AN ULTRAVIOLET STUDY OF STAR-FORMING REGIONS IN M31

We present a comprehensive study of star-forming (SF) regions in the nearest large spiral galaxy M31. We use Galaxy Evolution Explorer (GALEX) far-UV (1344-1786 angstrom, FUV) and near-UV (1771-2831 angstrom, NUV) imaging to detect young massive stars and trace the recent star formation across the galaxy. The FUV and NUV flux measurements of the SF regions, combined with ground-based data for estimating the reddening by interstellar dust from the massive stars they contain, are used to derive their ages and masses. The GALEX imaging, combining deep sensitivity and coverage of the entire galaxy, provides a complete picture of the recent star formation in M31 and its variation with environment throughout the galaxy. The FUV and NUV measurements are sensitive to detect stellar populations younger than a few hundred Myr. We detected 894 SF regions, with size >= 1600 pc(2) above an average FUV flux limit of similar to 26 ABmag arcsecond(-2), over the whole 26 kpc (radius) galaxy disk. We derive the star formation history of M31 within this time span. The star formation rate (SFR) from the youngest UV sources (age <= 10 Myr) is comparable to that derived from H alpha, as expected. We show the dependence of the results on the assumed metallicity. When star formation detected from IR measurements of the heated dust is added to the UV-measured star formation (from the unobscured populations) in the recent few Myr, we find the SFR has slightly decreased in recent epochs, with a possible peak between 10 and 100 Myr, and an average value of SFR similar to 0.6 or 0.7 M-circle dot yr(-1) (for metallicity Z = 0.02 or 0.05, respectively) over the last 400 Myr.