Observational Evidence for a Thick Disk of Dark Molecular Gas in the Outer Galaxy

Abstract

We present the serendipitous discovery of an extremely broad ($\Delta V_{LSR}\sim 150$ km/s), faint ($T_{mb}<10\text{mK}$), and ubiquitous 1667 and 1665 MHz ground-state thermal OH emission towards the 2nd quadrant of the outer Galaxy ($R_{gal}$ > 8 kpc) with the Green Bank Telescope. Originally discovered in 2015, we describe the redundant experimental, observational, and data quality tests of this result over the last five years. The longitude-velocity distribution of the emission unambiguously suggests large-scale Galactic structure. We observe a smooth distribution of OH in radial velocity that is morphologically similar to the HI radial velocity distribution in the outer Galaxy, showing that molecular gas is significantly more extended in the outer Galaxy than previously expected. Our results imply the existence of a thick ($-200<z<200$ pc) disk of diffuse ($n_{H_2}$ $\sim$ 5 $\times$ 10${}^{-3}$ cm${}^{-3}$) molecular gas in the Outer Galaxy previously undetected in all-sky CO surveys.

14 pages, 6 figures, accepted to ApJ

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• Dependence of Chemical Abundance on the Cosmic Ray Ionization Rate in IC 348
• Abundance ratios of OH/CO and HCO+/CO as probes of the cosmic ray ionization rate in diffuse clouds
• SPLASH: The Southern Parkes Large-Area Survey in Hydroxyl -- Data Description & Release
• Kinetic tomography of the Galactic plane within 1.25 kiloparsecs from the Sun. The interstellar flows revealed by HI and CO line emission and 3D dust
• The CO-dark molecular gas in the cold HI arc
• GNOMES II: Analysis of the Galactic diffuse molecular ISM in all four ground state hydroxyl transitions using Amoeba
• Anisotropy in the carbon monoxide (CO) line emission across the Milky Way's disk