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4.2. Colors and spectra of cluster dE's

Given the strong evidence for intermediate-age and even young stars in Local Group dE's, it is worth asking whether cluster dE's show similar phenomena. Detecting intermediate age populations at the modest levels seen in most local dwarfs is virtually impossible without resolving the stellar population. However, there are numerous examples of dE-like galaxies with either slightly disturbed morphologies (Sandage and Binggeli 1984; Sandage and Hoffman 1991; Sandage and Fomalont 1993), unusual colors (Vigroux et al. 1985), or Balmer absorption features in their spectra (Bothun and Mould 1988; Gregg 1991). Clearly not all cluster dE galaxies have been inert for the last 10 Gyr.

In general, however, dE galaxies fall roughly on the extrapolation of the color-luminosity relation for giant ellipticals (Caldwell 1983; Caldwell and Bothun 1987; Prugniel et al. 1993). The colors of faint dE's are closer to those of globular clusters than giant ellipticals. Indeed, comparison to galactic globular clusters provides a possible test of whether the blue colors are entirely due to low metallicity, or whether youth also plays a role. To try to disentangle age and metallicity Thuan (1985) and Bothun et al. (1986) observed a subset of these dwarfs in the infrared J, H and K bands. Thuan concluded that the Virgo dE's have metallicities Zsmsun / 3 leq Z leq Zsmsun and ages since the last burst of star formation of 1 to 8 Gyr. However, the conclusion that dE's typically have an intermediate-age population is based on a rather dated set of models computed for stellar populations of solar metallicity (Struck-Marcell and Tinsley 1978). Bothun et al. (1985) noted that while dE's for the most part fall on the globular-cluster locus in the B - H vs. J - K plane, a few have bluer B - H colors than globular clusters with the same J - K, suggesting younger ages.

Ferguson (1994) compiled line strengths and colors for the rather heteroeneous sample available in the literature and compared dE's, giant E's, and globular clusters to two recent population synthesis models (Worthey 1992; Bressan et al. 1994). The zeroth-order question addressed was whether the majority of stars in dE's could be younger than those in globular clusters. While dE's and globular clusters show significant differences in several of the line-strength-color relations plotted (cf. Ferguson 1994, Fig. 2), the differences are not easily attributable to age; they are more likely due to calibration problems, selection effects, or deficiencies in the population-synthesis models used for comparison. In most diagrams, the dE's follow the globular cluster locus, albeit with much scatter.

Held and Mould (1994) obtained spectra for 10 nucleated dE's in the Fornax cluster and examined both kinematics and stellar populations. Their most robust result is that dE colors (UBV) are tightly correlated with metallicities derived from line strengths. For galaxies in a narrow range of luminosities, they see metallicities that vary over 0.7 dex, with no clear correlation with either luminosity or surface brightness. They echo the findings of Bothun and Mould (1988) that (nucleated) dE's show a larger range of Balmer-line strengths at fixed metallicity than globular clusters. Galaxies with strong Balmer absorption are thought to have intermediate age populations. However, the dE's basically follow the globular-cluster locus in the Hbeta vs. [Fe/H] plane, and, interestingly, both Bothun and Mould (1988) and Held and Mould (1994) find a few dE's with Balmer lines significantly weaker than globular clusters of the same metallicity. Possible explanations include different initial mass functions or horizontal branch morphologies for those galaxies. As the metallicities overlap the range where the ``second parameter effect'' operates in globular clusters (see Fusi Pecci et al. 1993 for a recent discussion), variations in HB morphology are not inconceivable and could in principle account for both strong and weak Balmer lines without requiring intermediate-age stars.

In summary, only a few cluster dE's (about 10%) show significant evidence for a young or intermediate-age population from their spectra or colors. Most of the dE's could have ages of the bulk of their population ranging anywhere from about 5 Gyr to the age of globular clusters.

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