Hydrogen and Metal Line Absorption Around Low-Redshift Galaxies in Cosmological Hydrodynamic Simulations

Amanda Brady Ford (1), Benjamin D. Oppenheimer (2), Romeel Davé (1), Neal Katz (3),
Juna A. Kollmeier (4), David H. Weinberg (5) ((1) Astronomy Department, University of Arizona, Tucson, AZ 85721, USA; (2) Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, Netherlands; (3) Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA; (4) Observatories of the Carnegie Institution of Washington, Pasadena, CA 91101, USA; (5) Astronomy Department and CCAPP, Ohio State University, Columbus, OH 43210, USA)

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 \leq 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.

Complete preprint ==> http://arxiv.org/abs/1206.1859

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