The colour distribution of galaxies at redshift five

We present the results of a study investigating the rest-frame ultra-violet (UV) spec-tral slopes of redshift z ≈ 5 Lyman-break galaxies (LBGs). By combining deep Hubble Space Telescope imaging of the CANDELS and HUDF fields with ground-based imag-ing from the UKIDSS Ultra Deep Survey (UDS), we have produced a large sample of z ≈ 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 ≈ 5, such that the rest-frame UV slopes (β) of brighter galaxies are notably redder than their fainter counterparts. We determine that the z ≈ 5 CMR is well described by a linear rela-tionship of the form: dβ = (−0.12 ± 0.02)dMUV, with no clear evidence for a change in CMR slope at faint magnitudes (i.e. MUV −18.9). Using the results of detailed simulations we are able, for the first time, to infer the intrinsic (i.e. free from noise) vari-ation of galaxy colours around the CMR at z ≈ 5. We find significant (12σ) 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 ≈ 5 galaxies increases from