“Dead” Galaxies Mysteriously Ran Out of Fuel To Make Stars in the Early Universe

MRG-M1341

Galaxy clusters MACS J1341. Credit: Lead Author: NASA, ESA, Katherine E. Whitaker (UMass), Image Processing: Joseph DePasquale (STScI)

Hubble Finds Early, Massive Galaxies Running on Empty

“Live fast, die young” could be the motto of six early, massive, “dead” galaxies that ran out of the cold hydrogen gas needed to make stars early in the life of the universe. These galaxies lived fast and furious lives, creating their stars in a remarkably short time. But then they literally ran out of gas and shut down star formation. Without more fuel to create stars, these galaxies were running on empty. Why this happened at such early times is a mystery.

NASA’s Hubble Space Telescope, together with the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile, found these odd galaxies while looking back billions of years to the peak of star birth in the universe. To locate the very distant galaxies, scientists combined the power of Hubble and ALMA with extremely massive foreground galaxy clusters acting as natural telescopes. Through a phenomenon called strong gravitational lensing, the immense gravity of a giant 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.

Gravitationally Lensed REQUIEM Galaxies

These images are 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.
Hubble, together with ALMA, found these odd galaxies when they combined forces with the “natural lens” in space created by foreground massive galaxy clusters. The clusters’ gravity stretches and amplifies the light of the background galaxies in an effect called gravitational lensing. This phenomenon allows astronomers to use massive galaxy clusters as natural magnifying glasses to study details in the distant galaxies that would otherwise be impossible to see.
The yellow traces the glow of starlight. The artificial purple color traces cold dust from ALMA observations. This cold dust is used as a proxy for the cold hydrogen gas needed for star formation.
Even with ALMA’s sensitivity, scientists do not detect dust in most of the six galaxies sampled. One example is MRG-M1341, at upper right. It looks distorted by the “funhouse mirror” optical effects of lensing. In contrast, the purple blob to the left of the galaxy is an example of a dust-and-gas-rich galaxy.
One example of the detection of cold dust ALMA did make is galaxy MRG-M2129 at bottom right. The galaxy only has dust and gas in the very center. This suggests that star formation may have shut down from the outskirts inward.
Credit: Lead Author: NASA, ESA, Katherine E. Whitaker (UMass), Image Processing: Joseph DePasquale (STScI)

When the universe was about 3 billion years old, just 20% of its current age, it experienced the most prolific period of star birth in its history. But when NASA’s 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 more fuel for star formation, these galaxies were literally running on empty. The findings are published in the journal Nature

“At this point in our universe, all galaxies should be forming lots of stars. It’s the peak epoch of star formation,” explained lead author Kate Whitaker, assistant professor of astronomy at the University of Massachusetts, Amherst. 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?”

This study is a classic example of the harmony between Hubble and ALMA observations. Hubble pinpointed where in the galaxies the stars exist, showing where they formed in the past. By detecting the cold dust that serves as a proxy for the cold hydrogen gas, ALMA showed astronomers where stars could form in the future if enough fuel were present.

REQUIEM Galaxies Compass Image

Galaxy clusters MACS J1341 and MACS J2129. Credit: Lead Author: NASA, ESA, Katherine E. Whitaker (UMass), Image Processing: Joseph DePasquale (STScI)

Using Nature’s Own Telescopes

The study of these early, distant, dead galaxies was part of the appropriately named REQUIEM program, which stands for Resolving QUIEscent Magnified Galaxies At High Redshift. (Redshift happens when light is stretched by the expansion of space and appears shifted toward the red part of the spectrum. The farther away a galaxy is with respect to the observer, the redder it appears.)

The REQUIEM team uses extremely massive foreground galaxy clusters 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 is called “strong gravitational lensing.”

Only by combining the exquisite resolution of Hubble and ALMA with this strong lensing was the REQUIEM team able to able to understand the formation of these six galaxies, which 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,” said 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,” said 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.”

Live Fast, Die Young

These sorts of dead galaxies don’t appear to rejuvenate, even through later minor mergers and accretions of nearby, small galaxies and gas. Gobbling up things around them mostly just “puffs up” the galaxies. If star formation does turn back on, Whitaker described it as “a kind of a frosting.” 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.

These six galaxies lived fast and furious lives, creating their stars in a remarkably short time. Why they shut down star formation so early is still a puzzle.

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.”

Reference: “Exhausted gas reservoirs drive massive galaxy quenching in the early universe” by Katherine E. Whitaker, Christina C. Williams, Lamiya Mowla, Justin S. Spilker, Sune Toft, Desika Narayanan, Alexandra Pope, Georgios E. Magdis, Pieter G. van Dokkum, Mohammad Akhshik, Rachel Bezanson, Gabriel B. Brammer, Joel Leja, Allison Man, Erica J. Nelson, Johan Richard, Camilla Pacifici, Keren Sharon & Francesco Valentino, 22 September 2021, Nature.
DOI: 10.1038/s41586-021-03806-7

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.

2 Comments on "“Dead” Galaxies Mysteriously Ran Out of Fuel To Make Stars in the Early Universe"

  1. Atheist way, explains everything in the known Universe & then some. It’s also great for kids who favorite question is “What if”?? So much better than doing the Deep dive. And no one judges you…except for you & you’re always good! 👍👍

  2. We’re looking back in time when we watch those galaxies. The right thing to do is to compare those distant galaxies with galaxies just a little bit closer situated and not compare them with anything else. In order to get a full understanding

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