Tuesday 25 February 2014
to 14:15 at
Andrew King (University of Leicester)
Astronomers can detect black holes through the high-energy emission (often X-rays) that results as gas
falls into them. The total energy in the cosmic X-ray background tells us that on average the centres of all but the smallest galaxies should contain black holes of 10 - 100 million solar masses or more. There is direct evidence of this from the motions of stars in the centre of the Milky Way, and astronomers can measure similar black hole masses in many galaxies. Despite their large masses, these central black holes directly gravitationally control only a relatively tiny central region of their host galaxies. Yet properties of these hosts on very large scales correlate tightly with the masses of their central black holes: the holes apparently have an overwhelming influence on the structure of the galaxies they inhabit. The key to understanding this apparent paradox is that in growing to their current masses black holes release more than enough energy to cause major disruption to their host galaxies. The observed relations between black hole mass and galaxy properties reveal that the holes initially gorge fairly quietly on their surroundings, but eventually halt their own growth by driving away their food.