A wide field survey at the Northern Ecliptic Pole II. Number counts and galaxy colours in Bj, R, and K

Mw. Kummel et Sj. Wagner, A wide field survey at the Northern Ecliptic Pole II. Number counts and galaxy colours in Bj, R, and K, ASTRON ASTR, 370(2), 2001, pp. 384-397
Citations number
Categorie Soggetti
Space Sciences
Journal title
ISSN journal
0004-6361 → ACNP
Year of publication
384 - 397
SICI code
We present a medium deep survey carried out in the three filters B-j, R and K. The survey covers homogeneously the central square degree around the No rthern Ecliptic Pole (NEP) down to a completeness limit (95%) of 24.25, 23. 0 and 17.5 mag in B-j, R and K, respectively. While the near infrared data have been presented in the first paper of this series, here we concentrated on the optical data and the results based on the combined BjRK-data. The u nique combination of area and depth in our survey allows to perform a varie ty of investigations based on homogeneous material covering more than ten m agnitudes in apparent brightness. We analyze the number counts for point-li ke and extended sources in B-j and R to determine the slopes in d log N/dm and to test for possible breaks therein. While we can confirm the slopes fo und in previous works with a higher statistical significance, the largest u ncertainty remaining for the amplitudes is galactic extinction. We determin e the colour distributions of galaxies in B-j - R and R - K down to B-j = 2 4.0 and K = 18.0 mag, respectively. The distributions in both colours are m odeled using galaxy spectral evolution synthesis. We demonstrate that the s tandard models of galaxy evolution are unable to reproduce the steady redde ning trend in R - K despite flawless fits to the colour distributions in th e optical (B-j - R). The BjRK data collected over a large area provides the opportunity to select rare objects like candidates for high-redshift galax ies and extremely red objects (EROs, R - K > 5.0) and to determine their su rface density. Our EROs are selected at an intermediate magnitude range and contain contribution from both galactic as well as extragalatic sources. A t K < 16.5 mag, where a morphological classification is possible, the stell ar component dominates the sample.