New Constraints on the Acceleration of AGN-driven Galactic Winds and on the Density Structure of the CGM

Jonathan Stern (Max Planck Institute for Astronomy)

April 21, 2016

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Abstract: A vital ingredient in our theoretical understanding of AGN-driven galactic winds is the nature of the physical mechanism which accelerates them, such as radiation pressure on dust grains or the ram pressure of an expanding hot gas bubble. I will demonstrate that the nature of the acceleration mechanism can be constrained from the ratios of emission lines which originate from the cool (T~10^4K) gas illuminated by the AGN. I will then show that observed line ratios in UV-selected quasars suggest that radiation pressure dominates at all scales (0.1 pc — 10 kpc). I will demonstrate that this result is apparently in conflict with the large momentum outflow rates measured by many previous studies, and discuss how this conflict might be resolved. In the second part of my talk I will argue for a new phenomenological model for the T~10^4K circum-galactic medium (CGM), where the cool CGM spans a large range of gas densities, and small high-density clouds are hierarchically embedded in large low- density clouds. I will then constrain the relation between gas density and physical size, or the “density structure”, using ionic column measurements from the COS-Halos survey of low-redshift ~L* galaxies. I will show that this density structure is inconsistent with self-gravity, thus its physical origin is unclear. I will also present a 3D model of the cool CGM based on our results, which can provide a benchmark for the CGM structure in hydrodynamic simulations.