The great galactic 'snuff out'

What makes galaxies stop producing stars? Contrary to what you might think galaxies don't just run out of star-forming gas, there has to be something that's dispersing the gas or there'd be many more stars in the sky.

Scientists believe two mechanisms play a role in 'quenching' star formation: exploding supernovae and Active Galactic Nuclei (AGNs) - the stormy centres of galaxies powered by supermassive black holes.

At a recent AAS meeting, Sugata Kaviraj of Oxford's Department of Physics presented the first observations showing the role of AGNs. What these observations show is that AGNs take over from exploding supernovae as the main mechanism by which gas is dispersed as galaxies reach the critical size of 10 billion times the mass of the Sun.

'Our models of galaxies are all based on the notion that Active Galactic Nuclei are involved in ‘snuffing out’ – quenching – star formation in galaxies which are too large for mechanisms based on supernovae to explain,' Sugata tells us.

'Astronomers believe that the jets produced by Active Galactic Nuclei are powerful enough to ‘blow away’ star-forming gas from even the largest galaxies but up until now we have not had the observations to back this up.'

'Our observations using ultraviolet light show, for the first time, the relationship between the mass of a galaxy and whether supernovae or Active Galactic Nuclei play a dominant role in quenching star formation.'

Quantifying the role that AGNs play in quenching is of prime importance to astronomers and astrophysicists as it would enable them to calibrate their models of galaxies.

The observations used in the study were of nearby galaxies, the challenge now is to widen the scope of the work to include a representative sample of galaxies.