1. Dwarf Irregular Galaxies Models Marta Gavilán Mercedes Mollá Ángeles Díaz NGC 6822 DDO 187

2. Dwarf Irregular Galaxies ● Low surface brightness m B > -23mag arcsec -2 ● Small radius: R < 5kpc ● Low mass: M < 10 10 Mo ● Large amount of gas ● Low Abundances: 12+log(O/H) ● Young population - Blue color SMC

3. Looking for answers Evolution Models Chemical Abundances Gas Fraction Spectrophotometric Colors Mass-Luminosity Relation

4. The Scenario: INFALL T col  hh The gas fall rate is inversely to the time scale   is the efficiency in forming molecular clouds  h is the efficiency in forming stars Continuous Star Formation. There are not bursts. infall model

5. The Model Photometry Spectroscopy G.C.E.M. INPUT OUTPUT Yields Efficiencies Time Scale SN Rates SFH Gas Abundances Synthesis PopulationModel Chemical Evolution Model: Ferrini et al. 1992 – Mollá & Díaz 2005 Synthesis Population Model Gª Vargas et al. (1998) Mollá et al. (2009) (submitted)

6. Grid of Models Different masses Different infall rates (collapse time) Different efficiencies

7. Data DWARF GALAXIES IN THE FIELD DDO 155 DDO 187 IC 1613 IC 4662 NGC 1559 NGC 2366 NGC 3109 NGC 4214 NGC 6822 LMC SMC AUTHORS Ferrara & Tolstoy (2000) Garnett (2002) Hunter & Elmegreen (2004) Karachentsev et al. (2004) Lee et al. (2003) Mallery et al. (2007) Sánchez-Almeida et al. (2008) Skillman et al. (1989) van Zee et al. (2000, 2006) DATA M B, (B-V), M HI, M*, M tot, 12+log(O/H), log(N/O)

8. Medium-high Very low Low Medium High Very high Efficiencies color criterium Galaxy mass increases Scale factor? M6/M1 = 375 SFR 6 /SFR 1 =500 Star Formation History

9. Gas fraction Lowest mass galaxy Highest mass galaxy M1 M2 M3 M4 M5 M6 Mass increases Models colors criterium

10. Gas fraction gas+stars gas m=m= High infall - Max efficiency Medium infall - Low efficiency Intermediate case Problem: Mass-Luminosity relation

11. Gas fraction IC 1613NGC 1569

12. Nitrogen evolution

13. Mallery et al. 2007 - GALEX UV – Star forming galaxies enriched gas outflow High (N/O) with low (O/H) values are not reached by our models...enriched gas outflow?

14. Nitrogen evolution Data from van Zee et al. (2006)

15. Nitrogen evolution

16. Luminosity-Metallicity relation different evolutionary states? Data seem to follow the evolution lines, more than the final values...different evolutionary states? Correlation with mass

17. Luminosity-Metallicity relation different evolutionary states? Data seem to follow the evolution lines, more than the final values...different evolutionary states? Correlation with mass

18. Luminosity-Metallicity relation different evolutionary states? Data seem to follow the evolution lines, more than the final values...different evolutionary states? Correlation with mass

19. Luminosity-Metallicity relation different evolutionary states? Data seem to follow the evolution lines, more than the final values...different evolutionary states? Correlation with mass

20. SFR dIrr - Q-BCD Brown dots: Sánchez-Almeida et al. (2008) Quiescent BCD In low brightness range: models do not match data trend: Data: lower SFR for the same color than models. post-star burst situation?

21. LMC Collase Time = 8 Gyr Efficiency: Low-Medium Age: 10 Gyr Collase Time = 20 Gyr Efficiency: Medium Age: 4 - 5 Gyr M=5 x 10 9 Mo

22. NGC 1569 Collase Time = 8 Gyr Efficiency: Low-Medium Age: 7 - 9 Gyr Collase Time = 20 Gyr Efficiency: Medium Age: 2 - 4 Gyr M=5 x 10 9 Mo

23. NGC 3109 Collase Time = 8 Gyr Efficiency: Low Age: 10 - 13 Gyr Collase Time = 8 Gyr Efficiency: Low - Medium Age: 4 - 6 Gyr M=5 x 10 9 Mo

24. NGC 2366 Collase Time = 8 Gyr Efficiency: Low-Medium Age: 4 - 6 Gyr Collase Time = 8 Gyr Efficiency: Low - Medium Age: 4 - 6 Gyr M=10 9 MoM=5 x 10 9 Mo

25. SUMMARY What can be explained with these models? Gas Fraction - Relation gas mass-stars mass-metallicity (N/O) abundances: Dispersion can be due to different SFH - efficiencies Luminosity-Metallicity relation: Data match with different evolutionary stages SFH and age: young objects