7 Summary

We have performed a wide-field, medium deep survey in the filters B_j, R and K with completeness limits (95%) 24.25, 23.0 and $17.5~{\rm mag}$, respectively in the central square degree around the Northern Ecliptic Pole.

We have derived the object number counts for point-like and extended sources in B_j and R and compared our results for the extended sources to other galaxy number counts found in the literature by fitting a power law to the data. In both filters we confirm the slope in d$\,$log$\,N$/dm found in other, smaller surveys with values around 0.48and 0.37 in B_j and R, respectively. Differences in the absolute numbers, represented by the amplitude of the power law fits, can largely be attributed to galactic extinction and the limited accuracy of reddening corrections.

We have determined the colour distribution of galaxies in B_j-R and R-K in a large range of 10 mag down to B_j=24.0 and $K=18.0~{\rm mag}$, respectively. In B_j-R the median colour remains constant to $B_j=22.3~{\rm mag}$and become bluer to fainter levels. This trend to bluer colours marks the onset of the so called "faint blue galaxies" (Ellis 1997). In R-K the galaxies become redder from a medium colour R-K=2.6 at $K=12~{\rm mag}$ to R-K=3.8 at $K=17.8~{\rm mag}$.

For the filter system used in our survey we have given lower limits for the expected colours (in B_j-R) of Lyman-break galaxies at $z\sim 3.8$. We derive the surface densities for candidates found in our survey. From the reddest objects in B_j-R found in our survey we have derived the lower limit for Lyman-break galaxies at z>4to $M_{{\rm AB}}(12\,280~{\rm\AA})>-25.0~{\rm mag}$(for q₀, $\lambda_0$, h₀ = 0.5, 0.0, 0.5).

We have determined the surface density of red objects ( $R-K>5.0~{\rm mag}$) down to $K=17.5~{\rm mag}$ on the basis of our large field of view ( $0.9~{\rm deg}^2$). Since we are unable to determine the morphology of most of the objects, the surface densities include late type stars (M 6 and later) as well as the extragalactic EROs. At the bright end ( $K<16.5~{\rm mag}$), which has reliable morphological classification, point-like objects dominate the sample. While the surface density of objects with R-K>6 declines by a factor of twenty from $20.0~{\rm mag}$ to $19.0~{\rm mag}$ (Eisenhardt et al. 1999; Thompson et al. 1999; Daddi et al. 2000), we find the surface density at $17.5~{\rm mag}$ to be reduced only by an additional factor of four. This comparison shows that either the steep decline in density from K=20 to $19~{\rm mag}$ levels off to K=17.5 or that a large fraction of our extremely red objects are stars.

We have compared the colour distribution of galaxies in B_j-R and R-K with theoretical colours based on spectral evolution synthesis. We have shown that it is not possible to find parameters (e.g. luminosity function and galaxy type mix) such that both colour distributions are reproduced simultaneously. While a type mix dominated by late type galaxies reproduces the optical colour B_j-R, the strong trend to redder colours in R-K with increasing magnitudes can not be reproduced by models. A model which fits the data at $K=17~{\rm mag}$ is too red at $K=14~{\rm mag}$, a model which fits at the bright end is too blue at the faint end.

Acknowledgements

This work was supported by the DFG (Sonderforschungsbereich 328 and 439) and the Studienstiftung des deutschen Volkes.