1. Low-metallicity galaxies at low redshifts Y. I. Izotov Main Astronomical Observatory, Kyiv, Ukraine

2. Luminosity-metallicity relation Dwarf galaxies: - elliptical - spheroidal - irregular - H II galaxies (BCDs) Kunth & Östlin (2000) Grebel (2004) Dwarf emission-line galaxies at low redshifts (z ≤ 0.1)

3. OUTLINE: - General characteristics of dwarf emission-line galaxies; - Evolutionary status; - Element abundances; - Stellar wind; - Ionizing radiation.

4. Dwarf emission-line galaxies: - low metallicity (12+log O/H = 7.1 - 8.3) - massive stellar population - blue spectra with strong emission lines - unevolved systems, rich in neutral gas SBS 0335-052E,W HST/GHRS VLA MMT

5. Surveys: Zwicky, Markarian, Second Byurakan, University of Michigan, Hamburg, Tololo, Case, KISS, 2dF, 6dF, SDSS (~10 6 spectra) SDSS images

6. Evolutionary status 7.5 8.0 8.5 12 + log O/H I Zw 18 HST/WFPC2 HST/ACS Izotov & Thuan (2002) Izotov & Thuan (2004)

8. - - 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).

9. 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)

10. 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)

11. 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)

12. - 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).

13. 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)

14. Schaerer & Vacca (1998) Guseva et al. (2000) I Zw 18 SE NW Brown et al. (2002)

15. - 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.

16. 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)

17. Schaerer (2002,2003) - massive stars (MS, WR); - high-mass X-ray binaries; - fast shocks. Thuan & Izotov (2005)

18. HαHα He II 4686

19. - 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.

20. 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.