2000A&A...363.1123M

We observed the nearby spiral galaxy M 31 with the Long-Wavelength Spectrometer on board the Infrared Space Observatory. Far-infrared line-to-continuum ratios of [C II]/100µm≃6x10^–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 M 31 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 M 31 are translucent (the mean hydrogen column density of each cloud is <N_H>≲10²¹cm^–2) for photons sufficiently energetic to heat the dust grains but not the gas. The smaller column density of the M 31 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 M 31 is likely to cause a lower pressure of the interstellar medium, and then the lower gas density.