- - the major fraction of dwarf emission-line galaxies in the local - Universe is older than 1 Gyr; - - the most metal-deficient dwarf emission-line galaxies might be - young (< 1 Gyr).
Element abundances Primordial He abundance: Systematic effects: - He I emissivities; - collisional and fluorescent enhancement; - underlying stellar absorption; - ionization structure of HII region; - temperature variations. N=93 Y p = 0.244±0.001 (old emissivities) 0.252±0.001 (new emissivities) WMAP, D: Y p = 0.248 Izotov et al. (2005b)
Heavy element abundances (ionized gas) SDSS DR3 (~500,000 spectra) ~ 1450 H II regions with the [O III] 4363 emission line which is detected at > 1σ level O, N, Ne, S, Cl, Ar, Fe Izotov et al. (2005a)
Heavy element abundances (neutral gas) Galaxylog(O/H) i log(O/H) n I Zw 18-4.82-4.7 - -5.3 SBS 0335-052E-4.70-5.0 Mrk 209-4.23-4.5 Mrk 59-4.01-5.0 NGC 1705-3.79-4.6 Mrk 59 (FUSE, Thuan et al. 2002) SBS 0335-052E (FUSE, Thuan et al. 2005) Low limit of log O/H ~ -5.0 (12+logO/H = 7.0)
- the primordial He mass fraction which may be used in the evolutionary models of low-metallicity stars is Y p ~ 0.25; - the oxygen abundance in the most metal-deficient extragalactic H II regions is 12 + log O/H ~ 7.1; - the metallicity of the neutral gas in dwarf galaxies is similar to the metallicity of the ionized gas in the most metal-deficient galaxies; - the α-element-to-oxygen abundance ratios are nearly constant in the wide range of metallicities; - the N/O abundance ratio is in agreement with the predictions of the rotating star models (Meynet & Maeder 2002).
Stellar wind 12 +logO/H = 7.5 (N V) 12+logO/H=7.3 (Si IV) 12+logO/H=7.2 (N III, C IV) I Zw 18 Izotov et. al. (1997), Thuan & Izotov (1997)
Schaerer & Vacca (1998) Guseva et al. (2000) I Zw 18 SE NW Brown et al. (2002)
- massive stars with a stellar wind are detected even in the most metal-deficient galaxies; - the relative numbers of O and Wolf-Rayet stars are in general agreement with the predictions of the population synthesis models.
Hardness of ionizing radiation Campbell et al. (1986) – ionizing radiation is harder in galaxies with lower metallicity He II 4686 (4 Ryd) [Fe V] 4227 (4 Ryd) [Ne V] 3346,3425 (7 Ryd) Kunth et al. (2003) Thuan & Izotov (2005) SSCs (several 10 3 O stars in compact clusters with diameter < 20-30 pc)
- ionizing radiation is much harder in the H II regions with low metallicity as evidenced by stronger high-ionization emission lines (He II 4686, [Fe V] 4227, [Ne V] 3425); - the origin of hard radiation is not yet clear. The most likely source of hard radiation is fast shocks.
Summary - dwarf galaxies at low redshifts are good targets to test stellar evolutionary models in low metallicity environment because of their proximity and low metallicity; - Y p ~ 0.25 – an input parameter to the stellar evolutionary models; - - no galaxy with the primordial composition of the neutral gas is found - in the local Universe; - massive stars with the stellar wind are found even in the galaxies with the lowest metallicity. This allows to study wind properties in the wide range of metallicities; - ionizing radiation in the lowest-metallicity galaxies is very hard.