The Structure of Dark Molecular Gas in the Galaxy. II. Physical State of "CO-dark" Gas in the Perseus Arm

We report the results from a new, highly sensitive (Delta T-mb similar to 3 mK) survey for thermal OH emission at 1665 and 1667 MHz over a dense, 9 x 9 pixel grid covering a 1 degrees x 1 degrees patch of sky in the direction of l = 105 degrees.00, b = +2 degrees.50 toward the Perseus spiral arm of our Galaxy. We compare our Green Bank Telescope 1667 MHz OH results with archival (CO)-C-12(1-0) observations from the Five College Radio Astronomy Observatory Outer Galaxy Survey within the velocity range of the Perseus Arm at these galactic coordinates. Out of the 81 statistically independent pointings in our survey area, 86% show detectable OH emission at 1667 MHz, and 19% of them show detectable CO emission. We explore the possible physical conditions of the observed features using a set of diffuse molecular cloud models. In the context of these models, both OH and CO disappear at current sensitivity limits below an A(v) of 0.2, but the CO emission does not appear until the volume density exceeds 100-200 cm(-3). These results demonstrate that a combination of low column density A(v) and low volume density n(H) can explain the lack of CO emission along sight lines exhibiting OH emission. The 18 cm OH main lines, with their low critical density of n* similar to 1 cm(-3), are collisionally excited over a large fraction of the quiescent galactic environment and, for observations of sufficient sensitivity, provide an optically thin radio tracer for diffuse H-2.