Gas-Guzzling Galaxies May Reveal Clues About The Early Universe

In a galaxy more than 12 billion light years from Earth, stars are being created at a rate 500 times faster than in our Milky Way. That cosmic fertility comes at a price: the galaxy is quickly running out of fuel. But it's also teaching us more about galaxy formation in the early universe.

The galaxy, known as ALESS65—which was first observed by the Atacama Large Millimeter Array (ALMA) in Chile—is on its way to becoming "red and dead." It will no longer form stars and it will appear red because the existing stars have aged.

Massive "red and dead" galaxies have been discovered to exist at more than 11.5 billion light years away, only 2 billion years or so after the Big Bang. That puzzles astronomers, since galaxies are thought to form in hierarchical mergers, with large galaxies forming by combining with smaller ones. Since it takes time for massive galaxies to build up, use up all their stellar fuel, and for their stars to age, there shouldn't be many of them so early in the universe.

A team of Australian astronomers theorized that ALESS65—and other "starbursting" galaxies at 12 billion light years away—evolve more rapidly than those closer to Earth.

They conducted the investigation by searching for traces of carbon monoxide. Stars produce carbon and oxygen through fusion processes. Carbon monoxide is then formed in the cold dense galactic clouds of gas and dust when conditions are right—making it the second most abundant molecule in galaxies (after molecular hydrogen).

Carbon monoxide is easily excited into a high-energy state via collisions with the hydrogen molecules. As it returns to the unexcited state it releases the energy as a photon, which can be detected by ground based radio telescopes. Because of this, it is widely used as a tracer of the molecular gas in galaxies.

As Minh Huyhn, one of the investigating astronomers, explains:

From the carbon monoxide detection we were able to estimate how much molecular gas, the raw fuel for star formation, is in ALESS65. Our own Milky Way will take several billion years to use up its fuel and become a "red and dead" galaxy.

ALESS65 appears to have only tens of millions years left of fuel, which is very fast in astronomical terms. It's a "gas guzzler" compared to the Milky Way, because of the huge rate at which it is forming stars.

We also combined our observations of ALESS65 with the original data from ALMA to work out how similar ALESS65 is to the galaxies nearer to Earth. ALMA detected atomic carbon emission, which is due mostly to young stars with lots of ultra violet (UV) light emission that energizes the edges of molecular clouds.

The astronomers discovered that the UV radiation in ALESS65 is weaker than starbursting galaxies that are younger and closer. That finding suggests that one key difference between the two types of galaxies is that the stars in ALESS65 are forming over a larger area.

The astronomers intend to expand this investigative technique to other distant "red and dead" galaxies, to gain more clues about their physical traits and rapid evolution, which could help explain how the first galaxies formed in the aftermath of the Big Bang.

[Read the research paper, "Detection of molecular gas in an ALMA [C II]-identified submillimetre galaxy at z = 4.44," at the Monthly Notices of the Royal Astronomical Society]