VLT/FORS Surveys of Wolf-Rayet Stars in the Nearby Universe

摘要

We present results from a series of VLT/FORS narrow-band imaging and spec-troscopic surveys of Wolf-Rayet (WR) stars in nearby spiral galaxies and compare observed populations in high- and low metallicity environments. The metal-rich galaxy M 83 is seen to host an exceptional WR content, with over 1000 WR stars being detected. N(WC)/N(WN) 1.2 and late-type WC subtypes dominate the WC population. At low metallicity, 100 stars has been identified within NGC 1313, with N(WC)/N(WN) 0.5. In contrast to M83, the WC population of NGC 1313 comprises solely early subtypes plus a WO star (the first WO star to be identified beyond the Local Group). Consequently, the dominant WC subtype may serve as a crude metallicity diagnostic for WR galaxies. In addition, the WR content of the blue compact dwarf galaxy NGC 3125 is examined. Pre-vious UV and optical spectroscopic studies of knot A in NGC 3125 derive WR populations which differ by more than an order of magnitude. New VLT observations and archival HST spectroscopy reconcile this discrepancy via the use of LMC WR spectral templates and a re-duced nebular-derived interstellar extinction. Empirical N(WR)/N(0) ratios for clusters within NGC 3125 are a factor of two higher than evolutionary synthesis predictions but are consistent with those observed for other young massive clusters.