Recent star formation in the inner Galactic Bulge seen by ISOGAL

I. Classification of bright mid-IR sources in a test field

¹ Institut d'Astrophysique de Paris, CNRS and Paris-VI University, 98bis Bd. Arago, 75014 Paris, France e-mail: schuller@iap.fr
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
⁴ Observatoire de Besançon, CNRS, 41bis Av. de l'Observatoire, BP 1615, 25010 Besançon Cedex, France
⁵ Air Force Research Laboratory, ODASD, 1851 s. Bell St., CM3, Suite 7000, Arlington, VA 22202, USA
⁶ Air Force Research Laboratory, Space Vehicles Directorate, 29 Randolph Road, Hanscom AFB, MA 01731, USA

Received: 16 February 2005
Accepted: 23 February 2006

Abstract

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  arcmin² field centred at observed at 7 and 15 μm 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 Hii 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 μm, can be used to determine whether the ISOGAL sources brighter than 300 mJy at 15 μm (or [15] ≤ 4.5 mag) are young objects or late-type evolved stars.