Star formation history of early-type galaxies in low density environments

Abstract

This paper is the first of a series (Longhetti et al. 1997a,b) dedicated to the study of the star formation history in early-type galaxies which show fine structures and/or signatures of interaction . It presents nuclear line–strength indices for a sample composed of 21 shell galaxies, from the the Malin & Carter (1983) southern survey, and 30 members of isolated interacting pairs, from the Reduzzi & Rampazzo (1995) catalogue, located in low density environments . The spectral range covers 3700 A 5700 A at 2.1 A FWHM resolution. We measure 16 red ( 4200 A) indices defined by the Lick Group. Measures have been transformed into the Lick-IDS standard” system. The procedure has been tested on a set of 5 elliptical galaxies selected from the Gonzalez (1993) sample. We derive also three blue ( 4200) indices, namely Δ (4000 A) defined by by Hamilton (1985), H+K(CaII) and Hδ /FeI defined by Rose (1984, 1985). Blue indices are correlated to the age of the last starburst occurred in a galaxy (Leonardi & Rose 1996). The determination of these indices, the estimate of the measurement errors and the correction for the galaxies velocity dispersions are discussed in detail. In the Appendix A we present the indices for a set of hot stars ( K) which may be used for extending W92 fitting functions toward high temperatures.