EDP Sciences
Free access
Issue
A&A
Volume 449, Number 2, April II 2006
Page(s) 519 - 532
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20053923
A&A 449, 519-532 (2006)
DOI: 10.1051/0004-6361:20053923

Peculiarities and populations in elliptical galaxies

III. Dating the last star formation event
R. Michard

Observatoire de Paris, LERMA, 77 Av. Denfert-Rochereau, 75015 Paris, France
    e-mail: raymond.michard@obspm.fr

(Received 26 July 2005 / Accepted 17 November 2005 )

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 ${\it Nop}$ 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 qV, the fractional mass qM being found therefrom. The model is designed for "blueish" peculiar galaxies, i.e. the ${\it Pec}$ 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 ${\it Nop}$ sample, without morphological peculiarities however, to support the occurence of a distinct perturbing event. A few reddish objects of the ${\it Pec}$ sample (NGC 3923 family) and of the ${\it Nop}$ sample are also modeled, in terms of an excess of high metallicity stars, or diffuse dust, or both, but the results are inconclusive.


Key words: galaxies: elliptical and lenticulars, CD

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© ESO 2006

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