Recent star formation in the inner Galactic Bulge seen by ISOGAL - I. Classification of bright mid-IR sources in a test field

Context. The stellar populations in the central region of the Galaxy are poorly known because of the high visual extinction and very great source density in this direction. Aims. To use recent infrared surveys for studying the dusty stellar objects in this region. Methods. We analyse the content of a similar to 20 x 20 arcmin(2) field centred at (l,b) = (-0.27 degrees, -0.06 degrees) observed at 7 and 15 mu m as part of the ISOGAL survey. These ISO observations are more than an order of magnitude better in sensitivity and spatial resolution than the IRAS observations. The sources are cross-associated with other catalogues to identify various types of objects. We then derive criteria to distinguish young objects from post-main sequence stars. Results. We find that a sample of about 50 young stellar objects and ultra-compact H II regions emerges, out of a population of evolved AGB stars. We demonstrate that the sources colours and spatial extents, as they appear in the ISOGAL catalogue, possibly complemented with MSX photometry at 21 mu m, can be used to determine whether the ISOGAL sources brighter than 300 mJy at 15 mu m ( or [15] = 4.5 mag) are young objects or late-type evolved stars.