Six galaxies live ‘fast and furious’ lives, run out of fuel to make stars

Scientists examining the solar system were stunned to find six massive galaxies that have apparently died during the peak of the universe's star formation. 

 “Star formation in half of massive galaxies was quenched by the time the Universe was 3 billion years old,” a paper in Nature magazine notes. 

The article, Quenching of star formation from a lack of inflowing gas to galaxies, suggests that “Very low amounts of molecular gas seem to be responsible for this, at least in some cases, although morphological gas stabilization, shock heating or activity associated with accretion onto a central supermassive black hole are invoked in other cases.”

NASA reports it more simply: noting that when the universe was 3 billion years old, at 20 percent of its current age, “it experienced the most prolific period of star birth in its history.” Still, when its Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile “gazed toward cosmic objects in this period, they found something odd: six early, massive, 'dead' galaxies that had run out of the cold hydrogen gas needed to make stars.”

Without the cold hydrogen gas as fuel to make stars, these galaxies were, as NASA calls it, “running on empty.”

"At this point in our universe, all galaxies should be forming lots of stars. It's the peak epoch of star formation," explains lead author Kate Whitaker, assistant professor of astronomy at the University of Massachusetts, Amherst, in the NASA news release. Whitaker is also associate faculty at the Cosmic Dawn Center in Copenhagen, Denmark. "So what happened to all the cold gas in these galaxies so early on?"

NASA calls the study “a classic example of the harmony between Hubble and ALMA observations.” According to NASA, the Hubble Space Telescope “pinpointed where in the galaxies the stars exist, showing where they formed in the past.” And ALMA showed astronomers where stars could form in the future if enough fuel were present “by detecting the cold dust that serves as a proxy for the cold hydrogen gas.”

The REQUIEM program (Resolving QUIEscent Magnified Galaxies at High Redshift) studies these “early, distant, dead” galaxies. NASA explains redshift as the stretching of light with the expansion of space and appears shifted towards the red part of the spectrum. That is to say, the farther away the galaxy is to an observer, the redder it appears.

NASA also explains “strong gravitational lensing,” the term given to an early, massive and very distant galaxy positioned behind a massive foreground galaxy cluster, appearing as “greatly stretched and magnified, allowing astronomers to study details that would otherwise be impossible to see.”

The REQUIEM team were able to understand the formation of these six dead galaxies by combining the incredible resolution of Hubble, ALMA and the strong gravitational lensing. The six galaxies, NASA says ,”appear as they did only a few billion years after the Big Bang.”

"By using strong gravitational lensing as a natural telescope, we can find the distant, most massive, and first galaxies to shut down their star formation," says Whitaker. "I like to think about it like doing science of the 2030s or 40s – with powerful next-generation space telescopes – but today instead by combining the capabilities of Hubble and ALMA, which are boosted by strong lensing." 

"REQUIEM pulled together the largest sample to date of these rare, strong-lensed, dead galaxies in the early universe, and strong lensing is the key here," says Mohammad Akhshik, principal investigator of the Hubble observing program. "It amplifies the light across all wavelengths so that it's easier to detect, and you also get higher spatial resolution when you have these galaxies stretched across the sky. You can essentially see inside of them at much finer physical scales to figure out what's happening."

According to the NASA news release, even minor mergers and accretions of nearby small galaxies and gas are not enough to revive these kinds of dead galaxies; their addition to the dead galaxy “mostly just ‘puffs up’ the galaxies.” Whitaker describes a possible scenario of star formation turning back on as “a kind of frosting.” NASA notes that about 11 billion years later in the present-day universe, “these formerly compact galaxies are thought to have evolved to be larger but are still dead in terms of any new star formation.”

NASA calls the reason the galaxies shut down star formation so early “a puzzle,” saying the six galaxies “lived fast and furious lives” – creating their stars in an unbelievably short window of time.

Whitaker proposes several possible explanations: "Did a supermassive black hole in the galaxy's center turn on and heat up all the gas? 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."

The NASA news release notes that “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, DC.”

Thumbnail and headline images: Composites from NASA's Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA). The boxed and pullout images show two of the six, distant, massive galaxies where scientists found star formation has ceased due to the depletion of a fuel source – cold hydrogen gas. Processing: Joseph DePasquale (STScI)