On the OVI abundance in the circumgalactic medium of low-redshift galaxies

Abstract

We analyze the mass, temperature, metal enrichment, and OVI abundance of the circumgalactic medium (CGM) around z ∼ 0.2 galaxies of mass 10 9 M < M < 10 11.5 M in the Illustris simulation. Among star-forming galaxies, the mass, temperature, and metallicity of the CGM increase with stellar mass, driving an increase in the OVI column density profile of ∼ 0.5 dex with each 0.5 dex increase in stellar mass. Observed OVI column density profiles exhibit a weaker mass dependence than predicted: the simulated OVI abundance profiles are consistent with those observed for starforming galaxies of mass M = 10 10.5−11.5 M , but underpredict the observed OVI abundances by 0.8 dex for lower-mass galaxies. We suggest that this discrepancy may be alleviated with additional heating of the abundant cool gas in low-mass halos, or with increased numerical resolution capturing turbulent/conductive mixing layers between CGM phases. Quenched galaxies of mass M = 10 10.5−11.5 M are found to have 0.3-0.8 dex lower OVI column density profiles than star-forming galaxies of the same mass, in qualitative agreement with the observed OVI abundance bimodality. This offset is driven by AGN feedback, which quenches galaxies by heating the CGM and ejecting significant amounts of gas from the halo. Finally, we find that the inclusion of the central galaxy's radiation field may enhance the photoionization of the CGM within ∼ 50 kpc, further increasing the predicted OVI abundance around star-forming galaxies.

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

“…In contrast to Suresh et al (2015b), who argue that the decline of O VI around passive versus star-forming galaxies of the same stellar mass owes primarily to AGN feedback removing mass from the haloes of passive galaxies, our exploration indicates that it is primarily an ionization effect that reduces O VI columns. Passive COS-Halos galaxies reside in higher mass haloes than their active counterparts, which have virial temperatures that are too high for O VI.…”

“…This model could invalidate our conclusion that O VI traces the virial temperature of L * haloes and not that of group haloes, because photo-ionization requires <10 5 K gas, although it could be argued that both cold gas and ionizing photons from star formation decline in group haloes. Suresh et al (2015b) show that O VI can be significantly enhanced by local sources of ionizing radiation, but only within 50 kpc using a 100 per cent escape fraction for soft X-rays from a SFR = 10 M yr −1 galaxy, which are unrealistic assumptions for COS-Halos.…”

“…Stinson et al 2012;Hummels et al 2013;Shen et al 2014;Gutcke et al 2016) have found an intimate link between galaxies and their metal-enriched CGM. Simulations that specifically compared with the COS-Halos survey find lower O VI column densities than observed around star-forming galaxies (Suresh et al 2015b;Ford et al 2016). The Ford et al (2016) simulations do not include active galactic nucleus (AGN) feedback and do not reproduce the O VI bimodality, while Suresh et al (2015b) argue that AGN feedback clears out the CGM around passive galaxies thus reducing O VI column densities around passive galaxies.…”

“…Kereš et al 2005;Dekel et al 2009;van de Voort et al 2012) feeding star formation, which may subside around passive galaxies. Enhanced photoionization from young stars could also ionize O VI around star-forming galaxies (Oppenheimer & Schaye 2013b;Vasiliev, Ryabova & Shchekinov 2015;Suresh et al 2015b). Oppenheimer et al (2012) demonstrated that increasing the ionization background strength at the O VI ionization potential can result in photoionized O VI tracing higher densities, even in circumgalactic halo gas.…”

Metal flows of the circumgalactic medium, and the metal budget in galactic haloes

“…The relation between the metallicity of the CGM and galaxy mass appears to be more complex in our simulations than in the simulations of Suresh et al (2017), who found that CGM has a roughly constant metallicity offset from the ISM and is well correlated with galaxy mass. The main difference appears to be the increasing offset between the CGM and ISM metallicity at late times in some of our L* galaxies.…”