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