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5.1. Young Populations and Chemical Homogeneity

The gas in Irrs and dIrrs is fairly well mixed, and mixing must proceed rapidly considering how homogeneous present-day HII region abundances at different locations within the same galaxy are. Nebular abundances of ionized gas trace the youngest populations and the chemical composition of the star-forming material. Why there is such a high degree of homogeneity is not fully understood, nor is it clear how the mixing proceeds; mechanisms may include winds and turbulence. Inhomogeneities are only expected to be detectable very shortly after the responsible stellar population formed (e.g., when pollution by Wolf-Rayet stars occurs; see Kobulnicky et al. 1997). Note that such global chemical homogeneity appears to be less pronounced in gas-deficient dSph galaxies, which seem to have experienced different star formation and chemical enrichment histories than dIrrs (see, e.g., Harbeck et al. 2001; Grebel et al. 2003).

If Irrs and dIrrs are chemically homogeneous, one would expect to measure comparable abundances in HII regions and young stars of a given dIrr, since both trace the same population. Due to their proximity, in the Magellanic Clouds supergiants, giants, and even massive main sequence stars can be analyzed with high-dispersion spectroscopy and individual element ratios can be measured. Indeed, in the Magellanic Clouds good agreement is found between nebular and stellar abundances (e.g., Hill, Andrievsky, & Spite 1995; Andrievsky et al. 2001). Star-to-star variations in the overall metallicity of young stars (B to K supergiants and B main sequence stars) are small ( ± 0.1 dex: Hill 1997; Luck et al. 1998; Venn 1999; Rolleston, Trundle, & Dufton 2002), and there is no evidence for a significant Population I metallicity spread in either Cloud. Also, the differences between the stellar abundances in young clusters and field stars are small (Gonzalez & Wallerstein 1999; Hill 1999; Korn et al. 2000, 2002; Rolleston et al. 2002). The mean metallicity of the young population in the LMC is [Fe/H] approx -0.3 dex and ~ -0.7 dex in the SMC.

Very good agreement between young stellar and nebular abundances is also found in the more distant dIrrs NGC6822 (B supergiants: Muschielok et al. 1999; A supergiants: Venn et al. 2001, both yielding [Fe/H] = -0.5 dex), GR8, and SexA (Venn et al. 2004). The two blue supergiants analyzed in WLM, on the other hand, have clearly higher metallicities than found in its HII regions (Venn et al. 2003). The reasons for this discrepancy in WLM are still unknown.

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