Astronomers from the Comic Dawn Center have discovered six galaxies in the early Universe that have exhausted all their gas and formed into new stars. The astronomers used the Hubble Space Telescope, the ALMA radio telescope, and the “cosmic telescope” for the discovery.
The Cosmic Dawn Center is a collaboration between Niels Bohr Institute at the University of Copenhagen and DTU Space at the Technical University of Denmark. The astronomers are yet to find the way they ran out of gas.
Galaxies are full of stars, and stars are made from gas. Currently, certain galaxies have exhausted all their gas due to several reasons and, therefore, stopped forming new stars.
Meanwhile, others have reused gas from the old stars by accreting new gas from intergalactic space and have proceeded to form new stars.
A majority of the stars were created in the early Universe when it was one-fifth of its current age. This is about two to four billion years post-Big Bang.
As the light from distant galaxies takes time to travel, the research looks further back in time the farther it observes into space. When observing 10 to 12 billion years back in time, it is possible to see strongly star-forming galaxies.
Now, a team of astronomers led by Kate Whitaker, an assistant professor from the University of Massachusetts Amherst, and associate faculty at the Danish Cosmic Dawn Center at the Niels Bohr Institute and DTU Space has recorded six massive galaxies in this period which have already exhausted their gas due to unknown reason and ceased forming stars.
The observation contradicts the previous understanding of star formation and galaxy evolution.
For a long time, we have suspected that running out of gas is one of the main reasons that some galaxies cease to form star. But now we have the evidence.
Georgios Magdis, Associate Professor, Cosmic Dawn Center
The observations revealing the exhaustion of gases in these galaxies have been conducted using a series of radio antennas in Chile, known as the Atacama Large Millimeter Array, or “ALMA.”
To discover the galaxies in the first place, the astronomers leveraged the Hubble Space Telescope’s high resolution.
Then, with the excellent phenomenon called “gravitational lensing,” the astronomers investigated galaxies of this kind in unmatched detail. The gravitational lenses in these six instances are intervening clusters of hundreds of galaxies. Thanks to their gigantic mass curve space, they focus the light from the distant galaxies towards the Earth.
The “cosmic telescope” is capable of magnifying the six galaxies and amplifying their light up to 30 times. This enabled the team to constrain the amount of gas to considerably lower limits than the earlier possible amount.
Requiem for a Galaxy
It is an open question in astronomy regarding the underlying reason for some galaxies ceasing the formation of stars and turning inactive. For stars to form, a galaxy must have sufficient gas and this gas must not be too hot. This is because it is not possible for hot gas to clump and collapse into stars.
As the discovery is a long-awaited one, it is not a big surprise for astronomers.
Gas detection in faraway dead galaxies is usually near to impossible, as the galaxies are too dull and too small to see adequately minor details. However, in an observational program, known as REsolving QUIEscent Magnified (REQUIEM) Galaxies, Kate Whitaker and her team have discovered some of these unique galaxies.
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.
Kate Whitaker, Assistant Professor, University of Massachusetts Amherst
The reason behind why the REQUIEM galaxies shut down is still unclear.
"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. Was the gas blown out of the galaxy? Or did the galaxy simply use it all up? These are some of the open questions that we’ll continue to explore with new observations down the road," considers Whitaker.
Whitaker, K. E., et al. (2021) Quenching of star formation from a lack of inflowing gas to galaxies. Nature. doi.org/10.1038/s41586-021-03806-7.