Abstract
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 A, FUV) and near-UV (1771-2831 A, 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 pc2 above an average FUV flux limit of ~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 Hα, 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 ~0.6 or 0.7 M ☉ yr–1 (for metallicity Z = 0.02 or 0.05, respectively) over the last 400 Myr.
Highlights
In the cold dark matter framework, large spiral galaxies are built hierachically, and there is much observational evidence of galaxy interactions to support this
When star formation detected from IR measurements of the heated dust is added to the UV-measured star formation in the recent few Myrs, we find the star formation rate (SFR) has slightly decreased in recent epochs, with a possible peak between 10 and 100 Myrs, and an average value of SFR ∼0.6 or 0.7 M⊙ yr−1 over the last 400 Myrs
In order to derive the physical parameters of the SF regions from the integrated photometry, we must take into account the interstellar extinction
Summary
In the cold dark matter framework, large spiral galaxies are built hierachically, and there is much observational evidence of galaxy interactions to support this. UV imaging data allow us to unambiguously identify the young stellar populations and to estimate their ages and masses from colors and extinction-corrected UV luminosities, respectively. These results provide the means of probing the history and modality of recent star formation in galaxies. As with the bulk of local spirals, M31 shows evidences for a formation and evolution history affected by merging and/or accretion events, including substructures in its halo (Hammer et al 2007, and references therein) In this respect, it is important to investigate the star-forming (SF) regions in M31. GALEX images have a sampling of 1.5 arcsec pixel−1 which corresponds to 5.67 pc, assuming a distance of 785 kpc (McConnachie et al 2005)
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