Emission line ratios for the Circumgalactic Medium and the "Bimodal" Nature of Galaxies

TL;DR

This study reveals a clear bimodal pattern in emission line ratios of circumgalactic gas, linked to galaxy stellar mass, indicating different ionization mechanisms and gas accretion behaviors for low and high mass galaxies.

Contribution

It demonstrates a mass-dependent bimodal nature of circumgalactic medium ionization states using SDSS spectroscopy, highlighting a new connection to galaxy evolution.

Findings

Lower mass galaxies have gas ionized by star formation.

Higher mass galaxies show gas ionized by AGN or shocks.

Distinct gas properties correlate with galaxy stellar mass threshold.

Abstract

We find significantly different diagnostic emission line ratios for the circumgalactic gas associated with galaxies of stellar masses above and below $10^{10.4}$ M$_\odot$ using SDSS spectroscopy. Specifically, in a sample of 17,393 galaxies, intersected by 18,535 lines of sight at projected radii between 10 and 50 kpc, we stack measured fluxes for nebular strong emission lines, [O {\small III}] $\lambda$5007, H$\alpha$ and [N {\small II}] $\lambda6583$, and find that the gas surrounding the lower mass galaxies exhibits similar line ratios to those of gas ionized by star formation and that surrounding the higher mass galaxies similar to those of gas ionized by AGN or shocks. This finding highlights yet another characteristic of galaxies that is distinctly different above and below this stellar mass threshold, but one that is more closely connected to the gas accretion behavior hypothesized to be responsible for this dichotomy.