Peculiarities and populations in elliptical galaxies. III. Dating the last star formation event
Abstract
Using 6 colours and 4 Lick line-indices we derive two-component models of the populations of ellipticals, involving a "primary" and a "juvenile" population. The first component is defined by the regressions of indices against the central velocity dispersion found in Papers I and II for the {Nop} sample of non-peculiar objects. The second one is approximated by an SSP, and the modeling derives its age A, metallicity Z and fractional V-luminosity q_V, the fractional mass qM being found therefrom. The model is designed for "blueish" peculiar galaxies, i.e. the {Pec} sample and NGC 2865 family in the terminology of Paper I. The morphological peculiarities and the population anomaly are then believed to involve the same event, i.e. a merger plus starburst. It is possible to improve the models in a few cases by introducing diffuse dust (as suggested by far IR data), and/or by taking into account the fact that Lick- and colour indices do not relate to identical galaxy volumes. In most of the cases, the mass ratio of young stars qM seems too small for the product of a recent major merger: the events under consideration might be minor mergers bringing "the final touch" to the build-up of the structure of the E-type object. The same modeling has been successfully applied to blueish galaxies of the {Nop} sample, without morphological peculiarities however, to support the occurence of a distinct perturbing event. A few reddish objects of the {Pec} sample (NGC 3923 family) and of the {Nop} sample are also modeled, in terms of an excess of high metallicity stars, or diffuse dust, or both, but the results are inconclusive.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- April 2006
- DOI:
- 10.1051/0004-6361:20053923
- Bibcode:
- 2006A&A...449..519M
- Keywords:
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- galaxies: elliptical and lenticulars;
- CD