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A Universal Density Structure for Circum-Galactic Gas | | Jonathan Stern
; Joseph F. Hennawi
; J. Xavier Prochaska
; Jessica K. Werk
; | | Date: |
7 Apr 2016 | | Abstract: | We develop a new method to constrain the physical conditions in the cool
(~10^4 K) circumgalactic medium (CGM) from measurements of ionic columns
densities, under two main assumptions: 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 (or ’stacks’) the
information available from different sightlines during the photoionization
modeling, thus yielding significantly tighter constraints on the 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 <
ho/
ho_mean < 5x10^5 (
ho_mean is the cosmic mean), while the physical size
of individual clouds scales as ~
ho^-1, from ~35 kpc of the low density OVI
clouds to ~6 pc of the highest density low-ion clouds. The cloud sizes are too
small for this density structure to be driven by self-gravity, thus its
physical source is unclear. We find a total cool CGM mass within the virial
radius of 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 are a benchmark for the
CGM structure in hydrodynamic simulations. Our results can be tested by
measuring the coherence scales of different ions, using absorption line
measurements along multiple sightlines towards lensed quasars. | | Source: | arXiv, 1604.2168 | | Services: | Forum | Review | PDF | Favorites | |
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