4.4.7 The Chemical Abundances of BCGs
Blue compact galaxies (BCGs) include galaxies with the lowest measured abundances as derived from H II regions. Since both BCGs and dIs in general possess bright H II regions, many investigations of nebular abundances deal with objects of both classes, which in any case are not always very distinct. Very little is known about the metallicity of the stellar population of BCGs. Schulte-Ladbeck et al. (1999), using a colour magnitude diagram, derive [Fe/H] = -1.76 for the old population in VIIZw403, one of the most nearby BCGs, which has a previously reported oxygen abundance of 12 + log(O/H) 7.7 (Tully et al. 1980, Izotov and Thuan 1999); it is included in Fig. 1. Mas-Hesse and Kunth (1999), from the correlation between the strength of the stellar absorption lines from massive stars (C IV and Si IV) and O/H as obtained from the nebular lines, argue that the metallicity of the young stellar populations is similar to the gas metallicity.
The metallicity of BCGs was first addressed by Searle and Sargent (1972), who showed that the abundances of oxygen and neon in IZw18 and IIZw40 were sub-solar. Their work has been followed by numerous investigations over the years which have established their nature as metal-poor galaxies, as regards oxygen: Alloin et al. (1978), Lequeux et al. (1979), French (1980), Kinman and Davidson (1981), Kunth and Sargent (1983), Kunth and Joubert (1985), Campbell et al. (1986), Izotov et al. (1991), Peña et al. (1991), Pagel et al. (1992), Gallego et al. (1997) and others. A recent investigation of a large sample of 80 H II-galaxies can be found in Masegosa et al. (1994). Izotov and Thuan (1999) present O, N, Ne, S, Ar and Fe abundances for 50 BCGs, and in addition C and Si abundances for 7 of these, thereby constituting the largest homogeneous high quality source of information for metal-poor BCGs.
Figure 3. The metallicity distribution of BCGs/H II-galaxies from Terlevich et al. (1991), as measured by their oxygen abundances. Note the peak at ~ 1/10:th Z, which at least partly may be due to selection effects. (Courtesy R. Terlevich)
The abundances of heavy elements in these objects range between 1/2 Z and 1/50 Z, making them among the least chemically evolved objects in the universe. Figure 3 shows the distribution for oxygen abundance among H II galaxies. Oxygen is assumed to be representative of the total metallicity of the entire galaxy, although claims have been reported that this may only concern the H II region because of incomplete mixing (Kunth and Sargent 1986, Kunth et al. 1994, Roy and Kunth 1995, see Sect. 3.5). Figure 3 reveals that the oxygen abundance in this class of objects peaks slightly above 1/10 of the solar value. This peak may be related to selection effects since many surveys have selected galaxies with strong forbidden oxygen emission lines, which happen to be strongest for an oxygen abundance of around ten percent of the solar value. There is indeed another strong selection bias in Fig. 3 because it includes only galaxies with "measurable" electron temperatures (through the use of [O III]4363) hence restricts the sample to abundances lower than 12 + log[O/H] 8.5. The apparent low abundance cut-off is bounded by IZw18 (Sect. 5). The utter lack of known galaxies with abundances smaller than IZw18, despite concerted observational effort (see Sect. 6) has been a puzzle. Kunth and Sargent (1986) suggested that IZw18 could indeed be a primordial galaxy in which the observed H II regions have been self-enriched in the current burst. This idea has been tested by several distinct approaches, but we will see (Sect. 5.1.2) that there are many indications that IZw18 is in fact old.
According to Izotov and Thuan (1999), nine BCGs more metal-poor than 1/20 of solar exists. These are: IZw18, SBS 0335-052, SBS 0940+544, SBS 1159+545, UGC 4483, CG 389 (Izotov and Thuan 1999); CG 1116+51 (French 1980); Tololo 65 and Tololo 1214-277 (Pagel 1993). However another half dozen examples can be found in the litterature, with the most metal poor being UM 382 12 + log(O/H) = 7.45 (Masegosa et al. 1994). In Table 3 we list all known BCGs with oxygen abundances ~ 1/20 solar and below. As we discuss in Sect. 6.1.5 the number of very metal-poor galaxies will probably increase significantly in the near future. In total there are more than a dozen very metal-poor (Z 1/20 Z) BCGs known, while the number of BCGs with Z 1/10 Z is several times larger.