Resolved 200  images of nearby galaxies - evidence for cold dust at large galactic radii

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

¹ Department of Physics & Astronomy, University of Wales, PO Box 913, Cardiff CF2 3YB, U.K.

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 Code SR, NASA Headquarters, 300 E.Street SW, Washington DC 20546, USA

⁵ Kapteyn Institute, SRON, PO Box 800, 9700 Groningen, The Netherlands

⁶ Dept of Physics and Astronomy, The University of Toledo, 2801 W.Bancroft, Toledo, OH 43606, USA

We present resolved 200m images for 8 nearby galaxies observed with the Infrared Space Observatory (ISO). By comparing the m observations with IRAS m and 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 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 m data for our sample, we derive a mean gas-to-dust ratio of 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.