Supermassive black holes not guilty of shutting down

star formation

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 22 January, 2009

According to a team lead by Yale University astronomers, galaxies cease star formation long before their supermassive black holes have the power to do the job themselves.

Until recently, astronomers believed that active galactic nuclei
(AGN) - the supermassive and extremely energetic black holes at the centres of many young galaxies - were responsible for shutting down star formation in their host galaxies once they reached a critical size. The theory went that the AGN feed on the surrounding galactic material, producing enormous amounts of energy and heat such that gas and dust can no longer cool and condense into stars.

Artist's representation of an AGN at the centre of a galaxy. Gas is pulled towards a supermassive black hole, generating massive amounts of radiation. Image: NASA/CXC/M.Weiss.

New analysis concludes that the AGN have been wrongly accused. The team of astronomers demonstrate that the shutting-down process appears to take place much earlier in the AGN's lifetime than previously assumed, well before it starts shining brightly. "This high-luminosity phase, when the AGN are at their biggest and brightest and most powerful, is not the phase responsible for the shutdown of star formation," says Kevin Schawinski, lead author of the study that is featured in the 10 February issue of the Astrophysical Journal Letters.

The team of astronomers, which also included contributors from the University of Hawaii and the University of Oxford, reached their verdict after studying images of 177 galaxies with AGN. The sample included galaxies where the AGN were both obscured by the galaxy's dust and gas, and ones where there was an unobstructed view of the AGN, as seen from the Earth. This selection was important, since until now, many astronomers have believed that the AGN in any galaxies that are still actively forming stars will be obscured by the galaxy’s own gas and dust. Schawinski and his team, however, are the first to show that in fact there are no bright AGN at the centres of star-forming galaxies at all.

Furthermore, by subtracting out the light from the AGN, the team discovered that all of the galaxies with bright AGN had stopped forming stars several hundred million years earlier. "The key result is the finding that there is a lack of AGN in galaxies that are currently forming stars," says Meg Urry, head of the Yale team and director of the Yale Center for Astronomy and Astrophysics. "That tells us the AGN doesn't turn on until long after the stars stop forming."

This is the first study to show a significant delay between the end of star formation and the onset of a luminous AGN. But as for the real culprit that is shutting down stellar birth, the jury is still out. "It's possible that an earlier, low-luminosity phase is responsible," suggests Schawinski.

In any case, and whatever the cause, the analysis clearly shows that the shutting-down process is a lot more complicated than previously thought.