The Recent Star Formation in NGC 6822: an Ultraviolet Study

We characterize the star formation in the low-metallicity galaxy NGC 6822 over the past few hundred million years, using GALEX far-UV (FUV, 1344-1786 angstrom) and near-UV (NUV, 1771-2831 angstrom) imaging, and ground-based H alpha imaging. From the GALEX FUV image, we define 77 star-forming (SF) regions with area > 860 pc(2), and surface brightness less than or similar to 26.8 mag (AB) arcsec(-2), within 0 degrees.2 (1.7 kpc) of the center of the galaxy. We estimate the extinction by interstellar dust in each SF region from resolved photometry of the hot stars it contains: E(B - V) ranges from the minimum foreground value of 0.22 mag up to 0.66 +/- 0.21 mag. The integrated FUV and NUV photometry, compared with stellar population models, yields ages of the SF complexes up to a few hundred Myr, and masses from 2 x 10(2) M-circle dot to 1.5 x 10(6) M-circle dot. The derived ages and masses strongly depend on the assumed type of interstellar selective extinction, which we find to vary across the galaxy. The total mass of the FUV-defined SF regions translates into an average star formation rate (SFR) of 1.4 x 10(-2) M-circle dot yr(-1) over the past 100 Myr, and SFR = 1.0 x 10(-2) M-circle dot yr(-1) in the most recent 10 Myr. The latter is in agreement with the value that we derive from the H alpha luminosity, SFR = 0.008 M-circle dot yr(-1). The SFR in the most recent epoch becomes higher if we add the SFR = 0.02 M-circle dot yr(-1) inferred from far-IR measurements, which trace star formation still embedded in dust (age less than or similar to a few Myr).