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.

arXiv comment6 Pages, 3 Figures, (+13 Page Supplementary Materials with 7 Figures and 2 Tables) Published in Science Advances on July 3rd with free open access to the online article available at https://advances.sciencemag.org/lookup/doi/10.1126/sciadv.aay9711 Explainer Video: http://astronomy.dk/images/TiltedDisk/PressExplainer_vCaptions.mp4 (Duration - 1:53)

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