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Issue A&A
Volume 410, Number 2, November I 2003
Page(s) 461 - 470
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20031233



A&A 410, 461-470 (2003)
DOI: 10.1051/0004-6361:20031233

Galaxies in group and field environments: A comparison of optical-NIR luminosities and colors

M. Girardi1, F. Mardirossian1, C. Marinoni2, M. Mezzetti1 and E. Rigoni1

1  Dipartimento di Astronomia, Università degli Studi di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
    e-mail: mardiros,mezzetti,rigoni@ts.astro.it
2  Laboratoire d'Astrophysique de Marseille, Marseille, France
    e-mail: marinoni@astrsp-mrs.fr

(Received 19 May 2003 / Accepted 29 July 2003)

Abstract
We compare properties of galaxies in loose groups with those in field environment by analyzing the Nearby Optical Galaxy (NOG) catalog of galaxy systems. We consider as group galaxies objects belonging to systems with at least five members identified by means of the "friends of friends method", and, as field galaxies, all galaxies with no companions. We analyze both a magnitude-limited sample of 959 and 2035 galaxies (groups vs. field galaxies, respectively, B<14 mag, and 2000<cz<6000 km s -1) and a volume-limited sample ( $M_{B} <-19.01+5\,{\rm log_{10}}\,{h}$ mag, 2000<cz<4000 km s -1 369 group and 548 field galaxies). For all these galaxies, blue corrected magnitudes and morphological types are available. The cross-correlation of NOG with the 2MASS second release allow us to assign K magnitudes and obtain B-K colors for about half of the galaxies in our samples. We analyze luminosity and color segregation-effects in relation with the morphological segregation. For both  B and  K bands, we find that group galaxies are, on average, more luminous than field galaxies and this effect is not entirely a consequence of the morphological segregation. After taking into account the morphological segregation, the luminosity difference between group and field galaxies is about  $10\%$. When considering only very early-type galaxies ( T<-2) the difference is larger than  $30\%$. We also find that group galaxies are redder than field galaxies, $\Delta(B-K) \sim 0.4$ mag. However, after taking into account the morphological segregation, we find a smaller B-K difference, poorly significant (only at the c.l. of ~80%). We discuss our results considering that the analyzed groups define a very low density environment (projected mean density ~ 5-6  h 2 Mpc -2 galaxies).


Key words: galaxies: clusters: general -- galaxies: fundamental parameters -- galaxies: evolution -- cosmology: observations

Offprint request: M. Girardi, girardi@ts.astro.it




© ESO 2003

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