Composition of Low Redshift Halo Gas
  Halo gas in low-z (z<0.5) >0.1L* galaxies in high-resolution, large-scale
cosmological hydrodynamic simulations is examined with respect to three
components: (cold, warm, hot) with temperatures equal to (<10^5, 10^{5-6},
>10^6)K, respectively. The warm component is compared, utilizing O VI
\lambda\lambda 1032, 1038 absorption lines, to observations and agreement is
found with respect to the galaxy-O VI line correlation, the ratio of O VI line
incidence rate in blue to red galaxies and the amount of O VI mass in
star-forming galaxies. A detailed account of the sources of warm halo gas
(stellar feedback heating, gravitational shock heating and accretion from the
intergalactic medium), inflowing and outflowing warm halo gas metallicity
disparities and their dependencies on galaxy types and environment is also
presented. Having the warm component securely anchored, our simulations make
the following additional predictions. First, cold gas is the primary component
in inner regions, with its mass comprising 50% of all gas within
galacto-centric radius r=(30,150)kpc in (red, blue) galaxies. Second, at
r>(30,200)kpc in (red, blue) galaxies the hot component becomes the majority.
Third, the warm component is a perpetual minority, with its contribution
peaking at ~30% at r=100-300kpc in blue galaxies and never exceeding 5% in red
galaxies. The significant amount of cold gas in low-z early-type galaxies found
in simulations, in agreement with recent observations (Thom et al.), is
intriguing, so is the dominance of hot gas at large radii in blue galaxies.
