Interpreting the evolution of galaxy colours from $z = 8$ to $z = 5$
  We attempt to interpret existing data on the evolution of the UV luminosity
function and UV colours, $\beta$, of galaxies at $5 \leq z \leq 8$, to improve
our understanding of their dust content and ISM properties. To this aim, we
post-process the results of a cosmological hydrodynamical simulation with a
chemical evolution model, which includes dust formation by supernovae and
intermediate mass stars, dust destruction in supernova shocks, and grain growth
by accretion of gas-phase elements in dense gas. We find that observations
require a steep, Small Magellanic Cloud-like extinction curve and a clumpy dust
distribution, where stellar populations younger than 15 Myr are still embedded
in their dusty natal clouds. Investigating the scatter in the colour
distribution and stellar mass, we find that the observed trends can be
explained by the presence of two populations: younger, less massive galaxies
where dust enrichment is mainly due to stellar sources, and massive, more
chemically evolved ones, where efficient grain growth provides the dominant
contribution to the total dust mass. Computing the IR-excess - UV color
relation we find that all but the dustiest model galaxies follow a relation
shallower than the Meurer et al. (1999) one, usually adopted to correct the
observed UV luminosities of high-$z$ galaxies for the effects of dust
extinction. As a result, their total star formation rates might have been
over-estimated. Our study illustrates the importance to incorporate a proper
treatment of dust in simulations of high-$z$ galaxies, and that massive, dusty,
UV-faint galaxies might have already appeared at $z \lesssim 7$.
