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Astron. Astrophys. 353, 873-886 (2000)

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Gravo-thermal properties and formation of elliptical galaxies *

I. Márquez 1, G.B. Lima Neto 2,3, H. Capelato 4, F. Durret 3,5 and D. Gerbal 3,5

1 Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
2 Instituto Astronômico e Geofísico/USP, Av. Miguel Stefano 4200, São Paulo/SP, Brazil
3 Institut d'Astrophysique de Paris, CNRS, 98bis Bd Arago, 75014 Paris, France
4 Instituto de Pesquisas Espaciais, São José dos Campos/SP, Brazil
5 DAEC, Observatoire de Paris, Université Paris VII, CNRS (UA 173), 92195 Meudon Cedex, France

Received 21 September 1999 / Accepted 23 November 1999

Abstract

We have analyzed a sample of galaxies belonging to three clusters: Coma, Abell 85, and Abell 496 (real galaxies) and a sample of simulated elliptical galaxies formed in a hierarchical merging scheme (virtual galaxies). We use the Sérsic law to describe their light profile. The specific entropy (Boltzmann-Gibbs definition) is then calculated supposing that the galaxies behave as spherical, isotropic, one-component systems. We find that, to a good approximation ([FORMULA]%), both real and virtual galaxies have an almost unique specific entropy. Within this approximation the galaxies are distributed in a thin plane in the space defined by the three Sérsic law parameters, which we call the Entropic Plane. A further analysis shows that both real and virtual galaxies are in fact located on a thin line, therefore indicating the existence of another - and yet unknown - physical property, besides the uniqueness of the specific entropy.

A more careful examination of the virtual galaxies sample indicates a very small increase of their specific entropy with merging generation. In a hierarchical scenario, this implies a correlation between the specific entropy and the total mass, which is indeed seen in our data. The scatter and tilt of the Entropic Line, defined by Lima Neto et al. (1999a), are reduced when this correlation is taken into account. Although one cannot distinguish between various generations for real galaxies, the distribution of their specific entropy is similar to that in the virtual sample, suggesting that hierarchical merging processes could be an important mechanism in the building of elliptical galaxies.

Key words: gravitation – methods: N-body simulations – galaxies: clusters: individual: Abell 496 – galaxies: clusters: individual: Abell 85 – galaxies: clusters: individual: Coma – galaxies: formation

* Based on observations collected at the Canada France Hawaii Telescope and at the European Southern Observatory, La Silla, Chile

Send offprint requests to: I. Márquez (isabel@iaa.es)

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© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000
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