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Astron. Astrophys. 345, 419-429 (1999)

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1. Introduction

This paper is a sequel of a series dedicated to studying the star formation history of early type galaxies showing shells, ripples, fine structures etc. which are taken as signatures of past interactions (see Malin & Carter 1983; Schweizer 1992; Reduzzi et al. 1996) and complex star formation histories.

It is commonly accepted that dynamical interactions between galaxies leading to the formation of shell structures are in general accompanied by star formation if enough gas is at disposal to the interacting system. However, there are different views of this subject that emerge from dynamical models. In fact, numerical simulations of merging/accretion events with SPH codes, in which shell structures can develop, yield contrasting predictions as far as the accompanying stellar activity is concerned. The models by Kojima & Noguchi (1997) suggest that star formation in the satellite is turned off well before the shells develop. This explains why we see in shell galaxies signatures of post-star-burst. In contrast, the models by Weil & Hernquist (1993) show that star formation occurs (in the galaxy center) while the shell structure is forming. In both cases the shell structure lasts for time scales shorter than [FORMULA] Gyr.

In alternative to the merging/accretion hypothesis (cf. Barnes 1996 and references therein), dynamical simulations show that shell structures could also originate from "weak interaction" between galaxies (Thomson 1991 and reference therein). In this case, shells are expected to last much longer than with the previous alternative.

The first paper of the series (Longhetti et al. 1998a) presented the sample of galaxies to be examined, i.e. 21 shell galaxies and 30 members of isolated interacting pairs located in low density environments and derived for their nuclear region several line strength indices defined in the spectral range 3700 Å [FORMULA]5700 Å. The indices were further splitted in two groups: red indices ([FORMULA] 4200 Å) and blue indices ([FORMULA] 4200 Å). The red indices (H[FORMULA], Mg1, Mg2, etc..., 16 indices in total) are according to the definition by Worthey (1992) and Worthey et al. (1994) and have been transformed into the Lick-IDS standard system, whereas the blue indices are [FORMULA]4000 Å, H+K(CaII) and H[FORMULA]/FeI according to the definition by Rose (1984, 1985), Leonardi & Rose (1996) and Hamilton (1985). Finally, the spectra used to calculate the indices had a 2.1 Å FWHM resolution.

In the second paper (Longhetti et al. 1998b) the kinematics of the gaseous and stellar components of the galaxies under investigation were properly studied to correct the line-strength indices for velocity dispersion.

With the aid of this material, a thorough comparison of the above indices with those predicted by the fitting functions of several authors (Worthey 1992; Gorgas et al. 1993; Worthey et al. 1994; Idiart & de Freitas Pacheco al. 1995; Buzzoni et al. 1992, 1994) was made by Longhetti et al. (1998c) in a subsequent study. Furthermore, the properties of the galaxies in the sample were compared with those obtained by Gonzalez (1993), with particular attention to the distribution on the H[FORMULA] vs. [MgFe] plane. The main result was that no difference could be detected in this plane between interacting and post-interacting galaxies. In particular, Longhetti et al. (1998c) discussed the influence of a secondary burst of star formation induced by interaction/accretion events.

In the present paper we will investigate the capability of the blue indices in unraveling the strength and age of the last star forming episode. Indeed determining the strength and age of this episode would bear very much on our understanding of the formation of shell galaxies and the relationship between shell formation and star formation events.

The paper is organized as follows. Sect. 2 summarizes the basic properties of the "blue" indices. Sect. 3 presents models of Simple Stellar Populations (SSP) and describes how we calculate the line-strength indices by amalgamating the medium resolution empirical library of stellar spectra by Jacoby et al. (1984, J84) with the library of low resolution theoretical spectra by Kurucz (1992). In addition to this, Sect. 3 compares the line strength indices directly calculated from spectra with those obtained from fitting functions (see also Longhetti et al. 1998c). Sect. 4 presents the line-strength indices for composite stellar populations (CSP), i.e. model of galaxies in which a young stellar component is added to an old one in different proportions. This would mimic a recent burst of stellar activity taking place in an old galaxy (Leonardi & Rose 1996). The effects of emission lines and metallicity on the intensity of the blue indices are also discussed in some detail. The results of these simulations and how they compare with other similar models in literature are discussed in Sect. 5. Finally, some implications for dynamical models of shell formation and galaxy evolution are discussed in Sect. 6.

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© European Southern Observatory (ESO) 1999

Online publication: April 19, 1999
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