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Astron. Astrophys. 335, 807-822 (1998)

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Resolved 200µm images of nearby galaxies - evidence for an extended distribution of cold dust *

P.B. Alton 1, M. Trewhella 2, J.I. Davies 1, R. Evans 3, S. Bianchi 1, W. Gear 4, H. Thronson 5, E. Valentijn 6 and A. Witt 7

1 Department of Physics & Astronomy, University of Wales, P.O. Box 913, Cardiff CF2 3YB, UK
2 Infrared Processing and Analysis Center, Caltech, M.S. 100-22, Pasadena CA 91125, USA
3 University of Chicago Yerkes Observatory, 373, West Geneva St., Williams Bay, WI 53191, USA
4 Mullard Space Science Laboratory, University College London, Holmbury, Surrey RH5 6NT, UK
5 Code SR, NASA Headquarters, 300 E.Street SW, Washington DC 20546, USA
6 Kapteyn Institute, SRON, P.O. Box 800, 9700 Groningen, The Netherlands
7 Department of Physics and Astronomy, The University of Toledo, 2801 W.Bancroft, Toledo, OH 43606, USA

Received 13 February 1998 / Accepted 15 April 1998

Abstract

We present resolved 200µm images for 8 nearby galaxies observed with the Infrared Space Observatory (ISO). By comparing the [FORMULA]m observations with IRAS [FORMULA]m and [FORMULA]m data, we find that cold dust becomes more dominant at larger radii. We infer a grain temperature of 18-21 K for this cold component i.e. about 10 K lower than the warm dust detected by IRAS in external spirals. This value is close to theoretical predictions in the literature based on heating by the general interstellar radiation field. A comparison of the [FORMULA]m images with complementary B-band data also shows that the cold dust is radially more extensive than the stars. The gas-to-dust ratio of external spirals, derived using IRAS fluxes, has been claimed to be about an order of magnitude higher than the value infered for the Galaxy. By analysing the [FORMULA]m data for our sample, we derive a mean gas-to-dust ratio of [FORMULA] which is close to the value in the solar neighborhood (150-300). It is likely that IRAS may have `overlooked' the vast majority of grains residing in spiral disks.

Key words: ISM: dust, extinction – galaxies: ISM – infrared: galaxies

* 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.

Send offprint requests to: P.B. Alton, (paul.alton@astro.cf.ac.uk)

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998
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