1 Introduction

Understanding galaxy evolution relies on large and homogeneous sets of data. Inhomogeneities introduced by stacking surveys with different depth to maintain comparable numbers of sources over a large dynamic range are an important limitation in testing those models of galaxy evolution that are designed to reproduce the number counts in different filters. Such models have been developed extensively over the last decade (see Koo & Kron 1992). Models incorporating luminosity evolution have been found to explain the number counts down to the faintest levels (Metcalfe et al. 1995; Pozzetti et al. 1996). As pointed out by Gardner (1998) however, the colour distribution contains more information about the state of evolution than the pure number counts, and the modeling of the colour distributions is a good test for the evolution models which explain number counts in individual filters. Up to now such modeling of galaxy colours has mostly been done for bright, nearby samples (Bertin & Dennefeld 1997) or for deep samples with small number statistics (Pozzetti et al. 1996; McCracken et al. 2000).

We carried out medium-deep surveys in the optical and near-infrared regimes. They cover one square degree and were performed in the optical B_j- and R-bands as well as with a near-infrared K filter. The 95% completeness limits in B_j, R, and K are $24.25~{\rm mag}$, $23.0~{\rm mag}$, and $17.5~{\rm mag}$, respectively. The near-infrared survey and results based exclusively on K-data have been presented in Kümmel & Wagner (2000) (hereafter Paper I). The complete coverage of the K-survey by the R-data, both in depth and in the large area, allows us to study the colour evolution of our K-selected as well as optically selected sample with a high statistical significance. We report on tests of the models proposed for galaxy evolution, specifically, we study whether they can reproduce the galaxy colours and their variation with brightness.

 

 

Table 1: The coordinates in the ecliptic, galactic and supergalactic systems and measures of ISM column density towards the NEP

$\alpha_{{\rm 2000}}$:	18h$\:00$m$\:00.0$s		$\delta_{{\rm 2000}}$:	$66^{\circ}33'38.6''$
$l_{\rm II}$:	$96\hbox{$.\!\!^\circ$ }38$		$b_{\rm II}$:	$29\hbox{$.\!\!^\circ$ }81$
SGL:	$33\hbox{$.\!\!^\circ$ }30$		SGB:	$38\hbox{$.\!\!^\circ$ }34$
HI-column-density		=	$3.9{-}4.7\ 10^{20}~$cm-2
IR-100-$\mu$-cirrus		=	1.8-3.0 MJy/steradian
EB-V		=	0.05 mag

One particular field of interest is to determine the surface densities of extremely red objects (EROs) in an intermediate range of magnitudes. EROs are objects with $R-K>5.0~{\rm mag}$ (Cimatti et al. 1999; Daddi et al. 2000) and include galactic and extragalactic populations. While the galactic EROs population are late type stars (M 6 or even later, see Leggett 1992; Wolf et al. 1998), there is no unique explanation for the extragalactic EROs. Among the different scenarios (possibly all of which contribute on some not yet determined level) are galaxies with an old stellar population at high redshift with a strong $4000~{\rm\AA}$ break. For a redshift of z>0.85 this break falls between the R- and the K- filter bandpasses, resulting in very red colours. Another suggestion for EROs are starburst galaxies or active galactic nuclei at a redshift 1<z<2. In this case reddening by interstellar dust alters the observed SEDs (Thompson et al. 1999). A third possibility, which is probably less important in the magnitude range covered in our survey are very distant quasars, where the Lyman break is redshifted to $\lambda>700~{\rm nm}$. In all cases the objects lie at moderate to high redshifts and give important clues on galaxy evolution and their star formation history. Evidence that the largest fraction of the extragalactic component of the EROs-population are high-z ellipticals comes from the high clustering amplitude suggested in recent surveys (Daddi et al. 2000).

The EROs search in our medium deep and medium wide survey at the NEP bridges the gap between the large area multi colour surveys like DENIS (Epchtein et al. 1997) or 2MASS (Skrutskie et al. 1998) and the deeper surveys on smaller fields, e.g. CADIS (Thompson et al. 1999; Huang et al. 2001), or Daddi et al. (2000). While we can continue the DENIS-search for low mass stars to larger distances (Delfosse et al. 1999), we can detect the bright end of the extragalactic EROs population, which are identified in deep surveys like CADIS.

The specific field used for our studies is the Northern Ecliptic Pole (NEP), which is special in having been surveyed intensively by scanning satellites (ROSAT, IRAS). Our deep counts shall be used to identify sources in deep X-ray and far-infrared surveys (Brinkmann et al. 1999; Hacking & Houck 1987) and study their broad band energy distribution.