Astron. Astrophys. 346, L25-L29 (1999)

Letter to the Editor

Effect of the large scale environment on the stellar content of early-type galaxies ^*

Ph. Prugniel ¹, V. Golev ^1,2 and G. Maubon <sup>1

1</sup> CRAL-Observatoire de Lyon (CNRS: UMR 142), F-69561 St-Genis-Laval Cedex, France
² Department of Astronomy and Astronomical Observatory, University of Sofia, P.O. Box 36, BG-1504 Sofia, Bulgaria

Received 25 March 1999 / Accepted 17 April 1999

Abstract

In order to study the influence of the environment on the stellar population of early-type galaxies we have analyzed the relations between the density of the environment and the residuals to the - relation, , and between and the residuals to the Fundamental Plane, . Our sample of galaxies covers the range of densities, between field galaxies and poor clusters. We confirm the existence of a slight environmental bias on the mean stellar population and we show that this effect is due to a small fraction of galaxies containing a younger stellar sub-population while the majority of galaxies, in any environment, are equally old. The galaxies which are likely to contain a young stellar sub-population (negative and ) are preferentially found in the less dense environment.

This segregation of the stellar population may reflect the morphology-density relation or may result from a higher rate of recent star formation activity in sparse environments. Using the rotational support, , to assess the presence of a disk and to parameterize the morphology, we subtracted the contribution of the morphological segregation to population segregation. It is not yet possible to rule out that the morphology-density relation is at the origin of the population-density relation.

Key words: galaxies: elliptical and lenticular, cD galaxies: formation galaxies: fundamental parameters galaxies: general galaxies: stellar content

* Based on observations collected at the Observatoire de Haute-Provence

Send offprint requests to: Ph. Prugniel (prugniel@galaxies.univ-lyon1.fr

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Contents

• 1. Introduction
• 2. Analysis
  • 2.1. Parmeterizing the density
  • 2.2. Residuals to the - relation
  • 2.3. Residuals to the fundamental plane
• 3. The origin of the population segregation
  • 3.1. Evidence for recent merging events and star formation triggered by gravitational encounters
  • 3.2. Subtracting the morphological segregation
• 4. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999
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