Wednesday, October 13, 2021

These galaxies cannot form new stars and astronomers don't know why

Some of the oldest and most massive galaxies in the universe have mysteriously lost the ability to produce new stars and have become "blacked out".  Astronomers found them in a new study and wondered if this was due to some unknown process that disrupted stellar production, but they soon discovered something startling: galaxies actually never had enough gas for new stars.

 This "dead" galaxy can still generate a new large and massive elliptical galaxy

 How to measure distances between galaxies in a constantly expanding universe?

 It was through the Atacama Large Millimeter/submillimeter Array (ALMA) and the Hubble Space Telescope that the researchers discovered half a dozen primitive massive galaxies that ran out of “fuel”.  They're not the only galaxies considered to be faded, but given their ages, the absence of cold gas isn't exactly what scientists expected to see.

 To analyze galaxies so distant and dim, astronomers used a feature that the universe itself provides, gravitational lenses—spacetime distortions caused by the gravity of much closer galaxies positioned precisely between Earth and the farthest target.  With these lenses, light from distant objects is “forced” to follow the distortion path, resulting in magnification of the source.  In this case, the sources were the six galaxies described in the study.

 According to Kate Whitaker, lead author of the study and an assistant professor of astronomy at the University of Massachusetts, scientists deduced "that these extinct galaxies hit the brakes a few billion years after the Big Bang."  However, the most cautious observation led them to conclude "that the first galaxies didn't really apply the brakes, but were instead running in a vacuum".

 This conclusion is totally unexpected, because the oldest galaxies, formed in the first three billion years after the Big Bang, should be rich in cold gas, the raw material of the stars.  That was just the mystery—how did galaxies rich in cold gas stop stellar production?  Well, they weren't that rich.

 To better understand how these galaxies formed and died, the team made simultaneous observations with ALMA and Hubble, as part of a project called REQUIEM (short for REsolving QUIEscent Magnified galaxies), which focuses on using gravitational lenses to observe galaxies that, otherwise they would not be found.  The authors then analyzed light at millimeter wavelengths to measure the amount of gas in galaxies.

 Stars form in molecular clouds made mostly of hydrogen and helium, the most abundant elements in the universe.  When the most massive ones explode in a supernova, the resulting turbulence pushes those clouds away and, as a result, they become denser.  Increasingly compressed, this cold gas ends up collapsing under its own weight, and thus a star is born.  In this cycle that repeats itself over millions of years, new stars also incorporate the gas and dust released by supernovae—a kind of cosmic recycling.

 Thus, it is natural that galaxies always have some considerable amount of cold gas, but this is not the case for the six galaxies observed in this study.  They've had their gas tanks emptied, and it's still unclear why.  "We don't understand why this is, but possible explanations could be that the primary gas supply that fuels the galaxy has been cut off or perhaps a supermassive black hole is injecting energy that keeps the gas in the galaxy warm," said Christina C. Williams, co-author of the study.

 This means that galaxies cannot refuel the “fuel tank”, in Williams' words, and therefore cannot restart the processes of star production.  "The mere fact that these massive beasts of the cosmos formed 100 billion stars in about a billion years and then suddenly ended their star formation is a mystery we would all love to solve," Whitaker said.  In other words, where did the cold star-forming gas go?

 To find the answer to these nagging—or exhilarating, depending on your point of view—questions will require further studies.  But REQUIEM's research has already brought important advances to the study of galaxies through gravitational lenses.  In fact, this was the first time that scientists were able to make measurements of cold material from distant faint galaxies outside the local universe.


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