Tracing Molecular Gas in the Andromeda Galaxy

We present results from an Andromeda Galaxy (M31) survey of star-forming regions based on 24 μm luminosity for H2O masers, NH3(1,1) and NH3(2,2) lines, and Hydrogen recombination lines (H66α). Although five H2O masers were detected in the initial survey of 206 regions towards M31, we do not detect additional masers in a follow up survey of 300 similar luminous 24 μm regions. We do not detect NH3(1,1), NH3(2,2), or H66α lines in any of the 506 regions. The typical rms noise for 244 kHz channels in individual spectra is 2.5 mJy. Additionally, averaging and medianing 446 NH3(1,1), NH3(2,2), and H66α spectra and 441 H2O spectra, shifted to the correct radial velocity as indicated by CO and HI, yields no detection for any observed line. The typical rms noise for 244 kHz channels in stacked spectra is 0.13 mJy. The non-detection of NH3(1,1) provides an upper limit on the typical NH3(1,1) integrated flux in a M31 giant molecular cloud. A comparison with typical Milky Way (MW) NH3(1,1) integrated fluxes scaled to the distance of M31 shows an ambiguity in consistency between our upper limit on the M31 NH3 content and the typical MW NH3 content depending on the use of the near or far distance in our MW flux scaling method. To resolve this ambiguity, we also examine the relationship between NH3(1,1) integrated flux and Herschel 500 μm flux density (i.e., the NH3 abundance) for giant molecular cloud-sized regions in M31 and the MW. Comparing this relationship between M31 and the MW independently indicates that our upper limit on the typical M31 NH3 abundance is consistent with the typical MW NH3 abundance. The non-detection of H2O provides an upper limit on H2O maser peak flux density. Comparing this upper limit with typical MW H2O maser peak flux densities scaled to the distance of M31 indicates that our upper limit on the M31 H2O maser luminosity is consistent with the typical MW H2O maser luminosity. Subject headings: dark matter galaxies: individual (M31) galaxies: kinematics and dynamics ISM: clouds ISM: molecules Local Group masers proper motions radio lines: galaxies