Does Circumgalactic O VI Trace Low-pressure Gas Beyond the Accretion Shock? Clues from H I and Low-ion Absorption, Line Kinematics, and Dust Extinction
Abstract
Large O VI columns are observed around star-forming low-redshift ∼ {L}* galaxies, with a dependence on impact parameter indicating that most {{{O}}}5+ particles reside beyond half the halo virial radius (≳ 100 {kpc}). In order to constrain the nature of the gas traced by {{O}} {{vi}}, we analyze additional observables of the outer halo, namely {{H}} {{i}} to O VI column ratios of 1-10, an absence of low-ion absorption, a mean differential extinction of {E}B-V≈ {10}-3, and a linear relation between the O VI column and the O VI velocity width. We contrast these observations with two physical scenarios: (1) O VI traces high-pressure (∼ 30 {cm}}-3 {{K}}) collisionally ionized gas cooling from a virially shocked phase, and (2) O VI traces low-pressure (≲ 1 {cm}}-3 {{K}}) gas beyond the accretion shock, where the gas is in ionization and thermal equilibrium with the UV background. We demonstrate that the high-pressure scenario requires multiple gas phases to explain the observations and a large deposition of energy at ≳ 100 {kpc} to offset the energy radiated by the cooling gas. In contrast, the low-pressure scenario can explain all considered observations with a single gas phase in thermal equilibrium, provided that the baryon overdensity is comparable to the dark-matter overdensity and that the gas is enriched to ≳ {Z}⊙ /3 with an ISM-like dust-to-metal ratio. The low-pressure scenario implies that O VI traces a cool flow with a mass flow rate of ∼ 5 {{{M}}}⊙ {yr}}-1, comparable to the star formation rate of the central galaxies. The O VI line widths are consistent with the velocity shear expected within this flow. The low-pressure scenario predicts a bimodality in absorption line ratios at ∼ 100 {kpc}, due to the pressure jump across the accretion shock.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2018
- DOI:
- 10.3847/1538-4357/aac884
- arXiv:
- arXiv:1803.05446
- Bibcode:
- 2018ApJ...865...91S
- Keywords:
-
- galaxies: halos;
- intergalactic medium;
- quasars: absorption lines;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 26 pages, 11 figures. Accepted to ApJ