Balmer jump temperature determination in a large sample of low-metallicity HII regions

Aims.Continuing the systematic determination of the electron temperature of H II regions using the Balmer and/or Paschen discontinuities by Guseva et al. (2006, ApJ, 644, 890) we focus here on 3.6 m ESO telescope observations of a large new sample of 69 H II regions in 45 blue compact dwarf (BCD) galaxies. This data set spans a wide range in metallicity ( $Z_{\odot}/60 \la Z \la Z_{\odot}/3$) and, combined with the sample of 47 H II regions from Guseva et al. (2006), yields the largest spectroscopic data set ever used to derive the electron temperature in the H+ zone. Methods.In the same way as in Guseva et al. (2006) we have used a Monte Carlo technique to vary free parameters and to calculate a series of model spectral energy distributions (SEDs) for each H II region. The electron temperature in the H+ zones was derived from the best fitting synthetic and observed SEDs in the wavelength range ~3200-5100 Å, which includes the Balmer jump. Results.On the base of the present large spectroscopic sample we find that in hot ($T_{\rm e}$(H+) $\ga$ 11 000 K) H II regions the temperature of the O2+ zone, determined from doubly ionised oxygen forbidden lines, does not differ statistically from the temperature of the H+ zone. Thus, we confirm and strengthen the finding by Guseva et al. (2006). We emphasize that due to a number of modelling assumptions and the observational uncertainties for individual objects, only a large, homogeneous sample, as the one used here, can enable a conclusive study of the relation between $T_{\rm e}$(H+) and $T_{\rm e}$(O III).