How extreme are the Wolf-Rayet clusters in NGC3125?

Abstract

We reinvestigate the massive stellar content of NGC3125 (Tol3) using VLT/FORS1 imaging & spectroscopy, plus archival VLT/ISAAC, HST/FOC and HST/STIS datasets. Narrow-band imaging confirms that A and B knots represent the primary sites of Wolf-Rayet (WR) stars, whilst HST imaging reveals that both regions host multiple clusters. Clusters A1 + A2 within region A host WR stars. altough is not clear which cluster within region B hosts WR stars. Nebular properties infer an LMC-like metallicity. LMC template WN5-6 & WC4 spectra are matched to the observed optical WR bumps of A1 and B, permitting the contribution of WC stars to the blue bump to be quantified. We obtain N(WN5-6:WC4)=105:20, 55:0, 40:20 for clusters A1, A2 and B1+B2, a factor of 3 lower than optical studies, as a result of a lower E(B-V). Using Starburst99 models to estimate O star populations for individual clusters, we find N(WR)/N(O)=0.2 for A1 and 0.1 for A2 and B1+B2. From Halpha imaging, the O star content of the Giant HII regions A and B is found to be a factor of 5-10 times higher than that derived spectroscopically for the UV/optically bright clusters, suggesting that NGC3125 hosts optically obscured young massive clusters, further supported by VLT/ISAAC K band imaging. Archival HST/STIS UV spectroscopy confirms the low E(B-V) towards A1, for which we have determined an SMC extinction law, in preference to an LMC or starburst law. We obtain N(WN5-6)=110 from the slit-loss corrected HeII 1640 line flux, in excellent agreement with optical result. However, this is a factor of 35 times lower than that inferred from the same dataset by Chandar et al. due to their use of a starburst extinction law. Highly discrepant stellar populations may result in spatially resolved star forming regions from UV and optical studies through use of different extinction laws.