infrared satellite, reveals hot dust. MaNGA is an integral-field spectroscopic survey that supplies optical AGN signatures and spectra at multiple positions across each galaxy. This allows the team to map properties rather than just measuring them in aggregate. LoTSS, a state-of-the-art radio survey using the LOFAR telescope, traces synchrotron emission from jets. Together, these datasets enable the researchers to trace the evolution of each galaxy from its inner regions to its outskirts.

The first differences appear in the central regions. Galaxies hosting infrared AGN exhibit the clearest boost in central star formation, forming stars at a faster rate near their nuclei than similar galaxies without AGN activity. These infrared and optical AGN hosts also have younger stellar populations at their centres, as determined by a spectral indicator of stellar age. In contrast, radio AGN hosts have old central populations and reduced central star formation, much like normal quiescent galaxies. The simultaneous presence of black-hole growth and central star formation in the same populations is consistent with both processes being fed by the same gas supply, though the data do not show one triggering the other.

The gas emission and kinematics also carry the AGN's signature. Infrared, broad-line and narrow-line AGNs all exhibit stronger gas ionisation towards the centre, and this excess compared to normal galaxies extends over several kiloparsecs before fading. Fast-moving, ionised winds are strongest in infrared and broad-line AGNs, and their average signal extends to around 2 kiloparsecs from the nucleus. Radio AGNs show no comparable outflow on average. These winds clearly disturb the ionised gas around the black hole. However, the data do not show that star formation is immediately shut down by the current AGN. The link between black-hole and stellar growth is strongest near the centre and gradually weakens towards the outskirts.

A schematic view of the proposed AGN population sequence created with the help of AI. Left: a radiative AGN with abundant gas and a prominent dusty torus, typical of star-forming hosts. Centre: a mixed AGN with compact radio emission alongside radiative signatures. Right: a radio-dominated AGN with extended jets, typical in quiescent hosts. This is a map of connected AGN-galaxy states, not a timeline for any individual galaxy. Credit: MPA/G. Jin

A Bridge Between AGN Modes

A small group of AGNs exhibits both optical/infrared and radio signatures. Several independent measurements show that these 'mixed' AGNs lie between the radiative and radio-dominated populations, providing an observational link rather than a clear boundary between the two. Their radial star-formation profiles lie between those of optical/infrared-only and radio-only AGNs. Their stacked spectra reveal a combination of strong emission lines and a significant 4000-Ångström break, suggesting ongoing black-hole accretion within an ageing stellar population. While their radio emission is present, it remains relatively compact compared with the extended jets of radio-only AGNs.

These findings suggest a possible population-wide sequence running from radiative AGNs in star-forming hosts to radio AGNs in quiescent ones, with mixed AGNs marking an intermediate stage. However, it is important to note that this sequence should be interpreted as a map of connected AGN-galaxy states rather than as a time-lapse of a single galaxy. AGN episodes last far less time than the galaxy evolution, and any one galaxy may experience repeated AGN cycles throughout its lifetime. Changes in large-scale gas supply may contribute to the pattern, but the available data cannot identify the underlying cause.

What's missing?

Several observations could clarify this picture. Molecular gas from CO observations would reveal the fuel reservoir directly. X-ray observations could reveal the hot atmospheres thought to exist around radio-mode AGNs. Higher-resolution radio images would clarify whether the compact jets in mixed AGNs are younger. Comparing these with spatially resolved simulations would reveal whether the observed states arise from one evolutionary route or several. Ultimately, these observations will reveal whether the empirical map uncovered here reflects a common evolutionary route, repeated cycles or multiple paths through black hole and galaxy growth.

Source: Max Planck Institute for Astrophysics/Research Highlights

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Authors:

Gaoxiang Jin
PhD student
Tel: 2298
Email: gxjin@mpa-garching.mpg.de/a>

Guinevere Kauffmann
Director
Tel: 2013
Email: gamk@mpa-garching.mpg.de

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Original publication

Jin et al.
'A spatially resolved evolutionary sequence of multi-wavelength AGN host galaxies
Monthly Notices of the Royal Astronomical Society, Volume 546, Issue 4

DOI

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A Supermassive Cosmic Dance

Artist's impression of a pair of supermassive black holes orbiting each other, surrounded by accreting material from a circumbinary disk. Image credit: NAOJ. Download Image

Over the past week, NuSTAR observed 4C +37.11, the only confirmed supermassive black hole binary in an active galaxy with spatially resolved orbital motion. This system was originally identified in 2004 by the Very Long Baseline Array (VLBA) as a pair of central, compact, flat-spectrum, variable radio sources with a measured separation of 7.3 pc—evidence of a pair of gravitationally bound supermassive black holes. Subsequent VLBA observations claim the detection of relative proper motion between the binary components. Numerical simulations predict that accretion in supermassive black hole binaries can generate excess hard X-ray emission above 20 keV through shocks and heated mini-disks fed by circumbinary gas streams. NuSTAR will test those predictions of a significant hard excess, or see if the X-ray spectrum follows a standard AGN power law. This test of binary accretion physics in a wide-separation supermassive black hole binary will establish whether a hard X-ray diagnostic that can be applied to unresolved binary AGN candidates identified by LSST and future time-domain surveys.

Author: Daniel Stern (NuSTAR Deputy Principal Investigator, Caltech)

Source: NASA's Nuclear Spectroscopic Telescope Array (NuSTAR)/Weekly Highlights

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Hitchhiking PAHs Caught Catching a Ride on Galactic Winds

JWST's view of the spiral galaxy IC 5332. JWST's infrared instruments are sensitive to emission from molecules called polycyclic aromatic hydrocarbons.Credit: NASA, ESA, CSA, STScI, PHANGS Team, Janice Lee (STScI), Thomas Williams (Oxford), Rupali Chandar (UToledo)

Title: JWST Discovery of Warm Dust in the Circumgalactic Medium of the Makani Galaxy
Authors: Sylvain Veilleux et al.
First Author’s Institution: University of Maryland
Status: Published in ApJ

The Windy Galaxy

Billions of light-years away lies Makani, a record-breaking galaxy discovered in 2019 by researchers at the Keck Observatory atop Maunakea, Hawaiʻi. The galaxy shows evidence of a large merger event, where two galaxies collided to form a larger galaxy. This event triggered starburst activity, or several waves of intense star formation, that changed the makeup of not just Makani, but the region around it.

There are several processes that can create galactic winds. An active supermassive black hole at the center of the galaxy can slingshot particles out of the galaxy at close to the speed of light, or supernovae leftover from previous starburst activity can help to expel material from the galaxy. Makani, whose name in Hawaiian just so happens to mean “wind,” has both factors contributing to its galactic winds: massive amounts of star formation and a very active black hole in its center.

This contributes to some of the strongest galactic winds discovered to date, stretching approximately 100 kiloparsecs (330,000 light-years) beyond the galaxy, roughly ten times the extent of a typical galactic wind. These galactic winds remove gas from the galaxy and help to enrich the circumgalactic medium around the galaxy. There is still much to discover about how galactic winds change the makeup of such galaxies, and what exactly they take with them on their way out.

Figure 1: Makani’s spectrum overlaid on top of the JWST filters. The peaks in the spectrum are PAH features that neatly overlap with JWST’s observing bands. Credit: Veilleux et al. 2025

PAHs Gone Extragalactic

Using JWST, the authors scrutinized Makani for any signs of PAHs. They found that not only did PAHs exist within the galaxy, these molecules were found up to 30 kpc (~100,000 light-years) outside the galaxy. Makani’s strong galactic winds were ejecting the dust grains from the galaxy over an estimated timescale of a billion years.

Previously, astronomers had not expected to find high quantities of dust grains outside the protection of a galaxy. The journey across a galaxy is long and hot, with temperatures in the millions of degrees. A dust grain would likely not survive the trip, as it would be destroyed by high-energy photons. This suggests that the PAHs were somehow shielded from the hotter parts of the interstellar medium and circumgalactic medium in large enough volumes to make it beyond the edge of Makani.

This answered questions about how galaxies enrich the circumgalactic medium, but it also added several more. Since today’s authors were limited by the sensitivity and range of their instruments, they were unable to confirm if there were PAHs along Makani’s entire wind structure, or just the first 35 kpc (114,000 light-years) of it. They also would like to turn their sights towards the intergalactic medium to see if PAHs could make it even farther beyond the reach of their host galaxy.

Earth’s Favorite Carcinogen: Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs, pronounced P-A-Hs or “paws”) are small flat dust grains scattered into every corner of the universe, from the atmosphere of Earth in the form of exhaust and wildfire smoke to the circumgalactic medium (the mostly empty space between galaxies). These PAHs contribute to regulating the temperature of the interstellar medium, but astronomers have long wondered how they migrate across galaxies.

Even though they are smaller than a human hair, PAHs are still easily observable even in very distant galaxies. These dust grains emit infrared light, and if there are enough PAHs in a region, they are bright enough to be observed with modern telescopes, such as JWST. By combining multiple observations across multiple filters, astronomers can calculate ratios that reveal information about the PAHs’ size, temperature, and charge, allowing reserachers to further discern properties about the regions in which these PAHs reside.

With redshifted galaxies like Makani, the light from PAHs is stretched beyond its usual wavelength, but fortunately for today’s authors, the light was stretched just enough to land known PAH features well within JWST filters, as seen in Figure 1. The authors were then able to use this information to track the distribution of these PAHs across Makani.

Original astrobite edited by Chloe Klare.

Source: American Astronomical Society/AAS-Nova

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About the author, Natalie Price:

As a first-year master’s student at Wesleyan University, I study how stellar winds interact with the local interstellar medium. Outside of the observatory, you can find me dancing, with my nose in a book, or running at non-relativistic speeds.

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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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NASA’s Hubble Tracks New Decagon Encircling Saturn’s South Pole

Decagon on Saturn’s South Pole (Color)

Recent NASA Hubble Space Telescope images show the gas giant Saturn and its southern pole, where astronomers have discovered a 10-sided atmospheric wave. Observations show the decagon extends through multiple layers of Saturn’s atmosphere.Credit Image: NASA, ESA, STScI, Agustin Sánchez-Lavega (UPV), Amy Simon (NASA-GSFC), Michael Wong (UC Berkeley); Image Processing: Alyssa Pagan

Decagon on Saturn’s South Pole (Single Filter)

A single filter from NASA’s Hubble Space Telescope distinctly shows a 10-sided wave encircling Saturn’s south pole, labeled “decagon”. An “X” denotes where data was not captured. Credit Image: NASA, ESA, STScI, Agustin Sánchez-Lavega (UPV), Amy Simon (NASA-GSFC), Michael Wong (UC Berkeley); Image Processing: Alyssa Pagan

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Recent observations with NASA’s Hubble Space Telescope have revealed a giant, evolving, 10-sided atmospheric wave encircling Saturn's south pole. This discovery marks the first time a large, regular-sided jet pattern has been observed in the planet's southern hemisphere. The feature appears remarkably similar to Saturn's famous hexagon at its northern pole, but is also distinctly different, suggesting scientists may be witnessing a new atmospheric phenomenon develop on the iconic gas giant.

The results published Wednesday in the journal Science Advances.

By piecing together several years of Hubble observations dating back to 2023, researchers found subtle hints of the structure beginning to emerge before it became a clearly defined pattern. Those observations were taken as part of Hubble’s Outer Planet Atmospheres Legacy (OPAL) program, which has photographed the outer planets annually for more than a decade.

"We've never seen anything quite like this in Saturn's southern hemisphere," said Amy Simon, study co-author and OPAL principal investigator, NASA's Goddard Space Flight Center in Greenbelt, Maryland. "The northern hexagon has been there every time we've looked for more than 40 years. This feature is different — it appears to be strengthening, giving us the rare opportunity to watch a giant atmospheric pattern develop."


The discovery was possible because Saturn's changing seasons gradually brought the planet's south pole back into view from Earth, where astronomers who collectively analyze images of Saturn from ground-based observatories first identified it.

Agustín Sánchez-Lavega, lead author of the new study, is a researcher at the University of the Basque Country in Spain. The university manages a website, called Planetary Virtual Observatory Laboratory, that accepts ground-based images of solar system planets contributed by observers all over the world. It was in those images, first in 2024, that Sánchez-Lavega and amateur astronomers Trevor Barry and Jean-Paul Oger noticed a subtle undulating band along the southern pole. Additional 2025 imagery taken from the ground hinted even more strongly toward this decagon structure.

That’s when the Hubble observations come into the picture. Hubble’s view from space offers unmatched image sharpness and spatial resolution over full rotations of Saturn, without smearing by Earth’s atmosphere.

“Given Saturn’s symmetry in its north-south jet stream system, we have been searching for a counterpart to Saturn’s northern hexagon on the south pole in Hubble images since 1990,” Sánchez-Lavega said. “Images from NASA’s Cassini spacecraft, which orbited Saturn between 2004 and 2017, showed no inkling of a long-lived formation, either. The Hubble data confirmed the feature’s presence back to 2023.”

The wave sits within one of Saturn's powerful jet streams and extends through multiple layers of the atmosphere, indicating it is not just a cloud-level feature, but a vertically extended atmospheric structure. The decagon’s apparent position shifts slightly, because Hubble captures images from different wavelengths. Those different wavelengths probe different altitudes in Saturn's atmosphere.

“The most intriguing part to me is that this seems to have just formed recently,” said Simon. “The question is, why did it suddenly form now when we haven't seen one before?”

The authors say further study is needed from Hubble and NASA’s James Webb Space Telescope, as well as analysis of computer models, to understand how the decagon formed, how long it may last, and how it compares to the long-lived hexagon in the north.

Hubble’s long duration in operation has allowed astronomers to track changes over time in solar system planets and other astronomical objects as well.

Rather than providing a single snapshot, the OPAL program allows scientists to follow seasonal changes, track short-lived storms, and identify other atmospheric features that evolve slowly over time.


"When we started the OPAL program, we expected compelling surprises, but we didn't know what to expect specifically,” said Mike Wong, study co-author, University of California, Berkeley. “A lot of the discoveries we see coming from OPAL are not just based on one observation, but on years and years of data. Regular observations over time are enabling a lot of new findings."

The team plans to continue observing Saturn to determine whether the decagon settles into a long-lived, stable configuration like the northern hexagon or continues to evolve. Future observations also could help scientists determine what drives the wave, what it reveals about the atmospheric dynamics of giant planets throughout the solar system, and how they may relate to those we see here on Earth.

The Hubble Space Telescope has been operating for more than 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 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: NASA's Hubble Space Telescope/News

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Details:

Last Updated: Sep 02, 2026
Editor: Andrea Gianopoulos
Location: NASA Goddard Space Flight Center

Contact Media:

Claire Andreoli
NASA’s Goddard Space Flight Center
Greenbelt, Marylandv
claire.andreoli@nasa.gov

Hannah Braun
Space Telescope Science Institute
Baltimore, Maryland

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Hawai‘i Students Name Image of Glittering Galaxy Pair Nā ʻUhane Māhoe Huki Pū i ke Ola

PR Image noirlab2621a
Nā ʻUhane Māhoe Huki Pū i ke Ola

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Project Hōkūlani Interns Summer 2026

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Project Hōkūlani Galaxy Presentation

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Project Hōkūlani Stargazing on Maunakea

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Project Hōkūlani Interns Fall 2025

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Project Hōkūlani Interns Shadow Nighttime Operations

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Videos


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Pan on NGC 7253


PR Video noirlab2621b
Zooming into NGC 7253

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Project Hōkūlani interns researched and named this new Gemini North image featuring the stunning pair of interacting galaxies known as NGC 7253

Nā ʻUhane Māhoe Huki Pū i ke Ola is the Hawaiian name given to this image of NGC 7253 — a pair of spiral galaxies caught in each other’s gravity. Captured by the Gemini North telescope on Maunakea in Hawai‘i, this image created a spectacular symbolic display for this year’s interns participating in Project Hōkūlani.

Over 200 million light years away, deep within the constellation Ka Lupe o Kawelo (Pegasus), is NGC 7253 — a pair of gravitationally interacting galaxies separated by a distance of about four million light years. This glittering view of their meeting was captured by the Gemini North telescope, one half of the International Gemini Observatory, partly funded by the U.S. National Science Foundation (NSF) and operated by NSF NOIRLab. The new image offers viewers the closest look yet at NGC 7253, and has been named Nā ʻUhane Māhoe Huki Pū i ke Ola, which translates to “The Twin Spirits Pulling Together Creating Life.”

The creation of this image began in October 2025 with three local students from Waiākea High School and the Volcano School of Arts and Sciences participating in the Project Hōkūlani internship. Interns Raiyan Rahman, Rafan Rahman, and Alia Tamanha spent one week at the Gemini North Hilo Base Facility with three goals: learn about observatory operations and how scientific data is collected; deepen their own appreciation for Maunakea; and select an astronomical object for Gemini North to image.

“NGC 7253 is a fascinating object because it’s two galaxies that are merging into one over billions of years. But we mainly picked this object because it was one of the few that actually lay within a Hawaiian constellation, Ka Lupe o Kawelo,” says Raiyan Rahman. “After spending so much time on Maunakea, it was important to us to pick something that was both culturally and astronomically significant.”

In June 2026, three more local students from Keaʻau High School and Kamehameha Schools joined the Project Hōkūlani internship program. Mylin Wilson, Manu Silva-Sampaio, and Samantha Silva-Sampaio shared many of the same goals as their predecessors, and built upon their work by creating a name for the new image of NGC 7253.

The chosen name, Nā ʻUhane Māhoe Huki Pū i ke Ola, is inspired by the dual nature of the interacting galaxies and symbolizes the transformative experience the students had during their summer internship. Each portion of the name corresponds to a different part of the interns’ experience.

Nā ʻUhane Māhoe represents the two similarly sized galaxies and their wispy spiral arms. It also represents the twin nature of the International Gemini Observatory, composed of the Gemini North telescope in Hawai‘i and the Gemini South telescope on Cerro Pachón in Chile.

“ʻUhane (spirit) was an important theme for our group. Our experiences throughout the week — ʻoli (stargazing), visiting the summit, and learning about the awesome scientific discoveries made possible by Maunakea — ignited our own spirits as we prepare for life after high school,” says Samantha Silva-Sampaio.

“ʻUhane in this image reminds us that astronomy is not just science or spirit; it's both at the same time,” adds Manu Silva-Sampaio.

Huki Pū represents the physical and literal aspect of the galaxies being pulled together, driven by the force of each other's gravity. It also represents the communities that came together to make Project Hōkūlani a reality.

“Many people from around the world and different areas of expertise came together to make our internship possible, and we learned so much from everyone,” says Wilson. “Our experience, and astronomy itself, is a kākou (collective) type of practice.”

Finally, i ke Ola represents the life that can be created by interacting galaxies. To choose a fitting name for this image, the interns researched interacting galaxies and galaxy mergers. They learned that when spiral galaxies begin to merge, the gas and dust in their spiral arms collide, causing a significant burst in star formation. Extremely hot radiation from the newly born stars ionizes, or energizes, the surrounding gas. When atomic hydrogen is ionized, it forms what are known as H II (pronounced “h-two”) regions, which are visible in this image as concentrations of bright pink light. Manu Silva-Sampaio says, “Stars are sources of life and health for us as human beings, because our Sun is a star, and we cannot live without it.”

Project Hōkūlani operates through a partnership between the International Gemini Observatory and the Project Hōkūlani team at the University of Hawaiʻi at Mānoa. Project Hōkūlani supports middle and high school students in entering postsecondary science, technology, engineering, and math (STEM) fields through strengths- and work-based enrichment programs.

This image was taken as part of the NOIRLab Legacy Imaging Program — a continuation of the program started at the International Gemini Observatory in 2002, called the Gemini Legacy Imaging Program. Its aim is to use observing time on NOIRLab telescopes that is dedicated to acquiring data specifically for color images to share with the public.

Through both of these programs, six local high school students on Hawaiʻi Island get to contribute to the selection, imaging, and naming of a new astronomical image from the Gemini North telescope on Maunakea each year.

Source: NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab)/News

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More information

NSF NOIRLab, the U.S. National Science Foundation center for ground-based optical-infrared astronomy, operates the International Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), NSF Kitt Peak National Observatory (KPNO), NSF Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and NSF–DOE Vera C. Rubin Observatory (in cooperation with DOE’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona.

The scientific community is honored to have the opportunity to conduct astronomical research on I’oligam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence of I’oligam Du’ag to the Tohono O’odham Nation, and Maunakea to the Kanaka Maoli (Native Hawaiians) community.

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Links

• Explore Nā ʻUhane Māhoe Huki Pū i ke Ola in more detail
• Learn more about Project Hōkūlani
• Photos of the Gemini North telescope
• Videos of the Gemini North telescope
• Check out other NOIRLab Photo Releases

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Contacts:

Leinani Lozi
Hawaiʻi Education and Engagement Manager
International Gemini Observatory/NSF NOIRLab
Email: leinani.lozi@noirlab.edu

Josie Fenske
Public Information Officer
NSF NOIRLab
Email: josie.fenske@noirlab.edu

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An inside-out view of active supermassive black holes and their host galaxies September 01, 2026

Different telescopes reveal different signatures of active black holes. LOFAR (left) detects radio jets; WISE (centre) detects hot dust; SDSS/MaNGA (right) provides spatially resolved optical spectra of stars and gas. Credits: ASTRON, NASA/JPL-Caltech, SDSS, MPA

A schematic view of the proposed AGN population sequence created with the help of AI. Left: a radiative AGN with abundant gas and a prominent dusty torus, typical of star-forming hosts. Centre: a mixed AGN with compact radio emission alongside radiative signatures. Right: a radio-dominated AGN with extended jets, typical in quiescent hosts. This is a map of connected AGN-galaxy states, not a timeline for any individual galaxy. Credit: MPA/G. Jin

Host-galaxy properties change systematically from the nucleus outward for different AGN types. Each column shows one AGN class, with radial profiles of star formation (top), stellar age (middle), and gas ionisation (bottom); grey lines show reference profiles for normal galaxies. Infrared and optical AGN hosts show enhanced central star formation and younger stellar populations, while radio AGN hosts resemble quiescent galaxies at all radii. Credit: SDSS, MPA/G. Jin

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Active supermassive black holes do not all look the same. Some are obscured by hot dust, some are surrounded by fast-moving ionised gas, and some can eject giant radio jets extending over millions of light-years. By mapping about two thousand such objects from their galactic centres outward, researchers at the Max Planck Institute for Astrophysics found that these variations are related to the properties of their host galaxies. Infrared and optical AGNs are found in star-forming galaxies with young centres and ionised winds, whereas radio AGNs are mostly found in older, quieter systems. A small group showing both kinds of activity sits in between.

Seeing Active Black Holes in a Different Light

An active galactic nucleus (AGN) can appear in several forms, such as a hot, dusty torus that glows in infrared light; broad emission lines from gas moving close to the black hole; narrow lines from gas farther out; or a radio jet blasting away from the centre. Some of this diversity is simply due to our viewing angle. For example, an AGN seen edge-on through its dusty torus looks different from one seen face-on. However, orientation alone cannot explain everything. A long-standing question is whether these different 'faces' of AGN activity are also connected to what is happening in the surrounding galaxy, and if so, how.

From the Nucleus to the Outskirts

To find out, the team combined data on around ten thousand nearby galaxies, including nearly two thousand identified AGNs, taken from three surveys, each of which contributes a different piece of the picture. WISE, an infrared satellite, reveals hot dust. MaNGA is an integral-field spectroscopic survey that supplies optical AGN signatures and spectra at multiple positions across each galaxy. This allows the team to map properties rather than just measuring them in aggregate. LoTSS, a state-of-the-art radio survey using the LOFAR telescope, traces synchrotron emission from jets. Together, these datasets enable the researchers to trace the evolution of each galaxy from its inner regions to its outskirts.

The first differences appear in the central regions. Galaxies hosting infrared AGN exhibit the clearest boost in central star formation, forming stars at a faster rate near their nuclei than similar galaxies without AGN activity. These infrared and optical AGN hosts also have younger stellar populations at their centres, as determined by a spectral indicator of stellar age. In contrast, radio AGN hosts have old central populations and reduced central star formation, much like normal quiescent galaxies. The simultaneous presence of black-hole growth and central star formation in the same populations is consistent with both processes being fed by the same gas supply, though the data do not show one triggering the other. The gas emission and kinematics also carry the AGN's signature. Infrared, broad-line and narrow-line AGNs all exhibit stronger gas ionisation towards the centre, and this excess compared to normal galaxies extends over several kiloparsecs before fading. Fast-moving, ionised winds are strongest in infrared and broad-line AGNs, and their average signal extends to around 2 kiloparsecs from the nucleus. Radio AGNs show no comparable outflow on average. These winds clearly disturb the ionised gas around the black hole. However, the data do not show that star formation is immediately shut down by the current AGN. The link between black-hole and stellar growth is strongest near the centre and gradually weakens towards the outskirts.

A Bridge Between AGN Modes

A small group of AGNs exhibits both optical/infrared and radio signatures. Several independent measurements show that these 'mixed' AGNs lie between the radiative and radio-dominated populations, providing an observational link rather than a clear boundary between the two. Their radial star-formation profiles lie between those of optical/infrared-only and radio-only AGNs. Their stacked spectra reveal a combination of strong emission lines and a significant 4000-Ångström break, suggesting ongoing black-hole accretion within an ageing stellar population. While their radio emission is present, it remains relatively compact compared with the extended jets of radio-only AGNs.

What's missing?

Several observations could clarify this picture. Molecular gas from CO observations would reveal the fuel reservoir directly. X-ray observations could reveal the hot atmospheres thought to exist around radio-mode AGNs. Higher-resolution radio images would clarify whether the compact jets in mixed AGNs are younger. Comparing these with spatially resolved simulations would reveal whether the observed states arise from one evolutionary route or several. Ultimately, these observations will reveal whether the empirical map uncovered here reflects a common evolutionary route, repeated cycles or multiple paths through black hole and galaxy growth.

Source: Max Planck Institute for Astrophysics/Research Highlights

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Authors:

Jin Gaoxiang
PhD student
Tel: 2298
Email: gxjin@mpa-garching.mpg.de

Guinevere Kauffmann
Director
Tel: 2013
Email: gamk@mpa-garching.mpg.de

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Original publication

Jin et al.
'A spatially resolved evolutionary sequence of multi-wavelength AGN host galaxies
Monthly Notices of the Royal Astronomical Society, Volume 546, Issue 4

DOI

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A Busy Month in the Galactic Center

A deep X-ray image of the Galactic center taken with the Chandra observatory, with low-, intermediate-, and high-energy X-rays colored in red, green, and blue respectively. It shows thousands of point sources, all powered by accreting white dwarfs, neutron stars, or black holes, embedded in large clouds of hot gas. Sources from this enormous population will frequently go into outburst, prompting astronomers to trigger target-of-opportunity observations to track their evolution. Image credit: NASA/CXC/UMass/D. Wang et al. Download Image

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It has been a busy month for the NuSTAR observatory. NuSTAR has in the past performed on average six Target of Opportunity (ToO) observations each month, but the numbers have been steadily increasing over the summer, in part because the Sun has moved away from the position on the sky of the Galactic center, a particularly active part of the sky. So far this month there have been 22 ToO submissions to NuSTAR, five of which were ToO proposals from the Guest Observer (GO) program, and a further five approved Director's Discretionary Time proposals—a new record for time-domain observation requests! More than half of these have been requests to observe transient sources within 5 degrees of the Galactic center, including observations for three GO programs of a bright, previously unknown transient X-ray source, MAXI J1750-327, coordinated with NASA's IXPE mission. Other ToO requests have taken advantage of the new policy allowing short NuSTAR exposures, either splitting up GO observations to increase the number of visits or performing brief monitoring visits to well-known X-ray binaries to keep track of their behavior and to alert the community when an expected outburst begins. Up until recently, the Swift mission has performed the majority of this kind of monitoring. The increase in NuSTAR time domain observations will help to cover some of this lost capability, but it will not be able to replace the key role Swift has played in the X-ray astronomy ecosystem. We salute the Swift team for their extraordinary work over the past decades and the great teamwork the Swift and NuSTAR observatories have been able to achieve together, and wish them the best in their final months of operations.

Author: Karl Forster (NuSTAR Science Operations Lead, Caltech)

Source: NASA's Nuclear Spectroscopic Telescope Array (NuSTAR)/Weekly Highlights

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Do black holes pretend to have large masses?

A closer look at the gravitational-wave event GW231123

© Simulation: I. Markin (University of Potsdam), H. Pfeiffer (Max Planck Institute for Gravitational Physics), T. Dietrich (University of Potsdam and Max Planck Institute for Gravitational Physics); Collage: B. Knispel, M. Zumalacárregui (Max Planck Institute for Gravitational Physics)

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To the point

• The signal: On 23 November 2023, both LIGO observatories in the US detected gravitational waves from the most massive black hole merger to date.


• The mystery: According to previous studies, the two black holes had masses approximately 100 and 140 times that of our Sun, respectively. However, according to standard models of stellar evolution, black holes of these masses are difficult to form. How they could have evolved remains a mystery.


• A possible solution: Scientists at the Max Planck Institute for Gravitational Physics in Potsdam have now proposed an explanation for these unusual masses. The signal may have been magnified by a galaxy and diffracted by an object within it, making the black holes appear more massive than they really are.

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A potential explanation for a mysterious gravitational wave signal.

The gravitational-wave signal GW231123, detected by the two LIGO detectors in the US on 23 November 2023 from the merger of two black holes, continues to puzzle scientists. Given their large masses, the existence of these black holes cannot be explained with the standard understanding of stellar evolution. However, they are not only very massive, but also spin very quickly. This makes interpreting the signal particularly challenging and suggests an unusual formation history for the binary system.

In a new study, a research team at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in the Potsdam Science Park investigated whether the measured masses and spins appeared larger than they actually were due to the effect of a gravitational lens.

Light on Curved Paths and Distorted Gravitational Waves

Light traveling through the universe is deflected by massive objects, resulting sometimes in multiple images of the same astrophysical source. This gravitational lensing effect needs to be accounted for to correctly interpret observations. Lensing is routinely observed on electromagnetic sources such as stars and galaxies, and expected to occur in a fraction of gravitational-wave sources.

“Like light, gravitational waves can also be deflected, magnified and split into multiple signals by massive objects,” says Miguel Zumalacárregui, group leader in the Astrophysical and Cosmological Relativity Department at the AEI. “For gravitational waves, diffraction and interference effects give us an additional way to identify and study lensed signals.”

Detecting these subtle lensing diffraction effects requires sensitive detectors and novel data-analysis methods. To make this analysis possible, the team developed a mathematical description of gravitational-wave lensing and software fast enough to analyze the data.

“If we assume that GW231123 was deflected and distorted by a compact object of about 190 to 850 solar masses—or by an extended structure such as a globular cluster—we can understand the observed high masses,” says Srashti Goyal, co-lead author and postdoc at the AEI Potsdam when working on the topic. “Moreover, the lensing interpretation does not require unusually high spins.” Taking these effects into account, the total mass of the source would be around 140 solar masses rather than about 230 solar masses. The system would then be much less extreme and easier to accommodate within known black-hole formation scenarios.

Across the Universe

The study’s title, “Across the Universe”, is a nod to the Beatles — but here the journey really does change how the source appears. As gravitational waves travel through the expanding Universe, their wavelengths are stretched and their frequencies lowered (an effect known as cosmological redshift) — the equivalent of lowering a sound’s pitch. Just as a cello played back at a lower pitch might be mistaken for a double bass, a distant black-hole binary can appear more massive than it really is. Gravitational magnification adds to the illusion by making the distant source appear closer.

“Our analysis also suggests that the compact lens was embedded in a larger gravitational field, such as that of the galaxy hosting it,” adds Héctor Villarrubia-Rojo, co-leading author of the study and a postdoctoral scholar at the Universidad Complutense in Madrid, Spain. “By including this external potential, we can describe the small-scale diffraction and the large-scale magnification within a single framework.”

New insights from lensed gravitational waves

So far, researchers have not unambiguously detected any signals affected by gravitational lensing in the data from gravitational-wave detectors. According to the new study, GW231123 is a promising candidate for such an event.

“The nature of the lens remains a major mystery in our analysis, as individual compact lenses with 100–1,000 solar masses should be exceedingly rare,” adds Zumalacárregui. “Future work will need to establish whether such lenses can form, or whether an ensemble of lighter objects, including stars, can explain this event.”

Current data do not yet allow an unambiguous claim of gravitational lensing. Following further upgrades to the detectors, however, the scientists expect to be able to detect and interpret lensed gravitational waves using new data analysis methods.

The detection — or lack thereof — of gravitational waves deflected by the gravity of other objects will provide new insights into gravitational-wave astronomy: Gravitational magnification may reveal black-hole mergers beyond the distance current detectors can normally reach, while diffraction can probe compact objects and dark-matter structures along the line of sight. Consequently, these deflected gravitational waves could become a new method for exploring the universe.

Source: Max Planck Institute for Gravitational Physics

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Media contact:

Dr. Elke Müller
Press Officer AEI Potsdam, Scientific Coordinator
Tel: +49 331 567-7303
Email: elke.mueller@aei.mpg.de

Science contacts:

Dr. Srashti Goyal
Postdoc
Email: srashti.goyal@iucaa.in
Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, India

Dr. Héctor Villarrubia-Rojo
Postdoctoral scholar
Email: hectorvi@ucm.es
Universidad Complutense, Madrid, Spain

Dr. Miguel Zumalacarregui
Group Leader
Tel:+49 331 567-7322
Fax: +49 331 567-7298
Email: miguel.zumalacarregui@aei.mpg.de

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Publication

Goyal, S.; Villarrubia-Rojo, H.; Zumalacarregui, M.
Across the Universe: GW231123 as a magnified and diffracted black hole merger. The Astrophysical Journal Letters 1008, L12 (2026)

MPG.PuRe - DOI - pre-print

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Related information

1. Homepage of the “Astrophysical and Cosmological Relativity” department

2. YouTube video: The Next Frontier: Lensing of Gravitational Waves

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A Pulsar in a Nebula in a Supernova Remnant

Featured Image: A Pulsar in a Nebula in a Supernova Remnant

When a massive star collapses, the subsequent supernova can leave behind the star’s condensed core in the form of a neutron star. Some neutron stars are also pulsars, which rotate at incredible speeds, emit beamed radio emission, and expel winds of charged particles. A pulsar’s relativistic charged-particle winds billow around the pulsar as it travels through space — often at a few hundreds of kilometers per second or faster, having received a “kick” when its progenitor star exploded. When these winds interact with the surrounding supernova ejecta or the interstellar medium, the interaction creates a detectable pulsar wind nebula. The image above combines data from the Australian Square Kilometre Array Pathfinder (orange) and Wide-field Infrared Survey Explorer (cyan) to show a pulsar wind nebula, indicated with a white rectangle, within a larger supernova remnant. Sanja Lazarević (Western Sydney University) and collaborators discovered this pulsar wind nebula, which they’ve named “Thunder” in a nod to the supernova remnant’s moniker, “Nimbus.” The cometary shape of the pulsar wind nebula suggests that the pulsar is moving quickly, traveling outward from the center of the explosion that occurred some 30,000–45,000 years ago. To learn more about the discovery and characterizati,bron of this pulsar wind nebula, check out the article linked below.

Citation

“EMU Discovery of Thunder: A Bow-Shock PWN Powered by PSR J1631–4722 Escaping the Nimbus SNR (G336.7+0.5),” S. Lazarević et al 2026 ApJ 1007 159. doi:10.3847/1538-4357/ae7f11

Source: American Astronomical Society/AAS-Nova

Thermal Anomaly Discovered Below Mars's South Pole

Artist concept of Mars's warm southern interior
Credit: Artist concept: NASA / Theophilus Britt Griswold

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Researchers have discovered a thermal asymmetry deep beneath the surface throughout Mars's southern hemisphere.

Based on gravitational measurements that give clues to the Red Planet's interior structure, Mars's interior southern hemisphere is around 200 to 400 degrees Celsius warmer than the northern half of the planet and partially molten. The surprising finding adds additional context to Martian history and the periods of time in which it may have hosted conditions favorable for life.

The research was led by Caltech alumnus Alexander Berne (PhD '26), who is now a postdoctoral associate at the University of Arizona. The findings are reported in a paper appearing in the journal Nature on August 27.

During his graduate studies at Caltech, Berne developed a model that uses variations in gravitational data to infer the structure of a planetary body's interior. Berne and his collaborators then aimed to apply his model to understanding Mars's interior. Using data collected over decades from three different Mars missions—Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter—the team measured tiny variations in these spacecrafts' velocities and used them to reconstruct the gravitational field around Mars. The gravitational forces exerted by the Sun on Mars vary over seasonal timescales as a result of Mars's slightly elliptical orbit and its tilted rotation axis. A technique called tidal tomography measures how those gravitational signatures vary over time and results in a model of the planet's interior.

"Scientists usually assume that the interiors of planetary bodies are generally spherically symmetric, but this is not necessarily true," Berne says. "As we get more gravity data, we can determine the three-dimensional intricacies of a planet's interior structure. These inferences in turn give us a blueprint for designing future missions and scientific exploration of these worlds. Understanding the interior structure of planetary bodies helps us unravel the processes that shaped their formation and evolution."

On the surface, Mars is a geologically asymmetrical planet: Its southern hemisphere contains towering mountains and deep craters, whereas the northern hemisphere is composed of low flat lands. In the new study, the team was surprised to discover that Mars's interior is also thermally asymmetric—the southern hemisphere is hundreds of degrees hotter than the north.

The new observation also happens to suggest explanations for other phenomena observed on Mars, such as magnetic anomalies found in iron minerals in the south. A thermal anomaly in the southern mantle could mean that a magnetic field existed strong enough to cause magnetic differences between the north and south. Additionally, NASA's InSight mission had previously discovered that seismic waves dissipate more quickly in the south, which could be explained if the region were hotter.

"The dichotomy that we see between north and south is important to understand because it gives information about processes that may have influenced the hydrology of Mars, including the formation of basins that may have held water," says Amirhossein Bagheri, a postdoctoral scholar at Caltech and co-author on the paper. Bagheri is also a former member of the InSight team.

It is still unclear what created the thermal anomaly, and there are several hypotheses for its origins, including a giant impact releasing heat from the north, past spontaneous convection in the Martian southern mantle, and thick geological features trapping excess heat from escaping.

The paper is titled "Tidal Tomography Reveals a Thermal Anomaly Beneath Mars's Crustal Dichotomy." In addition to Berne and Bagheri, co-authors are Nicholas Wagner and Harriet Lau of Brown University, Isamu Matsuyama and Angela Marusiak of the University of Arizona, Sander Goossens of NASA Goddard Space Flight Center, Karwai Cheng of the Institute of Astronomy and Astrophysics at Academia Sinica in Taiwan, Antonio Genova of the University of Rome in Italy, Marc Rovira-Navarro of the Delft University of Technology in the Netherlands, Chuan Qin of UCLA, Douglas Hemingway of the University of Texas at Austin, Shijie Zhong of the University of Colorado Boulder, and Francis Nimmo of UC Santa Cruz. Funding was provided by NASA.

Source: Caltech/News

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Contact:

Caltech Media Relations
mr@caltech.edu

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Lenticular Galaxy NGC 4996

NGC 4996
Credit: NAOJ; Image provided by Masayuki Tanaka

Detail: Low Res. (46 KB) / Mid. Res. (860 KB) / High Res. (8.7 MB)

A lenticular galaxy, which means "lens-shaped," occupies an intermediate position between elliptical and spiral galaxies in the Hubble Classification scheme. These galaxies exhibit little or no ongoing star formation and lack the prominent spiral arms that characterize spiral galaxies. Instead, many lenticular galaxies feature a bright central bar and, in some cases, a faint outer ring, both of which are visible in this galaxy.

Astronomers believe that lenticular galaxies have lost much of the gas necessary for forming new stars. As star formation ceased, their spiral arms may have gradually faded from view. The bar and ring structures observed today are thought to be remnants of the galaxy’s former disk. Galaxies slowly change their appearance over time, and this lenticular galaxy may provide us with a glimpse of one stage in that evolutionary transition.

Distance from Earth: 260 million light-years
Instrument: Hyper Suprime-Cam (HSC)

Source: Subaru Telescope

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Calculating black hole scattering for any mass ratio

Gravitational two-body scattering event with gravitational waves.

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State-of-the-art predictions can now be used to create waveform models for next-generation gravitational-wave detectors.

An international team, including researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in the Potsdam Science Park, has calculated with record precision how two black holes deflect each other's paths when they fly past one another under the influence of their mutual gravitational attraction. This novel result covers any mass ratio.

Gravitational-wave observatories routinely detect ripples in spacetime from colliding black holes. Decoding these signals requires predictions of black-hole motion, and these predictions must be accurate enough to keep pace with ever more sensitive detectors. Recently, methods borrowed from particle physics — treating gravity with the tools of quantum field theory developed for colliders — have driven rapid progress.

Using their worldline quantum field theory approach and high-performance computers, the researchers computed the energy-conserving part of the deflection angle at the fifth order of approximation in the strength of gravity. The key advance is completing the mass dependence at this order, where earlier results applied only to highly unequal pairs. The explicit analytic answer involves exotic mathematical functions related to higher-dimensional generalizations of torii, and a subtle infinity at one special fly-by speed cancels in their refined definition of energy-conserving effects. This state-of-the-art prediction may now be used for the waveform models required for next-generation gravitational wave detectors.

Paper abstract

Using the worldline quantum field theory formalism, we compute conservative contributions to the scattering angle and impulse for classical black hole scattering at fifth post-Minkowskian (5PM) and second self-force (2SF) order. This four-loop calculation involves non-planar Feynman integrals and requires advanced integration-by-parts reduction, novel differential-equation strategies, and efficient boundary-integral algorithms to solve a system of hundreds of master integrals in four integral families on high-performance computing systems. The resulting function space includes multiple polylogarithms as well as iterated integrals with a K3 period, which generate a spurious velocity divergence at v/c = √8/3, γ = 3. This divergence is present in the potential region and must be canceled by contributions from the radiative memory region, while its dimensional-regularisation pole should cancel against the radiative tail region. As the standard use of Feynman propagators fails to ensure this cancellation, we instead propose a “(γ-3)” conservative prescription that realises both cancellations, leading to a physically sensible answer. All available low-velocity checks of our result against the post-Newtonian literature are satisfied.

Source: Max Planck Institute for Extraterrestrial Physics/Research Highlights

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Contacts:

Media contact:

Dr. Elke Müller
Press Officer AEI Potsdam, Scientific Coordinator
Tel:  +49 331 567-7303
Email: elke.mueller@aei.mpg.de

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Publication Driesse, M.; Jakobsen, G. U.; Mogull, G.; Nega, C.; Plefka, J.; Sauer, B.; Usovitsch, J.
Conservative Black Hole Scattering at Fifth Post-Minkowskian and Second Self-Force Order. Physical Review Letters 137, 081402 (2026)

MPG.PuRe - DOI - pre-print

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One-sided spiral

NGC 4654

A spiral galaxy. It has a prominent spiral arm on one side (lower left) and a wide, glowing core. Dark brown filaments of dust swirl through its disc, while blue clusters of stars are found mostly going out to its arm. On the opposite side to the arm (upper right), gas trails off from the disc, out of the view in this image. A matching spiral arm is not visible on this side. The galaxy lies on a dark background. Credit: ESA/Hubble & NASA, D. Thilker, J. Lee and the PHANGS-HST Team

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The subject of this ESA/Hubble Picture of the Month is a spiral galaxy struggling against some of the titanic forces that appear on galactic scales in space. This is NGC 4654, an intermediate spiral galaxy in the constellation Virgo (the Maiden). “Intermediate” means that it lies between the spiral galaxies that have a bar across the centre and those that don’t, with a weak bar structure in its centre. It is situated 72 million light-years from Earth in the Virgo Cluster, a particularly massive and populous galaxy cluster.

NGC 4654 is particularly asymmetric, with a rounded and clearly-defined edge on one side and a long tail of gas stretching out from the opposite side — beyond Hubble’s view in this image. The cause of this gaseous tail is the same as for many of the other galaxies jostling in the crowded Virgo Cluster: namely, ram pressure stripping. NGC 4654 moves with such high velocity through space that it sweeps up and rams through the intracluster medium, the hot, rarefied gas filling the space between the Virgo Cluster’s galaxies. The intracluster medium in turn exerts a “ram pressure” on the galaxy, compressing the galaxy’s leading edge and tugging at its gas, creating the elongated tail.

It’s not just the galaxy’s gas that is unevenly distributed: its stars are too, and this is more unusual for a spiral galaxy. While the spiral arm on its leading edge is rich with stars and gas, the opposite arm noticeably lacks stars, influencing the galaxy’s lopsided spiral shape. It’s thought that ram pressure alone is unlikely to have had this effect. Rather, NGC 4654 has also been subjected to the gravitational force of the fellow Virgo Cluster galaxy NGC 4639. While the two galaxies are far apart now, it’s thought that a fly-by interaction between them around 500 million years ago ripped away NGC 4654’s gas along one side, limiting star formation there and creating the asymmetry in its shape.

Many galaxies that undergo ram pressure stripping suffer reduced star formation rates as the cold gas that collapses to form their stars is pulled away and lost. NGC 4654, however, is still forming nearly two Suns’ worth of stars every year, a rate comparable to other galaxies of similar size. The active star formation can be seen in the latest Hubble data used in this image, which picks up on a wavelength of red light that’s emitted by the clouds of energised gas where newborn stars lurk. The bright pink bubbles appear all across NGC 4654, from its forward spiral arm, to around its weak bar, and out to the edge of its disc.

The data used for this image come from two observing programmes (#15654, #17502) that have the aim of linking gas in galaxies with star formation. By observing many prominent galaxies in the vicinity of our own, researchers hope to better understand how gas moves around in galaxies, where and when it collapses to form stars and star clusters, and what effect those new stars have on the gas around them.

Source: ESA/HUBBLE/potm.

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Links

• Pan video

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