Astron. Astrophys. Suppl. Ser. 130, 251-265

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

I. Nuclear line-strength indices

M. Longhetti¹ - R. Rampazzo¹ - A. Bressan² - C. Chiosi³

Send offprint request: M. Longhetti

1 - Osservatorio Astronomico di Brera, Via Brera 28, I-20121, Milano, Italy
2 - Osservatorio Astronomico di Padova, Vic. dell'Osservatorio, 5, I-35122 Padova, Italy
3 - Dip. di Astronomia, Univ. di Padova, Vic. dell'Osservatorio, 5, I-35122 Padova, Italy

Received July 1; accepted December 8, 1997

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 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 Å $< \lambda <$ 5700 Å at 2.1 Å FWHM resolution. We measure 16 red ($\lambda \gt$ 4200 Å) 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 Å) 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.

Key words: galaxies: fundamental parameters -- galaxies: elliptical and lenticular -- galaxies: interactions -- galaxies: evolution