M. Burgdorf 1, S. Price 2, S. Ott 1, M. Perault 3, C. Cesarsky 4, M. Egan 2, P. Gallais 4, Th. de Graauw 5, E. Palazzi 6, & R. Shipman 2
1 ISO Science Operations Centre, Astrophysics Division of ESA, Villafranca del Castillo, P.O. Box - Apdo. 50727, 28080 Madrid, Spain
2 Air Force Geophysics Laboratory, Bedford MA, USA
3 Radioastronomie, Ecole Normale Superieure, France
4 Service d'Astrophysique, CE-Saclay, France
5 SRON, PO box 800, 9700 AV Groningen, The Netherlands
6 TESRE, Bologna, Italy
A number of fields along the Galactic Plane were imaged by the Infrared Space
Observatory's camera ISOCAM between April and September 1996 with the
objective of observing the large scale properties of the Galaxy. Seven fields
in the inner Galactic Plane were imaged in the LW3 (12 - 18 microns) and LW7
(8.5 - 11 microns) filters; each image was obtained by raster scanning a 6x6
field with 100" spacing between samples. Five of the fields are at b = 0 deg
between 4 and 28 deg longitude; two others straddle the center at
.
Additionally, 38 single pointing observations were
obtained in
filters LW3, LW4 (5.5 - 6.5 microns) and LW7 of selected fields in the
Galactic Plane. They lie at various combinations of |b| = 0, 1 and 5 deg and
|l| = 5, 15, 30, 45 and 60 deg.
The data have been processed with version seven of the pipeline and potential
sources have been identified in the raster scanned fields using the ISOGAL
source extractor. Between 134 and 194 LW3 sources are extracted in the
fields, and 182 and 289 LW7 objects. Based on log N vs. log S
plots, the completeness limiting flux is approximately 10 mJy. That the number
of sources extracted is about the same in all fields strongly indicates that
the images are not confusion limited, a result that was confirmed by visually
inspecting the images. The systematically higher number of LW7 sources
compared to LW3 is expected owing to the stronger emission at shorter
wavelengths: Point sources that were only detected in LW7 have usually a
counterpart in LW3 which is just below the flux needed for a proven
detection. Other discrepancies between the source lists, for example very
different fluxes in the two filters for certain sources, are discussed. In
general, however, there is a good correlation between the fluxes measured in
the two filters in every field and only in a few isolated cases the source
extractor provides erroneous results by mistaking extended emission for a point
source, for example. Hence the source counts in the GPSURVEY data make a
fundamental contribution to the determination of the number of foreground
stars in the infrared sky.