Astron. Astrophys. 363, 1123-1133 (2000)

Far-infrared [CII]/continuum ratio toward the central kiloparsecs of M31 ^*

Comparison with that of our Galaxy

K. Mochizuki

Department of Astronomy, University of Tokyo, Japan (mochi@ir.isas.ac.jp)

Received 21 April 2000 / Accepted 19 September 2000

Abstract

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

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Results
  • 3.1. [CII] line emission
  • 3.2. 100 µm continuum emission
  • 3.3. [CII]/100 µm ratio
• 4. Discussion
  • 4.1. Factors affecting the [CII]/100 µm ratio
  • 4.2. Comparison with PDR models
  • 4.3. Molecular clouds in the M31 center
  • 4.4. Gas heating due to less energetic photons
  • 4.5. [CII] self-absorption
• 5. Conclusions
• Acknowledgements
• Appendices
  • A: gas cooling due to CO molecules
• References

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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