The ISOGAL Survey

Alain Omont  and the ISOGAL Collaboration

Institut d'Astrophysique de Paris, 98b bd. Arago, 75014 Paris, France

ISOGAL is a 7-15  ISOCAM survey with 6 pixels and sensitivity below 10 mJy, of in the Galactic Plane mostly interior to |l| = 30 deg. In combination with KJI DENIS data, the ISO images allow detailed studies of cold stellar populations and galactic structures in regions highly obscured throughout the inner Galaxy. Data reduction is particularly difficult because of the high density of strong sources, of memory effects and of short integration times. However, an improved data reduction is almost complete for all the fields observed, with the use of CIA software and of a special source extraction. The data quality is thus acceptable, as concerns the reliability, completeness and photometric accuracy of the sources, to allow a systematic scientific analysis. The first results, derived from a complete analysis of only a few fields among 200 observed, include:

1. A complete census of mass-losing AGB stars in fields of the inner Bulge. Even very weak mass-loss are very well characterised, down to the RGB tip. Such stars are by far the most numerous there. They form a very well defined sequence in infrared colour-magnitude diagrams.

2. Their 5-16  infrared spectra, obtained in complementary CVF observations of a few ISOCAM fields, display silicate features with a large range of width.

3. The same AGB sequence is also quite visible in 7-15 colour-magnitude diagrams in the Galactic Disk, even with a very large extinction. Such AGB stars are thus the most numerous among the 100,000 sources detected by ISOGAL. However, foreground red giants and young stars are also quite numerous.

4. Preliminary trends of the variation of these different classes of stars with the galactic location have been derived. The density of young stars in the different fields is quite variable.

5. The combination of ISOGAL and DENIS data allows a detailed estimate of the interstellar extinction for individual stars and lines of sight.

6. Many dark globules and filaments are visible on the 7 and 15  images, corresponding to visible extinction larger than 20-30.

7. The very rich structure of the infrared diffuse emission is often clearly associated with stars. In some cases the latter have been identified as probably AGB stars.

8. A few ISOGAL sources have already been found in association with various peculiar objects such as H₂O and OH masers, open clusters, planetary nebulae, etc. It is expected that their number will rapidly increase with the progress of the detailed analysis of the ISOGAL fields and of follow-up studies.

The aim is to publish, within less than one year, a complete catalogue of ISOGAL sources cross-identified with DENIS sources, as well as ISOGAL images of improved quality.