Line-Strength Indices in Bright Spheroidals: Evidence for a Stellar Population Dichotomy between Spheroidal and Elliptical Galaxies

Abstract

We present new measurements of central line-strength indices (namely Mg2, <Fe>, and Hbeta and gradients for a sample of 6 bright spheroidal galaxies (Sph's) in the Virgo cluster. Comparison with similar measurements for elliptical galaxies (E's), galactic globular clusters (GGC's), and stellar population models yield the following results: (1) In contrast with bright E's, bright Sph's are consistent with solar abundance [Mg/Fe] ratios; (2) Bright Sph's exhibit metallicities ranging from values typical for metal-rich GGC's to those for E's; (3) Although absolute mean ages are quite model dependent, we find evidence that the stellar populations of some (if not all) Sph's look significantly younger than GGC's; and (4) Mg2 gradients of bright Sph's are significantly shallower than those of E galaxies. We conclude that the dichotomy found in the structural properties of Sph and E galaxies is also observed in their stellar populations. A tentative interpretation in terms of differences in star formation histories is suggested.