ASTRONOMY COLLOQUIUM - FEBRUARY 7, 2006

Ralf Bender, Max-Planck-Institut fuer Extraterrestrische Physik

The Triple Nucleus and Supermassive Black Hole of M31

I report HST STIS observations of the nucleus of M31. It is shown that the red double nucleus P1+P2 is well explained by Tremaine's eccentric disk
model. Inside of P2 resides a third, very blue nucleus, P3, which contains the supermassive black hole. The blue light of P3 is due to A-type stars,
presumably formed in starburst 200 Myr ago. Photometric and dynamical modeling reveals that P3 has the structure of a thin disk in Keplerian rotation. The disk has an exponential profile and a scale length of about 1 lightyear. It rotates with >1000 km/s at this radius, resulting in a black hole mass of ~1.4*10**8 Msun, considerably higher than previous measurements indicated. I discuss black hole alternatives and show that clusters of dark objects are excluded.