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4.4.9 Summary on BCG Metallicities

In summary we find that the study of heavy elements in BCGs shows that these systems are chemically unevolved but does not allow to infer a young age in terms of galaxy formation. In the range 7.1 < 12 + log(O/H) < 8.3 more than 100 objects have good quality data. Alpha-elements (Ne, Si, S and Ar) have abundances relative to oxygen that show no dependence on oxygen abundance and are close to solar values and similar to that in halo stars and distant galaxies. At low metallicities, C/O is constant, independently of the oxygen abundance but more metal deficient galaxies should be observed to confirm the presence or absence of a trend. The behaviour of N/O indicates a primary origin as anticipated already by several investigators (see Matteucci 1996, and references therein). The conclusion that N, C, Fe and O are produced only by massive stars in the most metal-poor systems (Izotov and Thuan 1999) needs to be checked by independent observers with larger samples. There is a possibility that metals observed in the most metal-poor galaxies originate from previous population III star enrichments, previous bursts or continuous star formation at very low rate. In this case the minimum metallicity should be increasing with time, consistent with quasar absorption line data (Lu et al. 1996), although admittedly the connection to dwarfs is not clear. The question of young galaxies has to be further addressed before it can be settled, but as we have seen, most BCGs (including objects with less than 1/10 solar metallicity) are definitely not young. Even the best candidates IZw18 and SBS 0335-052 seems to be old although Izotov, Thuan and their collaborators still dispute this.

While abundances and abundance ratios are the footprints of the past chemical evolution, many factors need to be taken into account to unveil the enrichment history and chemistry alone is of limited value in constraining the history of galaxies. Still, the set of data presented by Izotov & Thuan (1999) is probably unique with respect to data quality, homogeneity and the large number of atomic species included, making it a most valuable tool.