Issue A&A Volume 463, Number 2, February IV 2007 Page(s) 427 - 443 Section Cosmology (including clusters of galaxies) DOI 10.1051/0004-6361:20066031

A&A 463, 427-443 (2007)
DOI: 10.1051/0004-6361:20066031

Planar distribution of the galaxies in the Local Group: a statistical and dynamical analysis

S. Pasetto and C. Chiosi

Department of Astronomy, Padova University, Vicolo dell'Osservatorio 2, 35122, Padova, Italy
    e-mail: [pasetto;chiosi]@pd.astro.it

(Received 13 July 2006 / Accepted 17 November 2006 )

Abstract
Aims.Basing on the projected distribution of the galaxies in the Local Group, Sawa & Fujimoto found that they all seem to distribute on a rather thin plane containing the two mayor local galaxies of the Local Group, Milky Way and Andromeda, and all their dwarf satellites. As their conclusion could be severely biased by projectional distortion effects, we re-analyse the whole issue using a different approach. In brief, adopting known data on positions and distances, we make use of the analytical geometry and look for the plane that minimizes the distances of all galaxies to it. A planar distribution is indeed found that, however, does not coincide with the plane found by Sawa & Fujimoto. Why? The second part of this study is devoted to answer this question and to find a dynamical justification for the planar distribution.
Methods.To this aim, we apply the Hamilton Method (Minimum Action) to investigate the dynamics of the two major system of the Local Group, Milky Way and Andromeda, under the action of external forces exerted by nearby galaxies or groups external to the Local Group.
Results.We find that the planar distribution is fully compatible with the minimum action and that the external force field is likely parallel to the plane. It pulls the galaxies of the Local Group without altering their planar distribution. Special care is paid to evaluate the robustness of this result.
Conclusions.In this paper we have examined the spatial distribution of galaxies in the Local Group. They are confined to a plane that can be statistically and dynamically understood as the result of the Minimum Action. The planar distribution seems to be stable for a large fraction of the Hubble time. The external force field, that has likely been constant over the same time interval, does not alter the planar distribution as it is nearly parallel to it. Effects due to undetected halos of sole Dark Matter are briefly discussed. They could be a point of uncertainty of the present study.

Key words: stellar dynamics -- Local Group -- galaxies: dwarf

© ESO 2007

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