We present the results of a study investigating the rest-frame ultraviolet (UV) spectral slopes of redshift z ≈ 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 ≈ 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 relationship 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) variation of galaxy colours around the CMR at z ≈ 5.
The width of the intrinsic colour distribution of galaxies at various luminosities. The first three panels relate to each of our three test methods, as denoted by the panel title and described in Section 5. In the first panel, × marks show the results of our Gaussian assumption test, while + marks denote our simulation-free check results. In all cases, each field contributes three bins of equal occupancy. The points are coloured by field, as in Fig. 6: salmon = UDS, blue = CGS, green = CGN, dark = HUDF. The error bars each enclose 68 per cent of the total p(β) where applicable. In each case, the yellow regions show the 1σ error limits of a linear fit to the measurements, weighted by the errors on β. In all three tests, brighter galaxies are drawn from a more varied intrinsic population than fainter galaxies. The final panel (with an expanded vertical range) shows measurements drawn from the literature over narrower dynamic ranges, from Bouwens et al. (2012, B12), Wilkins et al. (2011, W11) and Castellano et al. (2012, C12). The ML relation is shown in pale yellow in the final panel for comparison.
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 Δβ ≃ 0.1 at MUV = −18 to Δβ ≃ 0.4 at MUV = −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 β ≈ −2.1, combined with an increase in the fraction of red galaxies in brighter UV-luminosity bins.
The full paper can be found in Rogers et al. 2014, MNRAS, 440, 3714.