There are many large galaxies in the universe in which the formation of stars has almost completely ended – they are dead in a sense. However, the reasons for this were not clear. Now astronomers have discovered a galaxy that provides new insights. Because it removed nearly half of its gas supply in a galaxy collision. Researchers were able to observe this massive gas tail with the Atacama Large Millimeter / Submillimeter Array (ALMA). There is a loss of gas, due to the fact that star formation in this galaxy will end in a few tens of years for a long time.
In our Milky Way and most other galaxies, new stars are constantly forming. They are formed by the collapse of dense gas clouds and allow the galaxy to grow. According to the observations, the rate of star formation in the still young universe was much higher than it is today, often “older” galaxies. But there are also galaxies in which star formation stops for a long time or even stops completely during cosmic evolution. Our close cosmic atmosphere therefore consists of elliptical, old galaxies, largely “dead” – as good as no new star arises. But the reasons for this halt are still unclear: “We do not yet know what the processes behind neutralizing star formation in large-scale galaxies explain” the first author, Anagrazia Puglisi, of Durham University in the UK.
About half of the gas is removed
It is clear that the star formation in these “dead” galaxies is likely due to a lack of gas supply. According to this, there would have been processes that ejected some of the gases that would normally be abundant in such galaxies. “Strong star formation or active black hole winds have so far been considered the main culprits for such gas losses and for suppression of growth in large-scale galaxies,” says Puglisi. However, until now, there has been a lack of comments that can capture these processes and their implications in the act, so to speak. But now Puglisi and his team have accidentally discovered a galaxy that has just lost a normal amount of gas. He used the Atacama Large Millimeter / Submillimeter Array (ALMA) in Chile to study the properties of cold gas in over 100 galaxies.
By doing this, they discovered a galaxy about nine billion light years away that emits a huge stream of gas. Data from the observatory shows that gas extracted from the giant galaxy ID2299 is about 46 percent of its entire supply of cold gas. This output is at a high rate of about 10,000 solar masses per year, as reported by astronomers. The rate of star formation in this galaxy is still about five times higher than its “conspirators” from this cosmic age. But researchers believe it will not last long. According to their estimates, the remaining gas supply could be used over millions of years. The formation of the star would then come to a standstill – and it would probably be for several hundred million years.
Galaxy merger as trigger
“This is the first time that we have observed a specific large-scale star-forming galaxy in the distant universe, which is extinguished due to the tremendous rejection of cold gas,” Puglisi says. The new observations also provide information about why this gas was removed. According to the researchers, the rate at which the gas is coming out of the galaxy is very fast due to the formation of black holes or strong stars alone. The strong excitation of this gas is also not compatible with these general explanations. Instead, Puglisi and his colleagues believe that another scenario is more likely: the rejection of this gas from the turbulence of a previous gas collision. Accordingly, galaxy ID2299 may be the result of the very first merger of the two preceding galaxies.
According to the researchers, one indication of this is the connection of the emitted gas with a “tidal tail” – a long stream of stars and gas that extends far into interstellar space. Such “tails” are rarely easy to see in distant galaxies and can easily be mistaken for foul winds. Due to this phenomenon occurring relatively recently in the galaxy ID2299, researchers were able to identify it as a tidal tail. “Thus our study provides strong evidence that gas extracted from ID2299 was probably ejected by tidal forces when the two gas-rich spiral galaxies merged,” says Puglisi. This suggests that such collisions can also change the evolution of the galaxy and prevent its formation.
“ALMA has shed new light on the mechanisms that can lead star formation to a standstill in distant galaxies,” says co-author Chiara Circosta of University College London. “The observation of such a serious disturbance process adds an important piece to the complex puzzle of galaxy evolution.”
They: Anagrazia Pugliaci (Durham University, UK) et al., Nature Astronomy; doi: 10.1038 / s41550-020-01268-x
