Molecular Gas Velocity Dispersions in the Andromeda Galaxy
  In order to characterize the distribution of molecular gas in spiral
galaxies, we study the line profiles of CO:1-0 emission in Andromeda, our
nearest massive spiral galaxy. We compare observations performed with the IRAM
30m single-dish telescope and with the CARMA interferometer at a common
resolution of 23 arcsec ~ 85pc x 350pc and 2.5 km/s. When fitting a single
Gaussian component to individual spectra, the line profile of the single dish
data is a factor 1.5+-0.4 larger than the interferometric data one. This ratio
in line widths is surprisingly similar to the ratios previously observed in two
other nearby spirals, NGC 4736 and NGC 5055, but measured at ~0.5-1 kpc spatial
scale. In order to study the origin of the different line widths, we stack the
individual spectra in 5 bins of increasing peak intensity and fit two Gaussian
components to the stacked spectra. We find a unique narrow component of FWHM =
7.5+-0.4 km/s visible in both the single dish and the interferometric data. In
addition, a broad component with FWHM = 14.4+-1.5 km/s is present in the
single-dish data, but cannot be identified in the interferometric data. We
interpret this additional broad line width component detected by the single
dish as a low brightness molecular gas component that is extended on spatial
scales >0.5 kpc, and thus filtered out by the interferometer. We search for
evidence of line broadening by stellar feedback across a range of star
formation rates but find no such evidence on ~100 pc spatial scale when
characterizing the line profile by a single Gaussian component.
