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

## TL;DR

This study investigates how galaxy environment influences circumgalactic medium emission, revealing that proximity to massive galaxies reduces emission line flux, likely due to environmental effects like strangulation, with implications for galaxy evolution.

## Contribution

It provides the first observational evidence linking environment to CGM emission variations and introduces a simple model for environmental impact on CGM gas content.

## Key findings

- Lower Hα + [N II] flux near massive galaxies
- Significant environmental dependence after controlling for galaxy properties
- Approx. 80% of cool gas removed within 700 kpc of massive galaxies

## Abstract

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 H$\alpha$ + [N {\small II}] emission line flux from the CGM within 50 kpc of $\sim$ L$^*$ galaxies is lower both for galaxies that lie within a projected distance of $\sim$ 500 kpc from a massive ($M_*>10^{11} M_\odot$) 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.