Chemical abundances and ionizing clusters of Hii regions in the LINER galaxy NGC 4258

We present long-slit observations in the optical and near-infrared of eight Hii regions in the spiral galaxy NGC 4258. Six of the observed regions are located in the south-east inner spiral arms, and the other two are isolated in the northern outer arms. A detailed analysis of the physical conditions of the gas has been performed. For two of the regions, an electron temperature has been derived from the [Siii] λ6312 line. For the rest, an empirical calibration based on the red and near-infrared sulphur lines has been used. The oxygen abundances derived by both methods are found to be significantly lower (by a factor of 2) than previously derived by using empirical calibrations based on the optical oxygen lines. In the brightest region, 74C, the observation of a prominent feature caused by Wolf-Rayet (WR) stars provides an excellent constraint over some properties of the ionizing clusters. In the light of the current evolutionary synthesis models, no consistent solution is found to explain at the same time both the WR feature characteristics and the emission-line spectrum of this region. In principle, the presence of WR stars could lead to large temperature fluctuations and also to a hardening of the ionizing radiation. None of these effects is found in region 74C, for which the electron temperatures found from the [Siii] λ6312 line and the Paschen discontinuity at 8200Å are equal within the errors, and the effective temperature of the ionizing radiation is estimated at around 35300K. Both more observations of confirmed high-metallicity regions and a finer metallicity grid for the evolutionary synthesis models are needed in order to understand the ionizing populations of Hii regions.