Discovery of diffuse optical emission lines from the inner Galaxy: Evidence for LI(N)ER-like gas. (arXiv:2007.01872v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Krishnarao_D/0/1/0/all/0/1">Dhanesh Krishnarao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Benjamin_R/0/1/0/all/0/1">Robert A. Benjamin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Haffner_L/0/1/0/all/0/1">L. Matthew Haffner</a>
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
$overset{lower.5emcirc}{mathrm{A}}$/H$alpha$ $lambda$6563
$overset{lower.5emcirc}{mathrm{A}}$ increases from 0.3 to 2.5 with
Galactocentric radius; [OIII] $lambda$5007
$overset{lower.5emcirc}{mathrm{A}}$ and H$beta$ $lambda$4861
$overset{lower.5emcirc}{mathrm{A}}$ are also sometimes detected. The line
ratios for most Tilted Disk sightlines are characteristic of LI(N)ER galaxies.
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
$overset{lower.5emcirc}{mathrm{A}}$/H$alpha$ $lambda$6563
$overset{lower.5emcirc}{mathrm{A}}$ increases from 0.3 to 2.5 with
Galactocentric radius; [OIII] $lambda$5007
$overset{lower.5emcirc}{mathrm{A}}$ and H$beta$ $lambda$4861
$overset{lower.5emcirc}{mathrm{A}}$ are also sometimes detected. The line
ratios for most Tilted Disk sightlines are characteristic of LI(N)ER galaxies.
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