Using the Atacama Large Millimeter/submillimeter Array (ALMA), a team of astronomers revealed that molecular clouds in a galaxy called AzTEC/COSMOS-1, an extreme starburst (star-forming) galaxy located 12.4 billion light-years away, are highly unstable, which leads to runaway star formation. Extreme starburst galaxies, also known as ‘monster’ galaxies, are thought to be the ancestors of very massive elliptical galaxies in today’s Universe, therefore the findings pave the way to understand the formation and evolution of such galaxies.
Artist’s impression of the extreme starburst galaxy AzTEC/COSMOS-1. Image credit: National Astronomical Observatory of Japan.
‘Monster’ galaxies form stars at a startling pace — 1,000 times higher than the star formation in our Milky Way Galaxy. But why are they so active?
To tackle this problem, astronomers need to know the environment around the stellar nurseries. Drawing detailed maps of molecular clouds is an important step to scout a cosmic monster.
“A real surprise is that AzTEC/COSMOS-1 seen 12.4 billion years ago has a massive, ordered gas disk that is in regular rotation instead of what we had expected, which would have been some kind of a disordered train wreck that most theoretical studies had predicted,” said University of Massachusetts Amherst’s Professor Min Yun, co-author of the current study and a member of the team that discovered this galaxy in 2007.
“AzTEC/COSMOS-1’s gas disk is dynamically unstable now, which means the entire gas disk that makes up this galaxy is fragmenting and undergoing a gigantic episode of starburst, which helps to explain its enormous star formation rate, more than 1,000 times that of the Milky Way.”
Professor Yun and colleagues found that AzTEC/COSMOS-1 is rich in the ingredients of stars, but it was still difficult to figure out the nature of the cosmic gas in the galaxy.
They used ALMA’s high resolution and high sensitivity to observe the galaxy and obtain a detailed map of the distribution and the motion of the gas to make the highest resolution molecular gas map of a distant monster galaxy ever made.
“We found that there are two distinct large clouds several thousand light-years away from the center,” said study lead author Dr. Ken-ichi Tadaki, a postdoctoral researcher at the Japan Society for the Promotion of Science and the National Astronomical Observatory of Japan.
“In most distant starburst galaxies, stars are actively formed in the center. So it is surprising to find off-center clouds.”
“How these galaxies have been able to amass such a large quantity of gas in the first place and then essentially turn the entire gas reserve into stars in the blink of an eye, cosmologically speaking, was a completely unknown question about which we could only speculate. We have the first answers now,” Professor Yun said.
“Until this result came in from ALMA, nobody knew how Nature created massive, young galaxies formed only 1 billion years after the Big Bang.”
With the new observations, the researchers now believe that the monster galaxy is powered by ‘an extremely gas-heavy disk that is somehow kept stable until enough gas is amassed.’
“We still don’t know yet how so much gas is collected so quickly and what kept this enormous gas reserve from igniting and turning into stars, as gas is known to do in the local Universe,” Professor Yun said.
ALMA observations revealed dense gas concentrations in the disk of AzTEC/COSMOS-1 and intense stars formation in those concentrations. Image credit: National Astronomical Observatory of Japan.
The astronomers found that the gas clouds in AzTEC/COSMOS-1 are very unstable, which is unusual.
“In a normal situation, the inward gravity and outward pressure are balanced,” they said.
“Once gravity overcomes pressure, the gas cloud collapses and forms stars at a rapid pace. Then, stars and supernova explosions at the end of the stellar life cycle blast out gases, which increase the outward pressure.”
“As a result, the gravity and pressure reach a balanced state and star formation continues at a moderate pace. In this way star formation in galaxies is self-regulating.”
“But in AzTEC/COSMOS-1, the pressure is far weaker than gravity and hard to balance. Therefore this galaxy shows runaway star formation and has morphed into an unstoppable monster galaxy.”
The study authors estimate that the gas in AzTEC/COSMOS-1 will be completely consumed in 100 million years, which is 10 times faster than in other star forming galaxies.
Why the gas in this galaxy is so unstable is not clear yet, but a phenomenon called ‘galaxy merger’ is a possible cause.
Galaxy collision may have efficiently transported the gas into a small area and ignited intense star formation.
“At this moment, we have no evidence of merger in this galaxy. But by observing other similar galaxies with ALMA, we want to unveil the relation between galaxy mergers and monster galaxies,” Dr. Tadaki said.
The study is published in the journal Nature.
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K. Tadaki et al. 2018. The gravitationally unstable gas disk of a starburst galaxy 12 billion years ago. Nature 560: 613-616; doi: 10.1038/s41586-018-0443-1
