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Mem. S.A.It. Vol. 74, 496 c

° SAIt 2003 Memorie

della

On the star formation history of galaxies in rich clusters ?

M. Massarotti, F. La Barbera, P. Merluzzi, and G. Busarello

INAF, Osservatorio Astronomico di Capodimonte, Napoli, Italy

Abstract. The analysis of the Colour Magnitude Relation (CMR) of cluster galax- ies at different cosmological epochs is an effective tool to gain insight into the star formation history of galaxies in dense environments (Bower et al. 1992; Bower et al. 1998; Kodama & Bower 2001). By comparing the optical-NIR CMR (R−K, K) and the colour-colour (V − I, R − K) distribution of galaxies in the rich cluster AC 118 at z = 0.31, with the optical-NIR CMR (V − K, K) of Coma (z = 0.023), we find that half of the stars in galaxies form at z > 2, while star formation still continues at intermediate redshifts: 20% of the stars are formed at z < 1.

Key words. galaxies: evolution – galaxies: fundamental parameters – galaxies:

clusters: general

1. Data and models

The sample used for the present analysis consists of 252 galaxies in the cluster AC 118 at z = 0.31. The optical and infrared data are taken from Busarello et al. (2002) and from Andreon (2001) respectively. The sample is selected according to the follow- ing criteria: (1) galaxies are cluster mem- bers according to their photometric red- shift (Busarello et al. 2002; Massarotti et al. 2001) and (2) the sample is complete in the K−band (K = 18.25). Since a com- plete sample of cluster galaxies in the NIR was not available at z ∼ 0, we built a sim- ulated sample based on the Coma cluster,

Send offprint requests to: M. Massarotti

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Based on data taken at the European Southern Observatory

Correspondence to: INAF, Osservatorio Astro- nomico di Capodimonte, Via Moiariello 16, I- 80131 Napoli, Italy

from the data by de Propris et al. (1998) and Bower et al. (1992).

We introduce models of galaxy evolution based on the code of Bruzual & Charlot (1993). Each model is defined by three physical parameters: the metallicity Z, the time scale τ of the (exponential) Star Formation Rate (SFR), and the age T. In this way, any given value of (Z,T,τ ) may be related to a point in the colour-magnitude spaces at z ∼ 0 and z ∼ 0.3, and vice versa.

First, we looked for the ‘best’ value of

(Z,T,τ ) that minimizes the distance be-

tween observed and predicted magnitudes

for each galaxy of AC 118. As second step,

we used the ‘best’ parameters to predict

the distribution of AC 118 galaxies in the

V − K vs. K diagram at the redshift of

Coma. The model distributions at z ∼ 0

were compared with the distribution of the

local sample and the best model was found

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M. Massarotti et al.: Star formation history of cluster galaxies 497 by minimizing the differences of occurring

frequencies in the (V − K, K) plane.

Fig. 1. Distribution in colour-magnitude space of the observed cluster galaxies (open circles and solid line) compared with the

‘best fit’ model (filled circles and dashed line).

Fig. 2. Fraction of stellar mass already formed at a given redshift (solid line and dots).

2. Star formation history

The observed distributions and the best model are shown in Fig. 1. By using the values of (Z,T,τ ) of the best model we ob- tain the star formation history shown in Fig. 2 (see Merluzzi et al. 2002). We con- clude that half of the stars were born at high redshifts (z > 2), while star formation in cluster galaxies continues until interme- diate redshifts: 20% of the stars are formed at z < 1.

References

Andreon, S. 2001, ApJ, 547, 623

Bower, R. G., Lucey, J. R., & Ellis, R. S.

1992, MNRAS, 254, 601

Bower, R. G., Kodama, T., & Terlevich, A.

1998, MNRAS, 299, 1193

Bruzual, G. A., & Charlot, S. 1993, ApJ, 405, 538

Busarello, G., et al. 2002, A&A, 389, 782 de Propris, R. et al. 1998, ApJ, 503, L45 Kodama, T., & Bower, R. G. 2001,

MNRAS, 321, 18

Massarotti, M. et al. 2001, A&A, 368, 74 Merluzzi, P. et al. 2002, ApJ, submitted

(astro-ph/0206403)

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