Peculiarities and populations in elliptical galaxies

Abstract

Morphological peculiarities, as defined from isophote asymmetries and number of detected shells, jets or similar features, have been estimated in a sample of 117 E classified galaxies, and qualified by an ad hoc index. The overall frequency of “peculiar” objects (Pec subsample) is 32.5%. It decreases with the cosmic density of the environment, being minimal for the Virgo cluster, the densest environment in the sampled volume. This environmental effect is stronger for galaxies with relatively large . The Pec subsample objects are compared with “normal” objects (Nop subsample) as regards their basic properties. Firstly, they systematically deviate from the Fundamental Plane and the Faber-Jackson relation derived for the Nop subsample, being too bright for their mass. Secondly, the dust content of galaxies, as estimated from IRAS fluxes, are similar in both subsamples. Third, the same is true of the frequency of Kinematically Distinct cores (KDC), suggesting that KDC and morphological peculiarities do not result from the same events in the history of E-galaxies. Using the Nop sample alone, we obtain very tight reference relations between stellar population indicators (, , , , Mg2, Hβ, , Mgb) and the central velocity dispersion . The discussion of the residuals of these relations allows us to classify the Pec galaxies in two families i.e. the YP or NGC 2865 family, and the NP or NGC 3923 one. Galaxies in the first group show consistent evidence for a younger stellar population mixed with the old one, in agreement with classical results (Schweizer et al. [CITE]; Schweizer & Seitzer [CITE]). The second group, however, has “normal“, or reddish, populations. It is remarkable that a fraction (circa 40%) of morphologically perturbed objects do not display any signature of a young population, either because the event responsible for the pecularity is too ancient, or because it did not produce significant star formation (or eventually that the young sub-population has high metallicity). A preliminary attempt is made to interpret the populations of Pec objects by combining a young Single Stellar Population with a Nop galaxy, with only limited success, perhaps largely due to uncertainties in the SSP indices used.