2002MNRAS.337..540C

The ionizing stellar populations of 11 Hii regions in the spiral galaxies: NGC 628, 925, 1232 and 1637, all of them reported to have solar or oversolar abundances according to empirical calibrations, have been analysed using stellar population synthesis models.

Four of the observed regions in the sample show features which indicate the presence of a population of Wolf-Rayet (WR) stars with ages between 2.3 and 4.1 Myr. This population is sufficient to explain the emission line spectrum of the low-metallicity region H13 in NGC 628, taking into account the uncertainties involved in both observations and model computations. This is not the case for the rest of the regions for which a second ionizing population is required to simultaneously reproduce both the WR features and the emission line spectrum. Composite populations are also found for half of the regions without WR features, although in this case, the result is based only on the emission line spectrum analysis.

For two of the regions showing WR features, no consistent solution is found, as the population containing WR stars produces a spectral energy distribution which is too hard to explain the emission of the gas. Several solutions are proposed to solve this problem.