OPTICAL SPECTROSCOPY OF METAL-RICH H II REGIONS AND CIRCUMNUCLEAR HOT SPOTS IN M83 AND NGC 3351

摘要

We present optical spectroscopy of a sample of metal-rich extragalactic H II regions in the spiral galaxies M83 (NGC 5236), NGC 3351, and NGC 6384. The metal abundance, estimated from semiempirical methods using strong emission lines, is found to be above solar for most of the objects. The sample includes a number of circumnuclear H II regions (hot spots), which are found in general to have spectral properties similar to the H II regions located in the disks. Different age estimators [equivalent width of the Hβ emission line W(Hβ), Balmer line absorption profiles, UV spectra] provide consistently young ages (4-6 Myr) for the hot spots. We have detected the Wolf-Rayet (W-R) bump feature at 4650 A in five of the objects, and in fewer cases possibly the WC features in the red at 5696 and 5808 A. Six additional objects showing W-R features are drawn from our previous work on extragalactic H II regions. From the measured luminosity of the stellar He II λ4686 line we estimate a small number of WN stars (from one or two to about 30). We have compared the measured intensities and equivalent widths of the W-R bump to the predictions of recent evolutionary models for massive stellar populations. By assuming instantaneous bursts of star formation the ages derived from W(Hβ), between 3 and 6 Myr, are in agreement with the age span predicted by the models. The observed strength of the W-R bump is in agreement with the predictions for only half of the objects, the remainder showing lower I(4650)/I(Hβ) and W(4650) than the models of the appropriate metallicities. Our favored explanation is related to the finite number of stars formed in the clusters and to stochastic effects likely to be at work in star-forming regions of the size considered here. The He I λ5876 recombination line is used as an indicator of the equivalent effective temperature of the ionizing clusters, T_(eff). We have found that this temperature is not affected by the presence of W-R stars. From a comparison with published photoionization models based on synthetic cluster spectral energy distributions we find some evidence for an overestimation of the number of He ionizing photons in the model fluxes. In general, however, the massive star diagnostics considered in our work are in agreement with the predictions of recent evolutionary models calculated with a Salpeter initial mass function and a high upper mass limit. We find no compelling evidence for a depletion of massive stars (M > 40-50 M_☉) in the initial mass function of metal-rich clusters, contrary to our previous conclusions based on older evolutionary models.