A longstanding question about why some galaxies stopped producing stars during a period of the early universe otherwise noted for prolific star birth has now been answered, thanks to new research by an international team that includes a Penn State astronomer. The team discovered six of these early, massive “dead" galaxies that, it turns out, had run out of the cold hydrogen gas needed to make the stars. Their results appear September 22 in the journal Nature.
“When the universe was about 3 billion years old, most galaxies in our universe were experiencing spectacular bursts of star formation due to an abundance of available primordial gas,” said Joel Leja, assistant professor of astronomy and astrophysics at Penn State and a member of the research team. “Surprisingly however, we observed that many of the oldest, most massive galaxies had already gone quiescent—they stopped making stars— in spite of this abundance of fuel. There has not been consensus as to why this might be. In this study, we observed six of these quiescent galaxies to help shed light on this question.”
The research team used an innovative pairing of telescopes— NASA's Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile—to find and study these galaxies. The Hubble Space Telescope—which detects light that spans beyond what humans can detect with their eyes, from ultraviolet to near-infrared light—helped to pinpoint where quiescent galaxies exist. Because they are located so far away, the Earth is only just now seeing the light the galaxies emitted 10 to 12 billion years ago, providing a glimpse into the universe’s deep past. Then the research team used ALMA, which detects radiation invisible to the naked eye, to look for the cold dust that serves as a proxy for the cold hydrogen gas that fuels new star formation.
“There was copious cold gas in the early universe, so these galaxies, from 12 billion years ago, should have plenty left in the fuel tank,” said Kate Whitaker, assistant professor of astronomy at the University of Massachusetts, Amherst, who led the research team. “So what happened to all the cold gas in these galaxies so early on?”
The team found only traces of cold gas located at each galaxy’s center. This means that, within the first few billion years of the universe’s existence, these galaxies either burned through their energy supplies, or ejected them and, furthermore, that something may be physically blocking each galaxy’s replenishment of cold gas. Without more fuel for star formation, these galaxies were literally running on empty.
To better observe these galaxies, the team took advantage of extremely massive galaxy clusters located closer to Earth that act as natural telescopes. The immense gravity of a galaxy cluster warps space, bending and magnifying light from background objects. When an early, massive and very distant galaxy is positioned behind such a cluster, it appears greatly stretched and magnified, allowing astronomers to study details that would otherwise be impossible to see. This phenomenon is called strong gravitational lensing.
Then, to help the team understand and interpret the images from ALMA and the HST, Leja used the Penn State Institute for Computational and Data Sciences ROAR supercomputer to build models of the observed galaxies. These included stars and their emissions, dust, super-heated gas, as well as how the galaxies might feed black holes. ROAR generates millions and millions of these potential galaxies and their possible formation histories, then compares them to the real telescope images to see which of them may truly be occurring in these distant galaxies.
“By combining data from Hubble and ALMA and using strong gravitational lensing as a natural telescope, we found some of the first distant and massive galaxies to shut down their star formation,” said Leja. “This information helps us understand the early history of the universe as well as more generally how galaxies evolve.”
Star formation is one of the key ways galaxies grow, and these sorts of dead galaxies don’t appear to grow considerably after they become quiescent; merging with or adding nearby, small galaxies and gas are the only way for these dead galaxies to continue growing.
Why these galaxies ran out of cold gas so early is still a puzzle. The research team suggests several possible explanations.
"Did a supermassive black hole in the galaxy's center turn on and heat up all the gas?” said Whitaker. “If so, the gas could still be there, but now it's hot. Or it could have been expelled and now it's being prevented from accreting back onto the galaxy. Or did the galaxy just use it all up, and the supply is cut off? These are some of the open questions that we'll continue to explore with new observations down the road."
This research was part of the REQUIEM program (Resolving QUIEscent Magnified Galaxies at High Redshift). This research was supported by the Alfred P. Sloan Foundation, National Science Foundation, NASA Goddard Space Flight Center, European Research Council, Carlsberg Foundation, Villum Fonden, Canadian Space Agency, NASA ,and the Dunlap Institute for Astronomy & Astrophysics.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.