What sets the gas density in AGN hosts?

The pressure from the radiation of active galactic nuclei (AGN) can

exceed the typical gas pressure in the interstellar medium by many

orders of magnitude. Therefore, the radiation pressure will either

accelerate or compress the exposed gas. We address the compression

scenario, and find the analytical / numerical hydrostatic solution

for the compressed illuminated gas. This solution exhibits two

distinct properties:

1. The gas density of an illuminated gas cloud scales as the

distance of the cloud from the nucleus to the power of -2. We show

that this density vs. distance relation is observed over a dynamical

range of ~10^4 in distance and ~10^8 in gas density. Thus, the AGN

radiation pressure sets the density of the illuminated gas throughout

the host galaxy. This relation is potentially a direct observation of

AGN feedback.

2. The hydrostatic solution implies a unique cloud structure,

independent of the ambient pressure. This solution includes a low

density highly ionized surface layer, and a higher density lower

ionization inner layer. We compare this slab structure with

available observations.