4.2.2 Abundances of dEs Outside the Local Group
Unfortunately, very little is known about the chemical abundances of dEs outside the Local Group. The M81 group is near enough that the study of colour magnitude diagrams is feasible with powerful telescopes. Caldwell et al. (1998) report on HST photometry of two dEs in the M81 group, concluding that their metallicities are similar to those of Local Group dEs with the same luminosity. Of course two data points is far too little to assess whether the Local Group dE population is typical of dEs in general, but this investigation nicely shows that it is feasible to obtain CMDs of dEs outside the Local Group.
Held and Mould (1994) obtained integrated spectra of 10 nucleated dEs in the Fornax cluster, and derived metallicities in the range [Fe/H] = -1.4 to -0.7. They found that the metallicities are tightly correlated with the UBV-colours. The range in luminosity is too small to deduce any relationship with the metallicity, but interestingly the median metallicity, < [Fe/H] > = -1.1, and luminosity, < MV > = -16.1 (assuming a distance of 18.6 Mpc for the Fornax cluster, Madore et al. 1999), is in perfect agreement with the metallicity-luminosity relation for local dEs of Caldwell et al. (1998). The tight correlation between metallicity and colour is quite remarkable since the large metallicity spread at a given luminosity indicates very different enrichment histories. The colour-metallicity relation also offers some hope to obtain metallicity information from photometry of distant galaxy populations. One should, however, keep in mind that the relation was established from a biased sample of nuclei of galaxies with uniform luminosity and morphology.
Although the Virgo cluster is known to be very rich in dEs, their metallicities are to a large extent unknown. Based on optical and near infrared colours Thuan (1985) concluded that Virgo cluster dEs had metallicities in the range: 1/3 Z to Z. These conclusions were based on comparisons with models that are now outdated. Since the possibility of using photometry for metallicity estimates is of interest for the study of distant faint galaxies we decided to reassess this finding. To do this we have compared the photometric data by Thuan (1985), Bothun and Caldwell (1984), James (1991, 1994), and Zinnecker and Cannon (1986) with a new set of models by Bruzual and Charlot (2000). Special care was taken when modelling galaxies with inhomogeneous data (e.g. different aperture sizes). When using simple stellar populations (where all stars have the same age and metallicity) and a standard Salpeter or Scalo IMF, the resulting metallicities are in the range from less than 1/10 Z to Z, with a median around 1/3 Z, and a typical age of 3 Gyr. Models with exponentially decreasing SFR (e-folding time 3 Gyr) produced slightly better fits with similar metallicity and a median age of 11 Gyr. Assuming a distance of 16.2 Mpc to the Virgo cluster (Macri et al. 1999) we find a median luminosity < MV > = -15.3, which with a median metallicity of < [Fe/H] > -0.5 means that the Virgo dEs are more metal rich by 0.5 dex as compared to the MV - Z relation for local dEs by Caldwell et al. (1998). Thus, either the Virgo cluster dEs are overabundant, or perhaps more likely, the photometrically derived metallicities are too high. We find no clear trend between metallicity and luminosity in this analysis.
In conclusion, the photometric data on Virgo dEs indicate that these galaxies have metallicities typically around 0.3 Z (in rough agreement with Thuan's earlier result). Today it would be possible to probe fainter systems using CCDs and modern near-IR arrays, although it is not yet clear how powerful a tool optical/near-IR colours are in deriving metallicities. Note that recent spectroscopy for six dEs in the Virgo cluster gives metallicities ranging from a few tenths of solar to solar, with small radial gradients as compared to giant ellipticals (Gorgas et al. 1997). Their median inferred [Fe/H] and luminosity is in rough agreement with the relation for local dEs by Caldwell et al. (1998). Spectroscopic metallicity determination of intrinsically faint dEs outside the Local Group is now feasible with 8-10m class telescopes, and should be pursued.