Supernova-driven Gas Accretion

The relatively constant rate of star formation in galaxies like the Milky Way implies the need for continuum accretion of fresh gas. I present evidence that this accretion comes from the cooling of the virial-temperature coronae, that are thought to surround galaxy disks. This cooling is triggered by the passage of high-metallicity gas clouds ejected from the disks by supernovae. The prediction of this model is compared to neutral hydrogen observations of some local galaxies and the Milky Way. The gas accretion rates required to reproduce the data are about 1 M_Sun/yr, remarkably close to the star formation rates in these galaxies.

Getting the Most out of a Black Hole

Recent advances in computer simulations of black hole accretion systems provide us with unprecedented insights into the physics of jet production in active galactic nuclei (AGN) and other black hole accretion systems. For the first time, time-dependent 3D general relativistic numerical simulations allow us to determine the maximum efficiency with which radiatively inefficient accretion onto black holes can produce energy in the form of relativistic jets and winds. I will present the dependence of this maximum efficiency on black hole spin and accretion disk thickness and discuss the implications for radio loud/quiet dichotomy of AGN and recent observations of high jet efficiency in a number of AGN.