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

TL;DR

This study detects diffuse optical emission lines near the Galactic Center, revealing that the gas there exhibits characteristics similar to LI(N)ER galaxies, indicating ionization from older stellar populations.

Contribution

The paper presents the first detection of LI(N)ER-like optical emission lines within the inner Galaxy, linking the Tilted Disk's ionization state to LI(N)ER phenomena.

Findings

At least 48% of the hydrogen gas is ionized.

Line ratios increase with Galactocentric radius, resembling LI(N)ERs.

Optical emission lines are associated with the Tilted Disk of neutral 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.