The recent star formation in NGC6822: An ultraviolet study

Abstract

We characterize the star formation in the low-metallicity galaxy NGC6822 over the past few hundred million years, using GALEX far-UV (FUV, 1344-1786) and near-UV (NUV, 1771-2831) imaging, and ground-based Hα imaging. From the GALEX FUV image, we define 77 star-forming (SF) regions with area >860pc 2, and surface brightness ≲26.8mag(AB) arcsec-2, within 02 (1.7kpc) 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 0.22mag up to 0.66 0.21mag. 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 × 102 to 1.5 × 106 . 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 × 10-2 yr-1 over the past 100Myr, and SFR= 1.0 × 10-2 yr-1 in the most recent 10Myr. The latter is in agreement with the value that we derive from the Hα luminosity, SFR = 0.008 yr-1. The SFR in the most recent epoch becomes higher if we add the SFR = 0.02 yr-1 inferred from far-IR measurements, which trace star formation still embedded in dust (age ≲ a fewMyr). © 2011. The American Astronomical Society. All rights reserved.