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


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

V. Blue line-strength indices for the nuclear region *

M. Longhetti 1, A. Bressan 2, C. Chiosi  **  3 and R. Rampazzo 4

1 Institut d'Astrophysique, 98 bis Boulevard Arago, F-75014 Paris, France
2 Padua Astronomical Observatory, Vicolo dell'Osservatorio 5, I-35122 Padua, Italy
3 Astronomy Department, University of Padua, Vicolo dell'Osservatorio 5, I-35122 Padua, Italy
4 Brera Astronomical Observatory, Via Brera 28, I-20121 Milan, Italy

Received 15 December 1998 / Accepted 2 March 1999

Abstract

We analyze the star formation properties of a sample of 21 shell galaxies and 30 early-type galaxies members of interacting pairs, located in low density environments (Longhetti et al. 1998a, 1998b).

The study is based on new models developed to interpret the information coming from `blue' H[FORMULA]/FeI, H+K(CaII) and [FORMULA]4000 line-strength indices proposed by Rose (1984; 1985) and Hamilton (1985).

We find that the last star forming event that occurred in the nuclear region of shell galaxies is statistically old (from 0.1 up to several Gyr) with respect to the corresponding one in the sub-sample of pair galaxies ([FORMULA] Gyr or even ongoing star formation).

If the stellar activity is somehow related to the formation of shells, as predicted by several dynamical models of galaxy interaction, shells have to be considered long lasting structures.

Since pair members show evidence of very recent star formation, we suggest that either large reservoirs of gas have to be present to maintain active star formation, if these galaxies are on periodic orbits, or most of the pair members in the present sample are experiencing unbound encounters.

Key words: galaxies: elliptical and lenticular, cD – galaxies: evolution – galaxies: formation – galaxies: fundamental parameters – galaxies: interactions – galaxies: starburst

* Table 2 is available in electronic form only, at CDS: via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/345/419
** Visiting Scientist, Max-Planck-Institut für Astrophysik, K-Schwarzschild-Strasse 1, D-87540 Garching bei München, Germany

Send offprint requests to: M. Longhetti

© European Southern Observatory (ESO) 1999

Online publication: April 19, 1999

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