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(circle dot)/ 60 less than or similar to Z less than or similar to Z(circle dot)/ 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 similar to 3200 - 5100 angstrom, which includes the Balmer jump. Results. On the base of the present large spectroscopic sample we find that in hot ( T-e( H+) greater than or similar to 11 000 K) H ii regions the temperature of the O2+ zone, determined from doubly ionised oxygen forbidden lines, does not di. er 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-e( H+) and T-e( O III).