The temperature and ionization structure of the emitting gas in Hii galaxies: implications for the accuracy of abundance determinations

We propose a methodology to perform a self-consistent analysis of the physical properties of the emitting gas of Hii galaxies adequate to the data that can be obtained with the 21st century technology. This methodology requires the production and calibration of empirical relations between the different line temperatures that should supersede currently used ones based on very simple, and poorly tested, photoionization model sequences.As a first step to reach these goals, we have obtained simultaneous blue to far red long-slit spectra with the William Herschel Telescope (WHT) of three compact Hii galaxies selected from the Sloan Digital Sky Survey (SDSS) Data Release 2 (DR2) spectral catalogue using the INAOE Virtual Observatory superserver. Our spectra cover the range from 3200 to 10500A, including the Balmer jump, the [Oii]lambdalambda3727, 29A lines, the [Siii]lambdalambda9069, 9532A doublet as well as various weak auroral lines such as [Oiii]lambda4363A and [Siii]lambda6312A .For the three objects, we have measured at least four line temperatures, T([Oiii]), T([Siii]), T([Oii]) and T([Sii]), and the Balmer continuum temperature T(Bac). These measurements and a careful and realistic treatment of the observational errors yield total oxygen abundances with accuracies between 5 and 9 per cent. These accuracies are expected to improve as better calibrations based on more precise measurements, both on electron temperatures and densities, are produced.We have compared our obtained spectra with those downloaded from the SDSS DR3 finding a satisfactory agreement. The analysis of these spectra yields values of line temperatures and elemental ionic and total abundances which are in general agreement with those derived from the WHT spectra, although for most quantities they can only be taken as estimates since, due to the lack of direct measurements of the required lines, theoretical models had to be used whose uncertainties are impossible to quantify.The ionization structure found for the observed objects from the O(+)/O(2+) and S(+)/S(2+) ratios points to high values of the ionizing radiation, as traced by the values of the 'softness parameter'eta which is less than 1 for the three objects. The use of line temperatures derived from T([Oiii]) based on current photoionization models yields for the two highest excitation objects, much higher values of eta which would imply lower ionizing temperatures. This is, however, inconsistent with the ionization structure as pr