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

Abstract

The present paper is a companion of two others dedicated one to the measurement of the line–strength indices (Longhetti et al. 1997a) and the second to trace back the star formation history of a sample of early–type galaxies by comparing observed indices to the predictions of new spectro–photometric models (Longhetti et al. 1997b). The sample of 51 early-type galaxies in low density environments is composed of two sub-sets of galaxies: 21 shell galaxies from the Malin & Carter (1983) catalogue (one of which shows double nucleous and has been considered as two separate objects) and 30 members of isolated interacting pairs from the Reduzzi & Rampazzo (1995) catalogue. Most of the objects show fine structures. The paper collects nuclear kinematic data together with the velocity and velocity dispersion curves of the stellar and gaseous components as a function of the distance from the galaxies centres. The galaxies heliocentric systemic velocity compares within km s-1 with RC3 data, while their central velocity dispersion compares within km s-1 , km s-1 and km s-1 with Gonzalez (1993), Davies et al. (1987) and Carter et al. (1988) respectively. The detailed comparison between our velocity and velocity dispersion curves and those from several authors is discussed. 9 out of 22 shell galaxies nuclei show emission lines, 4 of which, using data in the literature, have line ratios characteristic of LINERs. 10 members of pairs out of 30 show emission lines. RR 331a has a Seyfert like nucleus, while for the remaining galaxies the ([O III] λ 5007)/Hβ ratio is characteristic of low ionization regions. In a small fraction of the objects the emission component is detectable outside the central value. None of the objects in the sample shows counter-rotation of the gaseous versus the stellar component. The two components appear associated, although, in two cases there is evidence that gas and stars lie on different planes. This latter phenomenon could be associated to accretion events. Emission lines in the central part of the RR 331a show a secondary component in the emission lines profile. E 2400100 has two nuclei embedded in the main body of the galaxy. The U-shape profile of the stellar velocity profile shows the ongoing interaction of the two nuclei. profile of shell galaxies is, finally, discussed in relation to the hypothesis of the accretion/merging origin of these galaxies.