Astronomy and Astrophysics – Astronomy
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.203i&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Astronomy and Astrophysics
Astronomy
Scientific paper
We present low-resolution Spitzer/IRS spectroscopic observations of a sample of 68 ISOGAL sources with unknown natures. The most common features in the spectra are silicate absorption bands, emission features from polycyclic aromatic hydrocarbons (PAHs), and forbidden fine structure lines from ionized species (essentially [ArII], [NeII] and [SIII]). In a small number of our sources, we detected the S(1) rotational line of H_2. Based on the detection of PAH or forbidden fine structure line emission and the slope of their spectra, the sources were classified as young objects (young stellar objects and HII regions) or late-type evolved objects. By fiting their spectral energy distributions and assuming a Galactic Center distance of 8 kpc, we obtained bolometric luminosity values L_bol for all young objects. Due to the dust cocoon around them young objects appear more extended at mid-infrared wavelengths than late-type evolved objects. Selection criteria which are based on the ISOGAL magnitude difference [7]-[15] and the spatial extent parameter sigma_15 can reliably distinguish between these two groups of sources. We applied these criteria to all ISOGAL sources within |l| =1.5 deg and |b|= 0.5 deg of the Galactic Center and selected 452 sources. Furthermore, 658 MSX sources with flux density ratios F_E/F_D ~ 2 in the E (21 µm) and D (15 µm) bands were added to the CMZ source list. After obtaining L_bol/F_15 conversion factors, with F_15 being the flux density at 15 µm, we calculated the bolometric luminosity values for the CMZ sample and subsequently the masses of the sources. The mass distribution roughly follows a Salpeter IMF. Since low-mass objects are missing from the sample, we extrapolated a modified Salpeter IMF to masses of 0.08 M_sol, resulting in a total mass of ~ 44000 M_sol. Assuming a formation time of 0.5 Myr, we determined an average star formation rate of 0.09 M_sol/yr for the central molecular zone.
Immer Katharina
Menten Karl. M.
Schuller Fr.
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