Multicolour faint galaxy number counts with the Hitchhiker parallel CCD camera

Abstract

We present the first deep observations with the new Hitchhiker parallel CCD camera. In parallel with scheduled observations, we have observed a field near the North Galactic Pole in four wavebands, with an equivalent total exposure time of 5 h, and obtained galaxy number counts to limiting magnitudes of ra5 = 26.4, mv=24.9, mR = 26.3 and ra7= 22.5. The data should be complete to about 0.5 mag above these faint limits. We also present B-V, B-R, B-I, V-R and R-I colour histograms for the faint galaxies as a function of 5, V or R magnitude. The observations are in good agreement with previous deep CCD data, showing a steep number-count slope in the blue, which gradually decreases for longer wavelength observations, and a gradual blueing (in terms of B-R) of the mean galaxy population at fainter magnitudes. Although the V frame is less deep, so limiting our sensitivity, there is no significant sign of equivalent blueing in V-R, suggesting that the main effect is at the shorter (observed) wavelengths, and supporting the idea that most of the excess blue galaxies are dwarfs at moderately low redshifts. We then present a simple model, based on significant numbers of dwarf galaxies, which accounts successfully for all the number-count and colour data with no evolution. The model underestimates the observed redshift distribution, and appears to be inconsistent with measures of the local luminosity function. We conclude that some level of evolution is required, unless these surveys are biased against dwarf galaxies.