Scientists Find Elusive Gas From Post-starburst Galaxies Hiding in Plain Sight

This news article was originally published on NRAO.edu on Apr. 25, 2022.

Scientists discovered that post-starburst galaxies condense their gas rather than expelling it, begging the question: what’s actually keeping them from forming stars?

Post-starburst galaxies were previously thought to scatter all of their gas and dust—the fuel required for creating new stars—in violent bursts of energy, and with extraordinary speed. Now, new data from the Atacama Large Millimeter/submillimeter Array (ALMA) reveals that these galaxies don’t scatter all of their star-forming fuel after all. Instead, after their supposed end, these dormant galaxies hold onto and compress large amounts of highly-concentrated, turbulent gas. But contrary to expectation, they’re not using it to form stars.

In most galaxies, scientists expect gas to be distributed in a way similar to starlight. But for post-starburst galaxies, or PSBs, this isn’t the case. PSBs are different from other galaxies because they are born in the aftermath of violent collisions, or mergers between galaxies. Galaxy mergers typically trigger massive bursts of star formation, but in PSBs, this outburst slows down and near-completely stops almost as soon as it begins. As a result, scientists previously believed that little or no star-forming fuel was left in these galaxies’ central star-forming factories. And until now, the belief was that the molecular gases had been redistributed to radii well beyond the galaxies, either through stellar processes or by the effects of black holes. The new results challenge this theory. 

“We’ve known for some time that large amounts of molecular gas remains in the vicinity of PSBs but haven’t been able to say where, which in turn, has prevented us from understanding why these galaxies stopped forming stars. Now, we have discovered a considerable amount of remaining gas within the galaxies and that remaining gas is very compact,” said Adam Smercina, an astronomer at the University of Washington and the principal investigator of the study. “While this compact gas should be forming stars efficiently, it isn’t. In fact, it is less than 10-percent as efficient as similarly compact gas is expected to be.”

In addition to being compact enough to make stars, the gas in the observed dormant—or quiescent—galaxies had another surprise in store for the team: it was often centrally-located, though not always, and was surprisingly turbulent. Combined, these two characteristics led to more questions than answers for researchers. 

“The rates of star formation in the PSBs we observed are much lower than in other galaxies, even though there appears to be plenty of fuel to sustain the process,” said Smercina. “In this case, star formation may be suppressed due to turbulence in the gas, much like a strong wind can suppress a fire. However, star formation can also be enhanced by turbulence, just like wind can fan flames, so understanding what is generating this turbulent energy, and how exactly it is contributing to dormancy, is a remaining question of this work.” 

Decker French, an astronomer at the University of Illinois, and a co-author of the research added, “These results raise the question of what energy sources are present in these galaxies to drive turbulence and prevent the gas from forming new stars. One possibility is energy from the accretion disk of the central supermassive black holes in these galaxies.”

A clear understanding of the processes that govern the formation of stars and galaxies is key to providing context to the Universe and our place in it. The discovery of turbulent, compact gas in otherwise dormant galaxies gives researchers one more clue to solving the mystery of how galaxies in particular live, evolve and die over the course of billions of years. And that means additional future research with the help of ALMA’s 1.3mm receiver, which sees the otherwise invisible with stark clarity. 

J.D. Smith, an astronomer at the University of Toledo, and a co-author of the research said, “There is much about the evolution of a typical galaxy we don’t understand, and the transition from their vibrant star-forming lives into quiescence is one of the least understood periods. Although post-starbursts were very common in the early Universe, today they are quite rare. This means the nearest examples are still hundreds of millions of light-years away, but these events foreshadow the potential outcome of a collision, or merger, between the Milky Way Galaxy and the Andromeda Galaxy several billion years from now. Only with the incredible resolving power of ALMA could we peer deep into the molecular reservoirs left behind ‘after the fall.’”

Smercina added, “It’s often the case that we as astronomers intuit the answers to our own questions ahead of observations, but this time, we learned something completely unexpected about the Universe.”

The results of the study are published today in The Astrophysical Journal.

Resource

“After The Fall: Resolving the Molecular Gas in Post-Starburst Galaxies,” Smercina et al (2022), The Astrophysical Journal, doi: 10.3847/1538-4357/ac5d5f

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About NRAO

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

About ALMA

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (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 Ministry of Science and Technology (MOST) 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.

 

NRAO Director Tony Beasley Appointed to New Five-Year Term

 

Dr. Tony Beasley, Director of the National Science Foundation’s National Radio Astronomy Observatory (NRAO), has been appointed to a new five-year term. The Board of Trustees for AUI— which operates NRAO under a cooperative agreement— and the NRAO Director Review Committee conducted a thorough review of Beasley’s leadership and performance earlier this year, and have appointed the Director to the new term through May 2027.

 

“Tony is an outstanding leader and stalwart champion for NRAO, the field of radio astronomy, the beauty of science, and the critical role of big facilities in the R&D ecosystem,” said Adam Cohen, President and CEO of AUI, which operates NRAO under a cooperative agreement. “He continues to support very innovative education and outreach programs to help build the workforce of the future, as well as programs and activities to enhance diversity, equity, and inclusion in our workplaces.” 

 

Over the course of more than two decades, Beasley’s leadership has shaped the present and future of NRAO’s leading-edge radio astronomy facilities, including the Atacama Large Millimeter/submillimeter Array (ALMA), Very Long Baseline Array (VLBA), and Very Large Array (VLA). More recently, he has collaborated on efforts to encourage cooperation between commercial spectrum users and research facilities and has created partnerships to explore the use of Green Bank Observatory’s radar systems in planetary science and defense applications. 

 

Beasley recently has generated significant support for the future of NRAO’s facilities, reaching major milestones in 2021. The observatory’s proposed next generation Very Large Array (ngVLA) received high priority for new ground-based observatories in the U.S. National Academy of Sciences’ Astronomy and Astrophysics Decadal Survey (Astro2020). Late last year, NRAO’s Central Development Laboratory (CDL) received approval and funding through the ambitious ALMA2030 Development Plan to upgrade its Band 6 receivers, which are ALMA’s most productive receivers. 

 

An ardent supporter of diversity, equity, and inclusion in astronomy and astrophysics, Beasley has elevated the efforts of NRAO’s Office of Diversity and Inclusion, Broader Impacts, and community development initiatives, including the National Astronomy Consortium (NAC), RADIAL, National and International Non-traditional Exchange (NINE), Research Experiences for Undergraduate students (REU), and most recently, grants for women in engineering fellowships and the development of a next generation Learning Center (ngLC). 

 

“Being a part of NRAO for more than 20 years has given me the opportunity to observe, contribute to, and lead growth and change in astronomy that positively impacts our facilities and allows us to collaborate with other like-minded institutions,” said Beasley. “I am proud of the work our teams have accomplished in research, engineering, outreach, and equity, and look forward to serving the NRAO community for another five years.”

 

Beasley, who holds a Doctorate in Astrophysics from the University of Sydney, was first appointed as NRAO Director in February 2012, after previously serving the observatory and the radio astronomy community in multiple capacities. He joined NRAO as a Postdoctoral Fellow in 1991 and served as Deputy Assistant Director in 1997 and Assistant Director from 1998 to 2000. He briefly left NRAO that year to become Project Manager for the Combined Array for Research in Millimeter-wave Astronomy (CARMA). In 2004, he returned to NRAO as Assistant Director, as well as Project Manager for the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. From 2008 to 2012, Beasley served as the Chief Operating Officer and Project Manager of NSF’s National Ecological Observatory Network (NEON). In addition to his role as NRAO Director, Beasley presently serves as the AUI Vice President for Radio Astronomy Operations. 

 

In January 2022, Beasley was honored as a Lifetime Fellow of the American Association for the Advancement of Science (AAAS) in recognition of his significant contributions to the field of radio astronomy. 

 

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