The Colour Distribution of Galaxies at Redshift Five

We present the results of a study investigating the rest-frame ultraviolet (UV) spectral slopes of redshift z approximate to 5 Lyman-break galaxies (LBGs). By combining deep Hubble Space Telescope imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and Hubble Ultra-Deep Field with ground-based imaging from the UKIRT Infrared Deep Sky Survey Ultra Deep Survey, we have produced a large sample of z approximate to 5 LBGs spanning an unprecedented factor of > 100 in UV luminosity. Based on this sample we find a clear colour-magnitude relation (CMR) at z approximate to 5, such that the rest-frame UV slopes (beta) of brighter galaxies are notably redder than their fainter counterparts. We determine that the z approximate to 5 CMR is well described by a linear relationship of the form: d beta = (-0.12 +/- 0.02)dM(UV), with no clear evidence for a change in CMR slope at faint magnitudes (i.e. M-UV >= -18.9). Using the results of detailed simulations we are able, for the first time, to infer the intrinsic (i.e. free from noise) variation of galaxy colours around the CMR at z approximate to 5. We find significant (12 sigma) evidence for intrinsic colour variation in the sample as a whole. Our results also demonstrate that the width of the intrinsic UV slope distribution of z approximate to 5 galaxies increases from Delta beta similar or equal to 0.1 at M-UV = -18 to Delta beta similar or equal to 0.4 at M-UV = -21. We suggest that the increasing width of the intrinsic galaxy colour distribution and the CMR itself are both plausibly explained by a luminosity-independent lower limit of beta approximate to -2.1, combined with an increase in the fraction of red galaxies in brighter UV-luminosity bins.