In the cosmos there are many large galaxies in which star formation has almost completely died out – they are, so to speak, dead. The reasons for this, however, were unclear. Now astronomers have discovered a galaxy that provides new insights. Because it ejected almost half of its gas supply in the course of a galaxy collision. The researchers were able to observe this enormous gas tail with the Atacama Large Millimeter / submillimeter Array (ALMA). The loss of gas leads, according to her, to the fact that star formation in this galaxy will be extinguished for a long time in a few tens of millions of years.
In our Milky Way and in most other galaxies, new stars are constantly being formed. They are created by the collapse of dense gas clouds and allow the galaxy to continue to grow. According to observations, the star formation rate in the still young universe was often much higher than in today’s, often “older” galaxies. But there are also galaxies in which star formation paused for a long time or even stopped completely in the course of cosmic evolution. In our closer cosmic environment there are therefore elliptical, old galaxies that are largely “dead” – in them as good as no new stars arise. But the reasons for this stop are still unclear: “We do not yet know exactly what processes are behind the deactivation of star formation in massive galaxies,” explains first author Annagrazia Puglisi from Durham University in the UK.
Almost half of the gas is ejected
It seems clear that the star formation in these “dead” galaxies has probably died out due to a lack of gas supplies. According to this, there must have been processes that ejected some of the gas that is normally abundant in such galaxies. “Winds of strong star formation or active black holes have so far been considered to be the main culprits for such gas loss and for the suppression of growth in massive galaxies,” says Puglisi. So far, however, there has been a lack of observations that could catch these processes and their effects in the act, so to speak. But now Puglisi and her team have accidentally discovered a galaxy that has just lost a normal amount of gas. They had used the Atacama Large Millimeter / submillimeter Array (ALMA) in Chile to study the properties of cold gas in more than 100 galaxies far away.
In doing so, they discovered a galaxy around nine billion light years away that emits a huge stream of gas. The data from the observatory shows that the gas ejected from the massive galaxy ID2299 accounts for around 46 percent of its entire supply of cold gas. This emission occurs at the high rate of around 10,000 solar masses per year, as the astronomers report. The star formation rate in this galaxy is still almost five times higher than that of its “conspecifics” from this cosmic epoch. But the researchers assume that this will not last much longer. According to their estimates, the remaining gas supply could be used up in tens of millions of years. Then star formation will come to a standstill – probably for several hundred million years.
Galaxy merging as a trigger
“This is the first time that we have observed a typical massive star-forming galaxy in the distant universe that is about to extinguish due to a tremendous ejection of cold gas,” says Puglisi. The new observations also provide valuable information on what caused this gas to be ejected. According to the researchers, the rate at which gas flows out of the galaxy is too fast to be caused by a black hole or the winds of intense star formation alone. The strong excitation of this gas is also not compatible with these common explanations. Instead, Puglisi and her colleagues believe another scenario is more likely: the ejection of this gas by the turbulence of a past galaxy collision. According to this, the galaxy ID2299 could be the result of a not long ago merger of two predecessor galaxies.
According to the researchers, an indication of this is the connection between the ejected gas and a “tidal tail – an elongated stream of stars and gas that extends far into interstellar space. Such “tails” are seldom easy to see in distant galaxies and can easily be mistaken for galactic winds. Because this phenomenon was relatively recent in the galaxy ID2299, the researchers were able to identify it as a tidal tail. “Our study thus provides convincing evidence that the gas ejected from ID2299 was probably ejected by tidal forces when two gas-rich spiral galaxies merged,” says Puglisi. This suggests that such collisions can also change the evolution of a galaxy and stop its star formation.
“ALMA has shed new light on the mechanisms that can bring star formation to a standstill in distant galaxies,” adds co-author Chiara Circosta from University College London. “The observation of such a serious disturbance process adds an important piece to the complex puzzle of galaxy evolution.”
Source: Annagrazia Puglisi (Durham University, UK) et al., Nature Astronomy, doi: 10.1038 / s41550-020-01268-x