Star Formation in HII Galaxies. Properties of the ionized gas

G. F. HÄGELE, A. I. DÍAZ, E. TERLEVICH, E. PÉREZ-MONTERO, R. TERLEVICH AND M. V. CARDACI

Kolymbari, Creta, Grecia

Workshop; Star-forming Dwarf Galaxies: Ariadne's Thread in the Cosmic Labyrinth; 2008

Academia Ortodoxa de Creta

We present two set of high signal-to-noise spectrophotometric observations of three and seven compact and luminous HII galaxies observed with the WHT and the 3.5m telescope at CAHA, respectively. All the observations have been made with the use of a double-arm spectrograph which provides spectra with a wide wavelength coverage, from 3200 to 10500 Å for the WHT data and 3400 to 10400 Å for the CAHA data, of exactly the same region of a given galaxy. These spectral ranges include the [OII]  3727,29 Å lines, the [SIII]  9069,9532 Å doublet as well as various weak auroral lines such as [OIII]  4363 Å and [SIII]  6312 Å. 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 XXI century technology. This methodology requires the production and calibration of empirical relations between the different line temperatures that should superseed currently used ones based on very simple, and poorly tested, photoionisation model sequences. Then, these observations are analysed applying a methodology designed to obtain accurate elemental abundances of oxygen, sulphur, nitrogen, neon, argon and iron in the ionsied gas. Four electron temperatures and one electron density are derived from the observed forbidden line ratios using the five-level atom approximation. For our best objects errors of 1% in te([OIII]), 3% in te([OII]) and 5% in te([SIII]) are achieved with a resulting accuracy between 5 and 9% in total oxygen abundances, O/H. These accuracies are expected to improve as better calibrations based on more precise measurements, both on electron temperatures and densities, are produced. For the objects observed with theWHT we have compared the measurements obtained for our spectra with those performed on the spectra downloaded from the SDSS DR3 finding a satisfactory agreement. The ionisation structure of the nebulae can be mapped by the theoretical oxygen and sulphur ionic ratios, on the one side, and the corresponding observed emission line ratios, on the other – the \eta and \eta’ plots –. The combination of both is shown to provide a means to test photo-ionisation model sequences currently applied to derive elemental abundances in HII galaxies. 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”  which is less than one for all the objects. The use of line temperatures derived from te([OIII]) based on current photo-ionization models yields for the two highest excitation objects much higher values of  which would imply lower ionizing temperatures. This is however inconsistent with the ionization structure as probed by the measured emission line intensity ratios. Finally, we have measured the T(Bac) for three of the HII galaxies and computed 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.