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

TL;DR

This study reports the discovery of a widespread, faint, and broad OH emission indicating a thick, diffuse molecular gas disk in the outer Galaxy, challenging previous notions of molecular gas distribution.

Contribution

It provides the first observational evidence for a thick, diffuse molecular gas component in the outer Galaxy, detected via OH emission, not seen in traditional CO surveys.

Findings

Detection of broad OH emission across large Galactic scales

Molecular gas extends much further in the outer Galaxy than previously known

Presence of a thick, diffuse molecular disk in the outer Galaxy

Abstract

We present the serendipitous discovery of an extremely broad ($\Delta V_{LSR} \sim 150$ km/s), faint ($T_{mb} < 10 \textrm{mK}$), 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.