2016ApJ...833..276Z

Using a sample of nearly half a million galaxies, intersected by over 7 million lines of sight from the Sloan Digital Sky Survey Data Release 12, we trace Hα + [N ii] emission from a galactocentric projected radius, r_p, of 5 kpc to more than 100 kpc. The emission flux surface brightness is \proptor_p^–1.9±0.4. We obtain consistent results using only the Hα or [N 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 3σ detection of Hα + [N ii] emission in the 50-100 kpc r_p bin. The mean emission flux within this bin is (1.10 ± 0.35) x 10^–20 erg cm^–2 s^–1 A^–1, which corresponds to 1.87 x 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_H, leads 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.