Probing the galactic ISM in OH absorption

We examine the four 2π3/2, J = 3/2 ground state transitions of the hydroxyl radical (OH) along 15 sightlines through the Milky Way disk towards bright background continuum sources. We find that the OH gas along these sightlines is optically thin, consistent with the findings from the Southern Parkes Large Area Survey in Hydroxyl (SPLASH) pilot region]. We present a partially automated method for Gaussian decomposition of our spectra which identified 55 components across 28 OH-containing clouds. Three of these OH clouds had no associated 12CO emission (CO data from NANTEN).We test two methods of finding excitation temperature: a modified ‘on-off’ method, and a method where the excitation temperatures and column densities are solved for numerically .Neither method constrained Tex well. Column densities were calculated based on a range of possibleexcitation temperatures (Tex (1667) = 5-15 K), and conversions to N(H2) were made. N(OH) values were found to range from 1 - 40 x 1014cm-2, corresponding to N(H2) values ranging from1 - 40 x 1021 cm-2. We discuss our results in the context of CO-dark H2, and outline future directions for our work.