A group of astronomers, led by Yale Postdoctoral fellow Kevin Schawinski, recently showed that luminous Active Galactic Nuclei (AGN) do not play a role in the suppression of star formation in their host galaxies. Instead, the group found that star formation ends long before the appearance of luminous AGN.
An AGN is a super-luminous region in the center of a host galaxy, created by a supermassive black hole accumulating mass and converting it to radiation (generally, the AGN and supermassive black hole are referred to interchangeably). A crucial ingredient in recent models of galaxy formation is the theory that luminous AGN coincide with and cause the suppression of star growth within their host galaxies. However, Schawinski says that his group’s observations show that this theory “doesn’t seem to quite work out in nature,” because the luminous AGN phase occurs about 100 million years after star formation shuts down.
Schawinski’s team used the Burst Alert Telescope on board the Swift satellite (Swift BAT) to detect all AGN in the local universe. They then analyzed images of these AGN taken from the Sloan Digital Sky Survey (SDSS) to estimate the average age of stars in the host galaxy.
This analysis revealed that all of the AGN host galaxies exist in the spectral “green valley,” with colors between blue (indicating younger stars) and red (indicating older stars). When these colors were translated into ages using star evolution models, it became evident that star formation in all AGN galaxies had been shut down for around 100 million years. Says Schawinski, “At no point do you have both star formation and an AGN that is in the process of shutting it down. By the time you see the AGN, star formation has already been shut down.”
So what is suppressing star formation in these galaxies? No one is really sure, but Schawinski proposes that perhaps lower luminosity AGN, not yet at the peak of their strength, are able to suppress star formation. Regardless, it is clear that theoretical models of galaxy formation need to be adjusted to agree with these new observations.