Astronomers have obtained unprecedented observational evidence of the role that supermassive black holes play in ending star formation in galaxies. The report was presented by Dr Sugata Kaviraj of Oxford University, UK, yesterday at the American Astronomical Society Meeting in St Louis, Missouri. This result is of special interest because it provides new insights into the role of black holes in the formation and evolution of massive galaxies.
A study by the Oxford scientists using ultraviolet light provides solid observational evidence that the stormy centers of galaxies powered by supermassive black holes — ‘Active Galactic Nuclei’ (AGN) — take over from exploding stars (supernovae) as the main mechanism by which the gas that fuels star formation is dispersed, as galaxies reach a critical size of 10 billion times the mass of the Sun. The results were recently published in the December 2007 issue of the Monthly Notices of the Royal Astronomical Society (Kaviraj et al., MNRAS, 2007, 382, 960).
“Our models of galaxy formation 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,” says Sugata Kaviraj, Leverhulme-Beecroft Fellow in Astrophysics at Oxford University, UK, who led the research.
“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 solid observational evidence to back this up. The jets produced by typical Active Galactic Nuclei would have enough energy to power ten billion stars like our Sun! Our study indicates, for the first time from a purely observational viewpoint, the relationship between the mass of a galaxy and whether supernovae or AGN play a dominant role in quenching star formation,” says Kaviraj.
Using a novel combination of ultraviolet data from the Galaxy Evolution Explorer (an orbiting space telescope launched by NASA in 2003) and optical data from the Sloan Digital Sky Survey (one of the largest observational surveys that uses a dedicated 2.5-meter telescope equipped with a 120 megapixel camera;), the scientists were able to measure the efficiency with which quenching takes place in individual galaxies with unprecedented accuracy.
“The work is very timely”, stresses Marc Sarzi, Research Fellow at the University of Hertfordshire and academic visitor at Oxford. “Post-starburst galaxies are extremely rare objects and it is only thanks to the large volume encompassed by the Sloan survey that we could harness a significant number of them.”
Kaviraj adds, “In the AGN regime, the quenching efficiency is expected to scale positively with galaxy mass, while in the supernova regime the opposite trend is expected. We also know 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!”
According to co-author Joe Silk, who is Savilian Professor of Astronomy at Oxford University and has pioneered the use of black holes in models of galaxy formation, “We simply do not understand the murky details of galaxy formation. Some form of ‘feedback’ is slowing down and quenching star formation in both nearby galaxies and the distant universe. These results point to the likely culprit being a combination of the effects of exploding stars and supermassive black holes, with black holes dominating in the massive galaxies, as envisaged in contemporary galaxy formation models”.
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. While the observations used in this study were of nearby galaxies, the challenge now is to confirm these preliminary results and widen the scope of the work to include a representative sample of galaxies: this would include those 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 percent of its current age!
Efforts, led by Kaviraj, are underway to combine several large observational surveys that trace the evolution of the universe over the last 10 billion years, to study the properties of post-starburst galaxy populations. The research is expected to yield significant insights into the quenching process and provide valuable constraints on the currently accepted galaxy formation paradigm.