Yu. I. Izotov

Excitation properties of galaxies with the highest [O iii]/[O ii] ratios - No evidence for massive escape of ionizing photons

The possibility that star-forming galaxies may leak ionizing photons is at the heart of many present-day studies that investigate the reionization of the Universe. We test this hypothesis on local blue compact dwarf galaxies of very high excitation. We assembled a sample of such galaxies by examining the spectra from data releases 7 and 10 of the Sloan Digital Sky Survey. We argue that reliable conclusions cannot be based on strong lines alone, and adopt a strategy that includes important weak lines such as [Oi] and the highexcitation Heii and [Ariv] lines. Our analysis is based on purely observational diagrams and on a comparison of photoionization models with well-chosen emission-line ratio diagrams. We show that spectral energy distributions from current stellar population synthesis models cannot account for all the observational constraints, which led us to mimick several scenarios that could explain the data. These include the additional presence of hard X-rays or of shocks. We find that only ionization-bounded models (or models with an escape fraction of ionizing photons lower than 10%) are able to simultaneously explain all the observational constraints.

SPECTROPHOTOMETRY OF 83 POSSIBLE BLUE COMPACT DWARF GALAXIES FROM THE SECOND BYURAKAN SURVEY

Spectrophotometric observations of 83 possible blue compact dwarf galaxies (BCDGs) and photoelectric UBVR (Johnson system) observations of 37 galaxies from this list are discussed. The observations have been made with the 6-meter telescope of Special Astrophysical Observatory of Russian Academy of Sciences. The general characteristics of BCDGs are considered. Most galaxies are shown to have very low heavy element abundance.

Determination of the physical parameters, Lyman continua, and chemical compositions of HII regions in blue compact dwarf galaxies

A new method for determining the physical parameters, ionizing Lyman continua (Lyc) and chemical compositions of HII regions in blue compact dwarf galaxies is developed. We propose our modified NLEHII method, which is independent on the initial mass function (IMF) for the determination of the Lyc spectra of the ionizing nuclei of HII regions. This method is based on the assumptions of ionization-recombination and thermal equilibria in the HII regions. It is used to calculate the Lyc spectra for optimization photoionization models (OPhMs) of HII regions in an iterative way, since the Lyc spectrum depends on parameters that can be found from photoionization modeling. We apply this method to determine the chemical composition of an HII region in the blue compact dwarf galaxy SBS 0940+544 and, in particular, the helium mass fraction Y.

The New Brilliant Blue Compact Dwarf Galaxy: A Test on the Early Stages of Galaxy Evolution

A blue compact dwarf galaxy SBS 0335 - 052 from the Second Byurakan Survey with extremely low heavy element abundance is found. The oxygen abundance in SBS 0335 - 052 is 77 times lower than the solar value and it is 1.7 times lower than that in I Zw 18. The electron temperature of gas is extremely high and equals 24800 K. SBS 0335 - 052 is a probable candidate for a young galaxy.