Star formation history of early-type galaxies in low density environments I. Nuclear line-strength indices

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 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 Angstrom < lambda < 5700 Angstrom at 2.1 Angstrom FWHM resolution. We measure 16 red (lambda > 4200 Angstrom) 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 (lambda < 4200) indices: namely Delta(4000 Angstrom) defined by Hamilton (1985), H+K(CaII) and H delta/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 (T > 10000 K) which may be used for extending W92 fitting functions toward high temperatures.