Hydrogen Emission from the Ionized Gaseous Halos of Low Redshift
  Galaxies
  Using a sample of nearly half million galaxies, intersected by over 7 million
lines of sight from the Sloan Digital Sky Survey Data Release 12, we trace
H$\alpha$ + [N{\small II}] emission from a galactocentric projected radius,
$r_p$, of 5 kpc to more than 100 kpc. The emission flux surface brightness is
$\propto r_p^{-1.9 \pm 0.4}$. We obtain consistent results using only the
H$\alpha$ or [N{\small II}] flux. We measure a stronger signal for the bluer
half of the target sample than for the redder half on small scales, $r_p <$ 20
kpc. We obtain a $3\sigma$ detection of H$\alpha$ + [N{\small II}] emission in
the 50 to 100 kpc $r_p$ bin. The mean emission flux within this bin is $(1.10
\pm 0.35) \times 10^{-20}$ erg cm$^{-2}$ s$^{-1}$ \AA$^{-1}$, which corresponds
to $1.87 \times 10^{-20}$ erg cm$^{-2}$ s$^{-1}$ arcsec$^{-2}$ or 0.0033
Rayleigh. This detection is 34 times fainter than a previous strict limit
obtained using deep narrow-band imaging. The faintness of the signal
demonstrates why it has been so difficult to trace recombination radiation out
to large radii around galaxies. This signal, combined with published estimates
of n$_{\rm H}$, lead us to estimate the temperature of the gas to be 12,000 K,
consistent with independent empirical estimates based on metal ion absorption
lines and expectations from numerical simulations.
