A new multi-wavelength study shows that some ultramassive galaxies shut down star formation early, while others remain active but hidden by dust
Media Contact: Meagan O’Shea
Maunakea, Hawaiʻi – An international team of astronomers has uncovered multiple evolutionary paths for the universe’s most massive galaxies. Observations of ultramassive galaxies, each containing more than 100 billion stars, show that less than two billion years after the Big Bang, some had already stopped forming stars and lost their dust, while others continued forming stars hidden behind thick dust clouds. Because dusty, star-forming galaxies can appear red and inactive, distinguishing truly “dead” galaxies from those still forming stars has long been a challenge—making the discovery of genuinely quiescent systems at such early times especially surprising.
“By combining multi-wavelength observations, we can tell which galaxies truly have limited ongoing star formation and which are still active but heavily hidden by dust,” said Wenjun Chang, lead author and graduate student at the University of California, Riverside. “Our far-infrared and (sub)millimeter measurements allow us to constrain how much dust these early massive galaxies contain.”
The study, led by Chang under the mentorship of Gillian Wilson, Vice Chancellor for Research, Innovation and Economic Development and Professor of Physics at the University of California, Merced, was presented at a media briefing during the 247th meeting of the American Astronomical Society on January 5, 2026, in Phoenix, Arizona.
Peering Through Cosmic Dust with Multi-Wavelength Observations
The findings come from MAGAZ3NE (the MAssive Galaxies at z ~ 3 NEar-infrared Survey), a long-running collaboration that uses spectroscopy and multi-wavelength observations to investigate how the most massive galaxies formed and evolved.
The team used more than 30 nights of Keck Observatory’s spectroscopy obtained through the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) instrument, which provided the precise redshifts and stellar mass measurements needed to interpret the multi-wavelength data.
Combining Keck Observatory optical data with longer-wavelength far-infrared and radio observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA) enabled the researchers to test whether massive galaxies previously classified as quiescent were truly inactive or instead hosting star formation or nuclear activity that is obscured at optical and near-infrared wavelengths.
“While optical and near-infrared data alone can severely underestimate obscured star formation in dusty massive galaxies, ALMA probes far-infrared wavelengths allowing improved constraints on the nature of these galaxies,” said Dr. Benjamin Forrest, a researcher at the University of California, Davis, who played a central role in the development of the MAGAZ3NE survey and its multi-wavelength analysis of ultramassive galaxies, and led the proposal that resulted in many of the ALMA observations used in this study.
“What is striking is not just that we can detect hidden activity, but that we see such diversity among galaxies with similar masses at the same epoch,” Wilson said. “That tells us that the shutdown of star formation in the most massive galaxies was neither uniform nor simple, and it places important new constraints on our understanding of galaxy formation and evolution in the early Universe.”
Rethinking How the Most Massive Galaxies Evolve
Understanding how massive galaxies stop forming stars is key to explaining why today’s universe looks the way it does. The team’s research helps clarify whether ultramassive galaxies in the early universe are ‘dusty or dead’ – their answer is not one or the other. The results show that most of the ultramassive galaxies studied are genuinely quiescent, indicating a rapid and efficient shutdown of star formation.
Within this quiescent population, several systems are among the most dust-poor massive galaxies ever identified at these early cosmic times. The remaining two galaxies exhibit residual dust emission, with one showing evidence for ongoing but heavily obscured star formation, and the other caught in the process of quenching. These results offer a rare glimpse into the diverse evolutionary paths taken by the universe’s most massive galaxies.
“What excites me most about this work is that we are able to study an extremely rare population in the universe by combining data from as many telescopes as possible,” said Chang. “By bringing together optical, near-infrared, and far-infrared observations, we can build a much more complete picture of what ultra-massive galaxies really look like at these early cosmic times.”
Members of the MAGAZ3NE collaboration involved in this work include Dr. Ian McConachie, postdoctoral researcher at the University of Wisconsin–Madison; Professor Allison Noble of Arizona State University; Professor Tracy Webb of McGill University; Professor Adam Muzzin of York University, Canada; Professor Michael Cooper of the University of California, Irvine; Professor Gabriela Canalizo of the University of California, Riverside; Professor Danilo Marchesini of Tufts University; Dr. Percy Gomez, Staff Astronomer at the W. M. Keck Observatory; Dr. Stephanie Urbano Stawinski of the University of California, Santa Barbara.
Contact:
Wenjun Chang, UC Riverside, Email: wchan148@ucr.edu
Gillian Wilson, UC Merced, Email: gwilson@ucmerced.edu
Benjamin Forrest, UC Davis, Email: bforrest@ucdavis.ed
AAS Press: Susanna Kohler susanna.kohler@aas.org
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ABOUT MOSFIRE
The Multi-Object Spectrograph for Infrared Exploration (MOSFIRE), gathers thousands of spectra from objects spanning a variety of distances, environments and physical conditions. What makes this large, vacuum-cryogenic instrument unique is its ability to select up to 46 individual objects in the field of view and then record the infrared spectrum of all 46 objects simultaneously. When a new field is selected, a robotic mechanism inside the vacuum chamber reconfigures the distribution of tiny slits in the focal plane in under six minutes. Eight years in the making with First Light in 2012, MOSFIRE’s early performance results range from the discovery of ultra-cool, nearby substellar mass objects, to the detection of oxygen in young galaxies only two billion years after the Big Bang. MOSFIRE was made possible by funding provided by the National Science Foundation.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaiʻi feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. For more information, visit: www.keckobservatory.org