Title:

Star-Forming Accretion Flows in Giant Elliptical Galaxies
and the Galactic Center



Jonathan Tan
University of Florida



Abstract:

The luminosities of the centers of nearby elliptical galaxies are very low
compared to models of thin disc accretion to their black holes at the
Bondi rate, typically a few hundredths to a few tenths of a solar mass per
year. This has motivated models of inefficiently-radiated accretion that
invoke weak electron-ion thermal coupling, and/or inhibited accretion
rates due to convection or outflows. Here we suggest an alternative: most
of the accreting gas does not reach the central black hole because it
condenses into stars in a gravitationally unstable disk. The star
formation occurs inside the Bondi radius (typically ~100pc in giant
ellipticals), but still relatively far from the black hole in terms of
Schwarzschild radii. Star formation depletes and heats the gas disk,
eventually leading to a marginally stable, but much reduced, accretion
flow to the black hole. We predict the presence of cold, dusty gas disks,
containing clustered H-alpha emission and occasional type II supernovae,
both resulting from the presence of massive stars. The model accounts
for several features of the M87 system: a thin disc, traced by H-alpha
emission, is observed on scales of about 100pc, with features reminiscent
of spiral arms and dust lanes; the star formation rates inferred from the
intensity of H-alpha and infrared emission are consistent with the Bondi
accretion rate of the system. We present SMA observations of CO in M87, 
a preliminary analysis of which confirm some predictions of the model.
Finally we discuss some implications of this model for the fueling of the 
Galactic center.