Fast winds drive slow shells: A model for the circumgalactic medium as galactic wind-driven bubbles

Abstract

Successful models of the low-redshift circumgalactic medium (CGM) must account for (1) a large amount of gas, (2) relatively slow gas velocities, (3) a high degree of metal enrichment, (4) the similar absorption properties around both star-forming and passive galaxies, and (5) the observationally inferred temperature and densities of the CGM gas.We show that galactic wind-driven bubbles can account for these observed properties.We develop a model describing the motion of bubbles driven by a hot, fast galactic wind characteristic of supernova energy injection. The bubble size grows slowly to hundreds of kiloparsecs over 5-10 Gyr. For high star formation rates or high wind mass-loading M˙w/ M˙*, the free-flowing hot wind, the shocked hot wind in the interior of the bubble, and the swept-up halo gas within the bubble shell can all radiatively cool and contribute to low-ionization state metal line absorption.We verify that if the free-flowing wind cools, the shocked wind does as well. We find effective mass-loading factors of (Mw + Mswept)/M* ~ 5 - 12 as the bubbles sweep into the CGM. We predict cool gas masses, velocities, column densities, metal content, and absorption line velocities and linewidths of the bubble for a range of parameter choices. This picture can reproduce many of the COS-Halos and Keeney et al. (2017) observations of low-ionization state metal absorption lines around both star-forming and passive galaxies.