On the Effect of Environment on Line Emission from the Circumgalactic Medium

We measure differences in the emission-line flux from the circumgalactic medium (CGM) of galaxies in different environments. Such differences could be a critical clue in explaining a range of galaxy properties that depend on environment. Using large samples of stacked archival spectra from the Sloan Digital Sky Survey, we find that the Hα + [N ii] emission-line flux from the CGM within 50 kpc of ∼L∗ galaxies is lower both for galaxies that lie within a projected distance of ∼500 kpc from a massive (M _∗ > 10¹¹ M _o) galaxy and for galaxies in richer/denser environments. The environmental differences are statistically significant even after we control for galaxy mass and morphology. We interpret these observations as a direct signature of environmentally caused strangulation. We present a simple, heuristic model for the effect of a massive parent galaxy. In this model, the CGM cool gas fraction within 50 kpc is significantly decreased for galaxies that lie within 700 kpc of a massive galaxy, with about 80% of the cool gas removed even when the galaxy is at a distance of 500 kpc from its massive parent. However, we discuss alternative physical causes for the observed behavior and discuss ways forward in addressing open questions.