Astron. Astrophys. 353, 479-486 (2000)

1. Introduction

In the optical band, the cluster luminosity function (hereafter LF) has three regimes: a bright end ( mag, km s^-1 Mpc^-1), a flat slope () down to luminosity of dwarf galaxies (López-Cruz et al. 1997; Garilli et al. 1999), and then a steep increase (Impey et al. 1988; Ferguson 1989; Thompson & Gregory 1993; Secker & Harris 1996; Secker et al. 1997). Often a dip is found in the otherwise flat part of the LF (see, e.g., Godwin & Peach 1977; Bucknell, Godwin & Peach 1979). A few well studied clusters (Smith et al. 1997), as well as a number of LFs published a long time ago (Schechter 1976) display an intermediate slope () instead of a flat LF.

The LF represents the zero-order statistics of galaxy samples and gives the relative number of galaxies as a function of the magnitude. Almost every quantity is, therefore, "weighted" by the LF, including obvious quantities, such as the galaxy color distribution, and also less obvious ones, such as correlations involving the luminosity (see, for example, the discussion on the impact of magnitude limits in the size-luminosity relation by Simard et al. 1999). When the sample is not complete in volume a further "weight" should be added: the selection function. Thus, an accurate knowledge of the LF is important when comparing galaxies of different luminosities at different redshifts.

From a physical point of view, the optical LF is the convolution of the number of galaxies of a given mass with their M/L distribution. Then, any measure of the optical LF traces a complex mix of galaxy mass and M/L distributions, so that evolution in luminosity or mass could not be easily disentangled from the measurement of the optical LF. A better estimate of the galaxy mass than the optical luminosity will certainly help to separate the two dependencies. Such a measure has a particular relevance in the determination of the density of the Universe: a possible way to proceed is to compute the cluster mass per unit luminosity times the Universe luminosity density. As stressed by Carlberg et al. (1996), this calculation assumes that cluster galaxies have the same LF as field galaxies. Observations suggest instead that galaxies change their optical luminosity during their infall in the cluster (see, for example, Bothun & Dressler 1986; Andreon 1996, and most of the papers by the CNOC collaboration, such as Balogh et al. (1998) and references therein), although the amplitude and the sign of the luminosity variation is not yet settled. Of course, it would be preferable to measure the M/L of clusters using a luminosity indicator weakly affected by possible bursts or halt of star formation induced by interactions with the hostile cluster environment.

The near-infrared luminosity has several advantages with respect to optical luminosities. It is tightly correlated to the galaxy mass (at least for spirals, Gavazzi et al. 1996) and, with respect to the optical luminosity, it is less affected by short and recent star formation events (Bruzual & Charlot 1993), possibly induced by interactions, and by dust absorption. Therefore, the near-infrared LF traces more directly the mass function and gives a Universe density less affected by possible systematic errors due to a differential star formation history between galaxies in clusters and in the field.

There are several additional advantages in observing galaxies in the near-infrared: K corrections are relatively small and well known, thus allowing to observe and to compare galaxies at different redshifts, up to high redshift values. In particular, K corrections are almost independent from the spectral type of galaxies, in such a way that statistics on a population of galaxies are less affected by changes of the morphological composition induced by differential corrections from type to type. Furthermore, galaxies that undergo a starburst are not selected preferentially, as instead happens in the optical, and therefore a sample selection in the near-infrared is less biased by episodic events of star formation.

It is therefore important to measure the near-infrared LF of clusters of galaxies over a magnitude range as wide as possible, in particular to characterize the properties of galaxies in the local Universe. So far, the near-infrared LF have been measured for a few clusters, but to bright limiting magnitudes (Barger et al. 1996, Trentham & Mobasher 1998, De Propris et al. 1999), and on a portion of the Coma cluster (De Propris et al. 1998), down to relatively faint magnitudes. According to De Propris et al. (1998), the Coma LF shows a flat slope, and a step increase () at faint magnitudes ( mag). However, this is presently the only LF determination attaining intermediate magnitudes, and such a survey could be improved in several respects. It is important to extend the study to other regions, and to reach deeper magnitudes. This is the aim of the present paper.

We present the near-infrared LF of the Coma cluster, based on independent observations, fully documented in a companion paper that also presents the photometric catalog. With respect to De Propris et al. (1998), this study has been performed on a different portion of the Coma cluster, slightly overlapping with their one, over an area which is 40% smaller, but it attains one magnitude deeper. All along this paper, we adopt km s^-1 Mpc^-1 and q₀= 0.1.

© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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