The Nature of the Warm/Hot Intergalactic Medium. I. Numerical Methods, Convergence, and O Vi Absorption

Abstract

We perform a series of cosmological simulations using Enzo, an Eulerian adaptive-mesh refinement, N-body + hydrodynamical code, applied to study the warm/hot intergalactic medium. The WHIM may be an important component of the baryons missing observationally at low redshift. We investigate the dependence of the global star formation rate and mass fraction in various baryonic phases on spatial resolution and methods of incorporating stellar feedback. Although both resolution and feedback significantly affect the total mass in the WHIM, all of our simulations find that the WHIM fraction peaks at z ∼ 0.5, declining to 35-40% at z = 0. We construct samples of synthetic O VI absorption lines from our highest-resolution simulations, using several models of oxygen ionization balance. Models that include both collisional ionization and photoionization provide excellent fits to the observed number density of absorbers per unit redshift over the full range of column densities (10 13 cm −2 N OVI 10 15 cm −2 ). Models that include only collisional ionization provide better fits for high column density absorbers (N OVI 10 14 cm −2 ). The distribution of O VI in density and temperature exhibits two populations: one at T ∼ 10 5.5 K (collisionally ionized, 55% of total O VI) and one at T ∼ 10 4.5 K (photoionized, 37%) with the remainder located in dense gas near galaxies. While not a perfect tracer of hot gas, O VI provides an important tool for a WHIM baryon census.

An HST/COS legacy survey of intervening Si III absorption in the extended gaseous halos of low-redshift galaxies

“…In conclusion, the AGN model from OWLS and the simulations from Smith et al (2011) predict absorber properties that are compatible with our observations and in line with the idea, that the majority of the Si iii absorbers at z ≤ 0.1 arise in the extended halos of low-redshift galaxies. For the future, we are planning to further explore the relation between intermediate-ion absorbers and galaxies at low z using higher-resolution simulations from the EAGLE (Evolution and Assembly of GaLaxies and their Environments) project (Schaye et al 2015).…”

“…The line statistics from synthetic absorption spectra generated from such simulations can be directly compared to QSO absorption-line studies to investigate the different gas phases in the IGM and CGM and their redshift evolution (Fang & Bryan 2001;Richter et al 2006;Fangano et al 2007;Tepper-García et al 2011, 2012, 2013Oppenheimer et al 2012;Smith et al 2011;Churchill et al 2015).…”

Bimodality of low-redshift circumgalactic O vi in non-equilibrium eagle zoom simulations

“…These observations could be confronted with cosmological hydrodynamical simulations that included metal enrichment from galactic superwinds to learn how the nucleosynthetic products of star formation enrich the diffuse CGM and IGM (e.g. Aguirre et al 2001;Schaye et al 2003;Springel & Hernquist 2003b;Oppenheimer & Davé 2006Tornatore et al 2010;Wiersma et al 2010;Cen & Chisari 2011;Smith et al 2011;Tepper-Garcia et al 2011;Tescari et al 2011;Oppenheimer et al 2012;Crain et al 2013;Rahmati et al 2016).…”

“…Alternatively, if O VI is primarily collisionally ionized at its 10 5.5 K peak (e.g. Heckman et al 2002;Cen & Chisari 2011;Smith et al 2011;Tepper-Garcia et al 2011;Faerman, Sternberg & McKee 2016), then it signals a hot halo component apparently associated with star-forming galaxies. Is this warm-hot component related to recent outflows or does it trace the temperature of a virialized, quasi-static halo?…”

Lensing and the Warm-hot Intergalactic Medium

“…Numerical simulations (Cen & Ostriker 1999;Davé et al 1999, Cen & Ostriker 2006, Smith et al 2011 indicated that only 10-20% of all baryons are in collapsed objects. Baryons in Intergalactic Medium (IGM) exist in a wide range of densities and temperatures.…”