Astronomers refer to galaxies that have ceased forming new stars as dead, even if their existing stars still produce plenty of light. Now the cause of a mass galactic die-off in the heart of the Virgo Cluster, the nearest really large concentration of galaxies to ourselves, has been explained.
It's been known since the 1970s that many galaxies in the Virgo Cluster are dead, as are others in densely packed environments, but the reasons were less obvious. In a paper due to be published in the Astronomical Journal Supplement (preprint on arXiv.org), a large international team has put the blame on superheated plasma choking the life out of the cluster's galaxies, or perhaps one might say, starving galaxies of food.
“We know that galaxies in the most extreme environments suffer tremendously, losing their gas reservoirs and eventually being unable to form any more stars. For a galaxy that's the equivalent of dying,” said co-author Dr Claudia Lagos of the International Centre for Radio Astronomy Research in a statement.
However, Dr Lagos told IFLScience, astronomers previously couldn't tell whether these galaxies were losing the reservoirs of cold gas they need to make stars entirely, or if the gas was still there but disturbed in some way that prevents star formation.
The paper confirms the galaxies have been stripped of their gas. “Gas stripping occurs when galaxies are moving so fast through hot plasma in the cluster, that vast quantities of cold molecular gas are stripped from the galaxy – as though the gas is being swept away by a huge cosmic broom,” said first author Dr Toby Brown of Canada's National Research Council.
Lagos explained to IFLScience galactic clusters lie on a cosmic web, and plasma a thousand times hotter than the Sun flows to concentrations of mass on the web like the Cluster.
In a sample of 51 galaxies the authors found some were already dead, but others may have 1-2 billion years of star formation to go. Nevertheless, even the live ones are losing their gas and will die young. Lagos says these galaxies have been captured by the Cluster; those that still have some life in them are probably the ones most recently caught. However, she acknowledged some may also have been particularly rich in cold gas initially, providing greater reserves to be stripped. “The good thing about having galaxies at different stages is we can use the observations to make an evolutionary timeline,” she told IFLScience.
The findings are a product of the Virgo Environments Traced in Carbon Monoxide (VERTICO) Survey, which has used the giant Atacama Large Millimeter/submillimeter Array to map the the relevant galaxies' carbon monoxide. Although it represents only a tiny proportion of the star-forming cold gas, CO is easier to observe than the more abundant hydrogen and is thought to serve as a reasonable tracer.
Previously, Lagos told IFLScience, astronomers were in a situation analogous to trying to explain rainfall anomalies. “We had the rain data and could see clear air, but now we can map clouds.”