Plant roots : New challenges in a changing world

Peer reviewe

Liberation of specific angular momentum through radiation and scattering in relativistic black-hole accretion discs

A key component of explaining the array of galaxies observed in the Universe is the feedback of active galactic nuclei, each powered by a massive black hole's accretion disc. For accretion to occur, angular momentum must be lost by that which is accreted. Electromagnetic radiation must offer some respite in this regard, the contribution for which is quantified in this paper, using solely general relativity, under the thin-disc regime. Herein, I calculate extremised situations where photons are entirely responsible for energy removal in the disc and then extend and relate this to the standard relativistic accretion disc outlined by Novikov & Thorne, which includes internal angular-momentum transport. While there is potential for the contribution of angular-momentum removal from photons to be >~1% out to ~10^4 Schwarzschild radii if the disc is irradiated and maximally liberated of angular momentum through inverse Compton scattering, it is more likely of order 10^2 Schwarzschild radii if thermal emission from the disc itself is stronger. The effect of radiation/scattering is stronger near the horizons of fast-spinning black holes, but, ultimately, other mechanisms must drive angular-momentum liberation/transport in accretion discs.Comment: 6 pages, 2 figures. Accepted for publication in PAS

Explaining Low Redshift Quasar Evolution

We have developed a flexible framework for constructing physical models of quasar evolution that can incorporate a wide variety of observational constraints, such as multi-wavelength quasar luminosity functions (QLFs), estimated masses and accretion rates of active black holes, space densities of quasar host galaxies, clustering measurements, and the mass function of black holes in the local universe. In this brief contribution we focus on the observed decline in the QLF break luminosity at $z<2$, which can be explained either by a shift toward lower characteristic accretion rates at low $z$ or by preferential suppression of activity in higher mass black holes.Comment: 2 pages, 1 figure, to be published in the Proceedings of "Multiwavelength AGN Surveys", Cozumel, Dec 8 - 12, 200