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

Year: 2018

Type: article

Abstract: Large O vi columns are observed around star-forming low-redshift galaxies, with a dependence on impact parameter indicating that most particles reside beyond half the halo virial radius ( ). In order to constrain the nature of the gas traced by , we analyze additional observables of the outer halo, namely to O vi column ratios of 1–10, an absence of low-ion absorption, a mean differential extinction of , 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 ( ) collisionally ionized gas cooling from a virially shocked phase, and (2) O vi traces low-pressure ( ) 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 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 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 , 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 , due to the pressure jump across the accretion shock.

Source: The Astrophysical Journal

DOI: 10.3847/1538-4357/aac884

Authors: Jonathan Stern, Claude-André Faucher-Giguère, Joseph F. Hennawi, Zachary Hafen, Sean D. Johnson, Drummond Fielding

Institutions: Northwestern University, Max Planck Institute for Astronomy, University of California, Santa Barbara, Carnegie Institution for Science, Princeton University, Carnegie Observatories, University of California, Berkeley

Language: English

Topic: Galaxies: Formation, Evolution, Phenomena

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: bronze

Funders: National Science Foundation, National Aeronautics and Space Administration, Research Corporation for Science Advancement, Alexander von Humboldt-Stiftung, Space Telescope Science Institute

OpenAlex ID: W2789271675