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

Abstract

We propose a methodology to perform a self‐consistent analysis of the physical properties of the emitting gas of H ii 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 H ii 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 10 500 Å, including the Balmer jump, the [O ii] λλ 3727, 29 Å lines, the [S iii] λλ 9069, 9532 Å doublet as well as various weak auroral lines such as [O iii] λ 4363 Å and [S iii] λ 6312 Å. For the three objects, we have measured at least four line temperatures, T([O iii]), T([S iii]), T([O ii]) and T([S ii]), 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+/O2+ and S+/S2+ ratios points to high values of the ionizing radiation, as traced by the values of the ‘softness parameter’η which is less than 1 for the three objects. The use of line temperatures derived from T([O iii]) based on current photoionization models yields for the two highest excitation objects, much higher values of η which would imply lower ionizing temperatures. This is, however, inconsistent with the ionization structure as probed by the measured emission‐line intensities. Finally, we have measured the T(Bac) for the three observed objects and derived temperature fluctuations. Only for one of the objects, the temperature fluctuation is significant and could lead to higher oxygen abundances by about 0.20 dex.

Long-slit spectrophotometry of the multiple knots of the polar ring galaxy IIZw71

“…found by Hägele et al (2006) for HII galaxies. The values of the derived electron temperatures for these two knots are listed in Table 5.…”

“…The atomic coefficients used here are the same as in Hägele et al (2006). We took as sources of error the uncertainties associated with the measurement of the emission-line fluxes and the reddening correction and we propagated them through our Table 4.…”

Integral field spectroscopy of nitrogen overabundant blue compact dwarf galaxies

“…In the study of galaxies with WR signatures from the Sloan Digital Sky Survey (SDSS) carried out by Brinchmann et al (2008), it is also found that WR galaxies show an elevated N/O relative to non-WR galaxies. Additionally, an N/O excess is reported in other six BCDs with log(N/O) > −1.3 and a prominent WR blue bump in their optical integrated spectra (Hägele et al 2006(Hägele et al , 2008Pérez-Montero et al 2010). On the other hand, the high N/O found in metalpoor halo stars indicates that another mechanism polluting the ISM with N, other than WR star winds, might exist.…”

“…To calculate the electron temperature t([Oiii]), we use the expressions from Hägele et al (2006Hägele et al ( , 2008 that are based on the TEMDEN task under the IRAF nebular package. The distribution functions and the maps of this temperature in the three observed galaxies are shown in Fig.…”

“…This makes determination of chemical abundances using the so-called direct method (i.e. using collisionally excited lines and electron temperatures) much more accurate than in other metallicity regimes (e.g., Pérez-Montero & Díaz 2003;Hägele et al 2006). This feature of BCDs, together with their compact aspect and gas richness, make them ideal scenarios for studying the star formation processes in the local universe and in low-metallicity environments resembling A&A 532, A141 (2011) those in a younger Universe.…”