Segregated optical-near-infrared colour distributions of Medium Deep Survey galaxies

Abstract

We present a K survey of 29 fields covering approximately 90 arcmin^2 from the Medium Deep Survey (MDS) catalogue down to a completeness magnitude of K=18.0 (limiting magnitude K=19.0). The morphology obtained by the MDS team using high-resolution images from HST/WFPC2 along with our near-infrared (NIR) observations allows colour-magnitude and colour-colour analysis that agrees in general with spectral evolution models, especially if a reasonable range of metallicities for the simple stellar populations used (0.2 < Z/Z_solar < 2.5) is considered. However, a significant population of spheroids was found, which appears bluer than expected, confirming previous observations. This blueness might possibly signal the existence of non-negligible star formation in ellipticals and bulges at medium redshift. A number counts calculation for different morphological types shows that discs become the dominant population at faint magnitudes. The median redshift of the sample is z ~ 0.2, from a photometric redshift estimation using V-K and I-K. A search for Extremely Red Objects (EROs) in the survey field was also performed, with no detection of objects having I-K>4.5, setting an upper limit to the number density of EROs d n_EROs/d Ο < 0.011 arcmin^-2 (K <= 18.0).