As galaxies reach a critically monstrous size of 10 billion times the mass of the Sun, their stormy centres, powered by supermassive black holes, or Active Galactic Nuclei, take over from supernovae as the main mechanism by which the gas that fuels star formation is dispersed, say scientists from Oxford University and the University of Hertforshire presenting their work at the 212th American Astronomical Society Meeting this week. 

“Our models of galaxy formation are all based on the notion that Active Galactic Nuclei (AGN) are involved in 'snuffing out' – quenching – star formation in galaxies which are too large for mechanisms based on supernovae to explain,” says Dr Sugata Kaviraj, who led the research. " Astronomers believe that the jets produced by AGN are powerful enough to 'blow away' star-forming gas from even the largest galaxies, but up until now we have not had solid observational evidence to back this up.” 

The galaxies studied where, by cosmological standards, nearby, at distances of 1.5 billion light-years or less.  The recent star formation episodes in these objects are driven by interactions wth other galaxies.  Image: Sloan Digital Sky Survey. 

Using a novel combination of ultraviolet data from the Galaxy
Evolution Explorer and optical data from the Sloan Digital Sky Survey, the astronomers studied a rare class of 'post-starburst' galaxies, which lack ongoing star formation but still formed a substantial fraction of their stellar mass in the very recent past, a sign that a vigorous period of recent star formation has been rapidly suppressed. 

By measuring the efficiency with which quenching takes place in individual galaxies the astrophysicists showed that in the AGN regime the quenching efficiency is greatest for large galaxies, whereas it is highest for low mass galaxies in the supernovae regime.  They also confirmed that AGN become significantly more abundant in galaxies with masses roughly above 10 billion times the mass of the Sun. “ Our results demonstrate that the expected dichotomy in the relationship between quenching efficiency and galaxy mass is indeed borne out by the data, exactly across the expected threshold mass - 10 billion times the mass of the Sun!"  says Kaviraj. 

The relationship between quenching efficency and galaxy mass in post-starburst galaxies.  The colour coding indicates the average age of the recently formed stars in each galaxy - bluer galaxies have typically formed a larger mass fraction in the recent star formation episode than their redder counterparts.  The top half of the image shows the trend of increasing quenching efficiency with increasing mass for AGNs, the lower half shows the trend for supernovae explosions.  Image: Oxford University. 

Quantifying the role that AGN play in quenching star formation is of prime importance to astrophysicists as it would enable them to calibrate their models of galaxy formation.  The observations used in this study were of nearby galaxies, so the challenge now is to widen the scope of the work to include galaxies that are much further away, dating back to the peak epoch of star formation some 10 billion years ago, when the Universe was only 25% of its current age. 

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