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Far-infrared [CII]/continuum ratio toward the central kiloparsecs of M31 *
Comparison with that of our Galaxy
Received 21 April 2000 / Accepted 19 September 2000
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 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 ) 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.
Key words: ISM: general Galaxy: center galaxies: individual: M31 galaxies: ISM infrared: ISM: lines and bands
* Based on observations with ISO, an ESA project with instruments funded by ESA member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.
Present address: Infrared Astrophysics Division, The Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
Online publication: December 5, 2000