We end this review by summarizing the star formation properties of early-type dwarfs, most notably of dwarf ellipticals (dEs) and dwarf spheroidals (dSphs; see Grebel 2003; Grebel et al. 2003). Typically located in high-density regions such as the immediate surroundings of massive galaxies or in galaxy clusters, this dense environment may have affected the evolution of these now gas-deficient galaxies (Section 3.3). Structural and kinematic studies suggest that early-type dwarfs are strongly dark-matter dominated (e.g., Odenkirchen et al. 2001b; Klessen et al. 2003; Wilkinson et al. 2004; Koch et al. 2007a, Koch et al. 2007b; Walker et al. 2007; , Gilmore et al. 2007; Wolf et al. 2010), and there are even indications of a constant dark-matter halo surface density from spirals to dwarfs (Donato et al. 2009). However, it is not yet clear whether the apparently constant total mass regardless of a dwarf's baryonic luminosity is universal (e.g., Adén et al. 2009).
All of dEs and dSphs studied in detail so far reveal varying fractions of old populations (Grebel 2004; Da Costa et al. 2010) that become dominant at low galactic masses, while intermediate-age populations (> 1 Gyr) are prominent at higher galactic masses. Still, no two dwarfs share the same evolutionary history or detailed abundance properties (Grebel 1997). Population gradients are found in many early-type dwarfs. Where present, younger and/or more metal-rich populations tend to be more centrally concentrated (e.g., Harbeck et al. 2001; Lisker et al. 2006b; Crnojevic et al. 2010).
The Galactic dSphs reach solar
[
/Fe] ratios at much lower
[Fe/H] than typical Galactic halo stars, which suggests low star
formation rates, the loss of metals and supernova ejecta, and/or a
larger contribution from SNe Ia (e.g.,
Shetrone et
al. 2001).
Abundance spreads of 1 dex in [Fe/H] and more are common. The scatter in
element abundance
ratios at a given
metallicity underlines the inhomogeneous, localized enrichment in the
early-type dwarfs, another characteristic expected of slow, stochastic
star formation and low star formation efficiencies
(Koch et al. 2008a,
Koch et al. 2008b;
Marcolini et
al. 2008).