On the Dark Molecular Gas as Revealed by 18CM OH Emission

We explore the usage of the 18cm hydroxyl OH lines as a novel tool for observing and characterizing molecular gas on Galactic (kpc) scales. We report the results from a highly sensitive ($\Delta T_{\rm mb} \sim 3 $mK) survey for thermal OH emission at 1665 and 1667 MHz over a dense, 9 x 9-pixel grid covering a $1^{\circ} \times 1^{\circ} $ patch of sky in the direction the Perseus spiral arm of our Galaxy. We explore the possible physical conditions of the observed features using a set of diffuse molecular cloud models. Both OH and CO emission disappear at current sensitivity limits below an A$_{\rm v}$ of 0.2 mag, 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$_{\rm v}$) and low volume density ($n_{\rm H}$) can explain the lack of CO emission along sight lines exhibiting OH emission. We then present the serendipitous discovery of an extremely broad ($\Delta V_{\rm LSR} \sim 150$ km s$^{-1}$), faint ($T_{\rm mb} 8 kpc) with the Green Bank Telescope. The longitude-velocity distribution of the emission unambiguously suggests large-scale Galactic structure. Our results imply the existence of a thick ($-200< z < 200$ pc) disk of diffuse ($n_{\rm H_{2}}$ $\sim$ 5 $\times$ 10$^{-3}$ cm$^{-3}$) molecular gas in the Outer Galaxy previously undetected in all-sky CO surveys. Finally, we present the first extragalactic detection of thermal (5:9 LTE ratio), ground-state 18cm (1667 and 1665 MHz) OH emission from the Southern galactic disk of M31. The velocity distribution of the OH emission is found to be similar to the HI distribution. At face value, these results may indicate the ubiquity of abundant `CO-dark` molecular gas at large galactic radii in spiral galaxies. We conclude this thesis with a technical overview of the field of radio observations of 18cm OH as a molecular gas tracer in the future and review upcoming telescopes which may enable large surveys of faint OH emission in the future