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.