Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

Cold dust in NGC 205

Martin Haas

Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

Received 07th April 1998

1. Introduction

NGC 205, one of the companions of M31, is classified as peculiar dE5. It shows a dozen bright early type stars and dark clouds, hence signatures of recent star formation (Hodge 1973). In H I it appears extended to about 3 arcmin (Young and Lo 1997). NGC 205 was also detected at 12-100 µm by IRAS with the spectral energy distribution rising towards longer wavelengths (Rice et al. 1988) indicating the presence of cool dust at a temperature around 25 K. Yet not solved is the question whether the dust emission is similarly extended as the H I gas or whether it is concentrated on the nucleus. NGC 205 is also the first among all ellipticals which was detected at 1.1 mm continuum (Fich and Hodge 1991).

Here new ISO observations between 120 and 200 µm are presented addressing the extension of the dust emission and the spectral energy distribution in the far infrared.

2. Results

The ISO mapping photometry covered a field of 13.5' × 13.5' with a sampling of 1.5'. At this spatial resolution no fine details could be resolved, nevertheless Fig. 1 shows the cuts through the ISO maps along north-south direction. These cuts clearly reveal that the dust emission originates from an extended area. It is elongated in N-S direction with a size of about 3' × 2'. It coincides with the bright emission regions seen on H I maps.

[Click here to see Fig. 1!]

Fig. 2 shows the spectral energy distribution. The ISO data reveal the maximum of the SED around 120-150 µm and the decline longward thereof. Fits with a modified blackbody (emissivity proportional to λ-2) yield refined dust parameters: a temperature of 21 K, a luminosity about 3.2·106 Lsun and mass about 104 Msun. For comparison Fich and Hodge predicted a temperature between 19 and 26 K and give a lower limit for the dust mass of about 3000 Msun.

[Click here to see Fig. 2!]

Despite this apparent broad agreement the new data suggest either a flatter emissivity or the existence of much more very cold dust at temperatures below 10 K: While the IRAS and ISO fluxes include the full 3' × 2' extended emission, the mm flux refers only to the central 18" and thus underestimates the total mm flux including that of the extended dust regions. In the case of a λ-2 emissivity a five times higher total mm flux (as indicated by the arrow in Fig. 2) would correspond to a 8 K dust component with a mass of more than 105 Msun. With such a dust mass a small galaxy like NGC 205 would become optically thick.

The presence of considerable amounts of cold dust in an elliptical galaxy is quite unusual and in the case of NGC 205 this could be caused by the dust rich neighbouring spiral M31.

References

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First version: 16thAugust,1998
Last update: 27thSeptember,1998
Jochen M. Braun   &   Tom Richtler
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