K_s NUMBER COUNTS IN THE GROTH AND COPPI FIELDS

摘要

We have used William Herschel Telescope/INGRID K_s images on two high-latitude fields, the Coppi and Groth strips, to obtain galaxy number counts over ～180 arcmin~2, to a depth of K_s ～ 21.0. Detection efficiency corrections as a function of object size have been calculated on each pointing. We have used a signal-to-noise threshold in two complementary half-exposure images to remove spurious detections. Our data cover the range from K_s = 14.5 to K_s = 21.0, so they are useful for investigating a previously reported change in the number count slope (d log N/dm) at K ～ 17. We find a slope γ_b = 0.54-0.63 for K < 17.5 and a slope γ_f = 0.25-0.29 for K > 17.5. A total contribution from galaxies to the extragalactic background light (EBL) in the K band of vI_v = 10.5 nW m~(-2) sr~(-1) has been calculated. This K-band EBL coming from galaxies accounts for only ～50% of the recent measurements of the diffuse EBL. Standard number count models fail to reproduce the observed slope change at K ～ 17.5 unless elliptical and spiral formation is pushed to z approx< 2.