The Recent Star Formation in NGC 6822: An Ultraviolet Study

Abstract

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 Å) and near-UV (NUV, 1771-2831 Å) 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 ≾ 26.8 mag (AB) arcsec^(-2), within 0°.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_⊙ to 1.5 x 10^6 M_⊙. 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_⊙ yr^(-1) over the past 100 Myr, and SFR = 1.0 x 10^(-2) M_⊙ yr^(-1) in the most recent 10 Myr. The latter is in agreement with the value that we derive from the Hα luminosity, SFR = 0.008 M_⊙ yr^(-1). The SFR in the most recent epoch becomes higher if we add the SFR = 0.02 M_⊙ yr^(-1) inferred from far-IR measurements, which trace star formation still embedded in dust (age ≾ a few Myr).

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