An H I absorption survey of M31 and M33 - The mixture of warm and cool phases in the interstellar media of spiral galaxies

The VLA in A-array is used to survey the nearby spiral galaxies M31 and M33 in the 21-cm line in order to measure absorption by the H I toward background continuum sources. Spectral line cubes with rms noise less than 1.2 mJy are obtained in spectral channels of 2.6 km/s. The warm neutral medium is found to be the dominant form of atomic hydrogen in M31 and M33. This phase is robust to variations in the interstellar environment represented by the differences among these galaxies. Compared with the Milky Way, M31 shows somewhat more cool atomic hydrogen (40 percent relative to 25 percent), whereas M33 shows less (15 percent). This suggests a trend with morphological type, with Sb galaxies exhibiting more cool H I relative to warm, and Sc galaxies, less, a trend similar to that seen by comparing molecular with atomic gas overall.