Comparison with that of our Galaxy

We observed the nearby spiral galaxy M31 with the Long-Wavelength Spectrometer on board the Infrared Space Observatory. Far-infrared line-to-continuum ratios of [C ii]/100 μm ' 6 × 10−3 were obtained for the central kiloparsec region along the major axis of the galaxy. These ratios are 2–3 times higher than those in the Galactic counterpart and are closer to those in the general Galactic plane. This indicates that the gas-to-dust heating ratio of the neutral interstellar medium in the center of M31 is not affected by the soft interstellar radiation field due to the late-type stars in the bulge. Comparing the observations with photon-dominated region models, we conclude that the molecular clouds in the central region of M31 are translucent (the mean hydrogen column density of each cloud is 〈NH〉 <∼ 1021 cm−2) for photons sufficiently energetic to heat the dust grains but not the gas. The smaller column density of the M31 clouds could result from a lower molecular-gas density suggested by previous millimeter observations. The lower recent star-forming activity in the inner part of M31 is likely to cause a lower pressure of the interstellar medium, and then the lower gas density.