A Universal Density Structure for Circum-Galactic Gas

TL;DR

This paper introduces a universal density structure model for the cool circumgalactic medium, constraining its properties across multiple sightlines and providing a benchmark for simulations.

Contribution

A novel method combining sightline data to model a universal density structure of the cool CGM, improving constraints over traditional individual sightline modeling.

Findings

Reproduces observed ion columns in 44 galaxies with a single model

Gas densities range from 50 to 5x10^5 times the cosmic mean

Cloud sizes scale inversely with density, from 35 kpc to 6 pc

Abstract

We develop a new method to constrain the physical conditions in the cool (~10^4 K) circumgalactic medium (CGM) from measurements of ionic column densities, by assuming that the cool CGM spans a large range of gas densities and that small high-density clouds are hierarchically embedded in large low-density clouds. The new method combines the information available from different sightlines during the photoionization modeling, thus yielding tighter constraints on CGM properties compared to traditional methods which model each sightline individually. Applying this new technique to the COS-Halos survey of low-redshift ~L* galaxies, we find that we can reproduce all observed ion columns in all 44 galaxies in the sample, from the low-ions to OVI, with a single universal density structure for the cool CGM. The gas densities span the range 50 < \rho/\rho_mean < 5x10^5 (\rho_mean is the cosmic mean), while the physical size of individual clouds scales as ~\rho^-1, from ~35 kpc of the low density OVI clouds to ~6 pc of the highest density low-ion clouds. The deduced cloud sizes are too small for this density structure to be driven by self-gravity, thus its physical origin is unclear. The implied cool CGM mass within the virial radius is 1.3x10^10 M_sun (~1% of the halo mass), distributed rather uniformly over the four decades in density. The mean cool gas density profile scales as R^-1.0, where R is the distance from the galaxy center. We construct a 3D model of the cool CGM based on our results, which we argue provides a benchmark for the CGM structure in hydrodynamic simulations. Our results can be tested by measuring the coherence scales of different ions.