The Recent Star Formation in NGC 6822 from Hubble Space Telescope Imaging

We present Hubble Space Telescope WFPC2 and STIS imaging of the low-metallicity galaxy NGC 6822, performed as part of a study of the young stellar populations in the galaxies of the Local Group. Eleven WFPC2 pointings, with some overlap, cover two regions extending over 19 and 13 arcmin², respectively, off the galaxy center. The filters used are F170W, F255W, F336W, F439W, and F555W. One 25^''×25^'' field observed with the STIS FUV- and NUV-MAMA includes Hodge's OB 8 association and the H II region Hubble V, contained in field 1 of Bianchi et al.; this previous study provides additional WFPC2 four-band photometry. We derive the physical parameters of the stars in the fields and the extinction by comparing the photometry to grids of model magnitudes. The environments studied in this work include one of the most luminous (in Hα) H II regions in the Local Group (Hubble V) with a compact star cluster, a typical OB association (OB 15), the sparse field population, and the outskirts of NGC 6822. In the WFPC2 fields, most of the hot massive stars are found in the Hodge OB 15 association, at about 5' (0.7 kpc) east of the galaxy center, extending ~90" (~200 pc) in our imaging. The color-magnitude diagram indicates a young age, <=10 Myr, for this association, where we measure 70 stars hotter than ~16,000 K (earlier than mid-B spectral type) according to their photometric colors. In the compact H II region Hubble V, we measure 80 stars brighter than m_NUV<22.5 mag and find most of them to have high temperatures. The density (per unit area) of hot massive stars in the core of the OB 8 association is higher than in OB 15 by a factor of 12, while the total stellar mass formed is similar (~4 or 7×10³ M_solar, when extrapolated to a mass range of 1-100 or 0.1-100 M_solar, respectively). In both OB 15 and OB 8 massive star candidates are found. In the general field outside of the OB 15 association, we find few hot massive stars (0.7 arcmin^-2) and several A-type supergiants. No massive star candidates are found in the WFPC2 fields outside the main galaxy body (~10.5 arcmin-14.5 arcmin [1.5-2 kpc] from the galaxy center), where the population is dominated by foreground stars, at least down to V~22. At fainter magnitudes, we measure in these outer fields a significantly larger number of stars than the model for Milky Way foreground objects would predict. The average extinction is found to vary among the three environments studied: E(B-V)=0.22 in the outer regions, E(B-V)=0.27 in the fields east of the galaxy main bar, and E(B-V)=0.40 in the H II region Hubble V. A quantitative discussion of the applicability of the reddening-free-index method for photometric determination of stellar parameters is provided for the filters used in this work, based on our grids of stellar models.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.