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8.2. Contribution to faint galaxy counts

The secondary star-formation episode in some of the Local Group dE's could have extended to redshifts as low as z approx 0.3. This suggests a possible link to the faint blue galaxy population that shows up in deep surveys. Several authors have suggested that dwarf galaxies form many (if not all) of their stars at late epochs (Silk et al. 1987; Babul and Rees 1992; Efstathiou 1992). Because of the uncertainties in the physics of star formation, these models do not make very detailed predictions. However, there are a few reasonably model-independent requirements that must be met if dwarf galaxies are to account for the majority of galaxies fainter than B approx 21.

At high redshifts, the B band samples the rest-frame UV and depends more on the star-formation rate than on the number of stars in the galaxy. At a redshift of z approx 0.4, a B magnitude of 24 corresponds to a star-formation rate of 5 Msmsun yr-1, for a Salpeter IMF. Even for a large dwarf galaxy, say 109 Msmsun, forming all its stars at a rate of 5 Msmsun yr-1 would require a relatively short burst of ~ 2 x 108 year duration. Less massive galaxies would require even shorter bursts. For comparison, the interval (0.3 < z < 1) incorporates a range of 4 Gyr in lookback time. This means that if dwarf galaxies are to supply the excess blue counts, they must form their stars in short bursts over a wide range of redshifts.

The widely varying star-formation histories of the Local Group dE's are perhaps consistent with this requirement, although galaxies even as bright as the Fornax dE are much lower in luminosity than those detected in the deepest redshift surveys. Cowie et al. (1991) estimate luminosities of 0.01 L* in the K band for the bulk of the galaxies at B = 24. This is closer to SMC luminosities than Local Group dE luminosities. However, the dE sequence in clusters continues up to approx 0.05 L*, so it may be that the lower luminosity dE's are just not showing up to the limits of the current redshift surveys. More problematical is that for the local dE's, the data suggest the star formation in the secondary episodes took place over a few Gyr (Mighell and Butcher 1992; van den Bergh 1994), star-formation rates lower than required by several orders of magnitude.