Discovery of diffuse optical emission lines from the inner Galaxy: Evidence for LI(N)ER-like gas

Abstract

Optical emission lines are used to categorize galaxies into three groups according to their dominant central radiation source: active galactic nuclei, star formation, or low-ionization (nuclear) emission regions [LI(N)ERs] that may trace ionizing radiation from older stellar populations. Using the Wisconsin H-Alpha Mapper, we detect optical line emission in low-extinction windows within eight degrees of Galactic Center. The emission is associated with the 1.5-kiloparsec-radius "Tilted Disk" of neutral gas. We modify a model of this disk and find that the hydrogen gas observed is at least $48\%$ ionized. The ratio [NII] $\lambda$6584 $\overset{\lower.5em\circ}{\mathrm{A}}$/H$\alpha$ $\lambda$6563 $\overset{\lower.5em\circ}{\mathrm{A}}$ increases from 0.3 to 2.5 with Galactocentric radius; [OIII] $\lambda$5007 $\overset{\lower.5em\circ}{\mathrm{A}}$ and H$\beta$ $\lambda$4861 $\overset{\lower.5em\circ}{\mathrm{A}}$ are also sometimes detected. The line ratios for most Tilted Disk sightlines are characteristic of LI(N)ER galaxies.