Recent star formation in the inner Galactic bulge seen by ISOGAL

Abstract

We present 5--38 $\mu$m spectroscopic observations of a sample of 68 ISOGAL sources with unknown natures, taken with the Spitzer Infrared Spectrograph. Based on the characteristics and the slope of their spectra we classified the sources as young or late-type evolved objects. These sources were selected to test selection criteria based on the ISOGAL [7]--[15] color and the spatial extent parameter $\sigma_{\rm 15}$. We revised these criteria until they reliably distinguished between young and late-type evolved objects and then applied them to all ISOGAL sources in the central molecular zone (CMZ), resulting in the selection of 485 sources believed to be young. Furthermore, we added 656 Midcourse Space Experiment (MSX) sources to the CMZ sample that fulfilled $F_{\rm E}/F_{\rm D} > 2$ with $F{\rm D}$ and $F_{\rm E}$ being the flux densities in the D (15 $\mu$m) and E (21 $\mu$m) bands. After obtaining $\frac{L_{\rm bol}}{F_{\rm 15}}$ conversion factors, we calculated the bolometric luminosity, $L_{\rm bol}$, values for the CMZ sample and subsequently the masses of the sources. Applying a Kroupa initial mass function, we derived the total mass in young objects that has been formed over the last 1 Myr, resulting in an average star formation rate of 0.08 solar masses per year for the CMZ.