Abstract
We study the physical conditions of the circum-galactic medium (CGM) around
z=0.25 galaxies as traced by HI and metal line absorption, using cosmological
hydrodynamic simulations that include galactic outflows. Using lines of sight
(LOS) targeted at impact parameters out to 1 Mpc around galaxies with a range
of halo masses, we study the physical conditions and their variation with
impact parameter b and line-of-sight velocity in the CGM as traced by HI, MgII,
SiIV, CIV, OVI, and NeVIII absorbers. All ions show a strong excess of
absorption near galaxies compared to random LOS. The excess continues beyond 1
Mpc, reflecting the correlation of metal absorption with large-scale structure.
Absorption is particularly enhanced within ~ 300 km/s and 300 kpc of galaxies,
roughly delineating the CGM; this range contains the majority of global metal
absorption. The different behaviour of low ionisation potential species versus
high ionisation potential species can be understood as low ionisation potential
species tracing denser areas closer to galaxies, versus high ionisation
potential species tracing more diffusely distributed gas. Photo-ionisation is
the driver of this trend where lower ionisation potential species decline
rapidly with increasing b while OVI and even weak NeVIII show comparatively
flat radial dependencies. In addition, collisionally ionised OVI and strong
NeVIII trace hot CGM gas when present in higher mass halos at b 100 kpc.
Lower ionisation potential metals show little temperature dependence with b,
while OVI and especially NeVIII trace hotter gas when present at lower b.
Larger halo masses generally produce more absorption. These findings arise
using our favored outflow scalings as expected for momentum-driven winds; with
no winds, the CGM gas remains mostly unenriched, while outflows with constant
velocity and mass loading factor show subtle differences.
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