Astronomers have long wondered how galaxies die and by what means. Now a team of researchers from the University of Cambridge and the Royal Observatory Edinburgh have found that the primary cause of galactic death is by the cut off of material needed to make new stars – a process known as strangulation.
Galaxies are roughly divided into two categories; those that are ‘alive’ and producing stars, and the other half which are ‘dead’ and star formation no longer occurs. How galaxies change from being an active, alive galaxy to one that has become passive and gas-poor has been the focus of many studies and two leading theories are often put forward to explain this shift in circumstances; one explanation is via a sudden ejection of gas whereby either internal or external forces ‘blow’ the gas out of the galaxy. The other cause is by strangulation. In this situation the supply of cold gas to the galaxy is halted, thus ‘strangling’ the galaxy of the material needed to produce new stars over a prolonged period of time.
To determine which of these processes could be the leading cause of galactic death, the team, headed by Dr Yingjie Peng of Cambridge’s Cavendish Laboratory and Kavli Institute of Cosmology, analysed metal levels from the Sloan Digital Sky Survey for more than 26,000 average-sized local galaxies – metals are defined in astronomy as anything heavier than helium and they are also a powerful tracer of the history of star formation. “The more stars that are formed by a galaxy, the more metal content you’ll see. So looking at levels of metals in dead galaxies should be able to tell us how they died.” explains Peng.
So just how do the levels stack up? If the cold gas is suddenly expelled from the galaxy, star formation would abruptly stop and the metal content of the dead galaxy should be the same as just before it died. Conversely, in a galaxy that has undergone death by strangulation, stars will continue to form until the existing material gets completely used up. In this scenario, the metal content of the galaxy would continue rising and eventually stop when star formation ceases.
But, why would the inflow of gas into a galaxy suddenly stop? “One mechanism could be the so-called environment effect, which means when a star-forming galaxy is falling into a hot massive halo, it halts the inflow of gas into the galaxy and leads to its death. Other mechanisms including various preventive feedback mechanisms, such as the energetic feedback from the supermassive black holes (in the form of radiation, winds, jets) that prevents cooling inflows,” explains Peng. Hot halos are large reservoirs of hot gas surrounding a galaxy. These galaxies are generally found in groups or clusters and it is not clear whether halos are associated with the galaxy itself or the intra-cluster medium (the gas that exists between galaxies) of the group or cluster to which the galaxy belongs to.
Feedback, such as active galactic nuclei (AGN) feedback is thought to be important in shaping galaxy evolution and many modern simulations of galaxy formation invoke energetic AGN feedback to explain the observed properties of massive galaxies. “AGN feedback can drive strong outflows and regulate the galaxy evolution, but it’s still in debate and not clear whether such feedback will directly shut down the star formation in a galaxy or not,” said Peng. The galaxies studied by Peng and her colleagues however were all average-sized. Does size matter then when it comes to determining if a galaxy dies by strangulation or not? “For more massive galaxies (with a stellar mass greater than 10^11 solar masses), the stellar metallicity, which is used as an indicator in our work to discriminate between different quenching mechanisms, loses its discriminating power (due to the small size of the gas content of these massive galaxies). Therefore we do not know whether strangulation works for the most massive galaxies or not.”
Accordingly Yeng and her colleagues point out that their next step of investigation is to test this theory on galaxies that do not fit the criteria of being moderate in size, “we need deeper observations in the future to explore if the strangulation works the same way for galaxies with lower masses as well as massive galaxies too.” For now though, it would seem that galaxies that are deemed as average are doomed to deteriorate through strangulation.