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