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Astron. Astrophys. 362, 138-150 (2000)

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Modelling the spectral energy distribution of galaxies

I. Radiation fields and grain heating in the edge-on spiral NGC 891

C.C. Popescu *  1, A. Misiriotis 2,3, N.D. Kylafis 2,4, R.J. Tuffs 1 and J. Fischera 1

1 Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
2 University of Crete, Physics Department, P.O. Box 2208, 710 03 Heraklion, Crete, Greece
3 Observatoire de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 4, France
4 Foundation for Research and Technology-Hellas, P.O. Box 1527, 711 10 Heraklion, Crete, Greece

Received 27 June 2000 / Accepted 26 July 2000


We describe a new tool for the analysis of the UV to the sub-millimeter (sub-mm) spectral energy distribution (SED) of spiral galaxies. We use a consistent treatment of grain heating and emission, solve the radiation transfer problem for a finite disk and bulge, and self-consistently calculate the stochastic heating of grains placed in the resulting radiation field.

We use this tool to analyse the well-studied nearby edge-on spiral galaxy NGC 891. First we investigate whether the old stellar population in NGC 891, along with a reasonable assumption about the young stellar population, can account for the heating of the dust and the observed far-infrared and sub-mm emission. The dust distribution is taken from the model of Xilouris et al. (1999), who used only optical and near-infrared observations to determine it. We have found that such a simple model cannot reproduce the SED of NGC 891, especially in the sub-mm range. It underestimates by a factor of 2-4 the observed sub-mm flux.

A number of possible explanations exist for the missing sub-mm flux. We investigate a few of them and demonstrate that one can reproduce the observed SED in the far-infrared and the sub-mm quite well, as well as the observed radial profile at [FORMULA]m.

For the models calculated we give the relative proportion of the dust radiation powered by the old and young stellar populations as a function of FIR/sub-mm wavelength. In all models we find that the dust is predominantly heated by the young stellar population.

Key words: ISM: dust, extinction – galaxies: individual: NGC 891 – galaxies: spiral – galaxies: stellar content – infrared: galaxies – submillimeter

* Research Associate at The Astronomical Institute of the Romanian Academy, Str. Cuitul de Argint 5, 75212, Bucharest, Romania

Send offprint requests to: C.C. Popescu (Cristina.Popescu@mpi-hd.mpg.de)

© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100