2021ApJ...914...72B

We present the serendipitous discovery of an extremely broad (ΔV_LSR ∼ 150 km s^–1), faint (T_mb < 10 mK), and ubiquitous 1667 and 1665 MHz ground-state thermal OH emission toward the second 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 H I 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 pc < z < 200 pc) disk of diffuse (n_H2_ ∼ 5 x 10^–3 cm^–3) molecular gas in the outer Galaxy previously undetected in all-sky ¹²CO surveys.