H$\alpha$ Emission and the Dependence of the Circumgalactic Cool Gas Fraction on Halo Mass

TL;DR

This study empirically analyzes how the fraction of cool gas in galaxy halos depends on halo and stellar mass, revealing a decreasing trend with increasing mass and highlighting the dominant role of cool gas in low-mass halos.

Contribution

It provides new empirical relationships for the cool gas fraction in galaxy halos as functions of halo and stellar mass, based on a large SDSS dataset.

Findings

Cool gas fraction decreases with increasing halo mass.

Cool gas can constitute over 90% of baryons in low-mass halos.

In massive groups and clusters, cool gas makes up less than 5% of the baryonic content.

Abstract

We continue our empirical study of the emission line flux originating in the cool ($T\sim10^4$ K) gas that populates the halos of galaxies and their environments. Specifically, we present results obtained for a sample of nearly half a million individual galaxies, groups, and clusters of galaxies, intersected by more than two million SDSS lines of sight at projected separations of up to a quarter of the virial radius. Adopting simple power law relationships between the circumgalactic (CGM) cool gas fraction and either the halo or stellar mass, we present expressions for the CGM cool gas fraction as a function of either halo or stellar mass, $f_{\rm cool}(M_h) = (0.25^{+0.07}_{-0.06}) \times (M_h/10^{12}M_\odot)^{(-0.39^{+0.06}_{-0.07})}$ or $f_{\rm cool}(M_{*}) = (0.28^{+0.07}_{-0.06}) \times (M_{\rm *}/10^{10.0}M_\odot)^{(-0.33\pm 0.06)}$. Where we can compare, our results are consistent with previous constraints from absorption line studies, our own previous emission line work, and simulations. The cool gas can be the dominant baryonic CGM component, comprising a fraction as high as $> 90\%$ of halo gaseous baryons, in low mass halos, $M_h\sim$ $10^{10.5} M_\odot$, and a minor fraction, $<$ 5\%, in groups and clusters, $M_h > 10^{14} M_\odot$.