The dusty, albeit ultraviolet bright infancy of galaxies
  The largest galaxies acquire their mass early on, when the Universe is still
youthful. Cold streams violently feed these young galaxies a vast amount of
fresh gas, resulting in very efficient star formation. Using a well resolved
hydrodynamical simulation of galaxy formation, we demonstrate that these
mammoth galaxies are already in place a couple of billion years after the Big
Bang. Contrary to local starforming galaxies, where dust re-emits a large part
of the stellar ultraviolet (UV) light at infrared and sub-millimetre
wavelengths, our self-consistent modelling of dust extinction predicts that a
substantial fraction of UV photons should escape from primordial galaxies. Such
a model allows us to compute reliably the number of high redshift objects as a
function of luminosity, and yields galaxies whose UV luminosities closely match
those measured in the deepest observational surveys available. This agreement
is remarkably good considering our admittedly still simple modelling of the
interstellar medium (ISM) physics. The luminosity functions (LF) of virtual UV
luminous galaxies coincide with the existing data over the whole redshift range
from 4 to 7, provided cosmological parameters are set to their currently
favoured values. Despite their considerable emission at short wavelengths, we
anticipate that the counterparts of the brightest UV galaxies will be detected
by future sub-millimetre facilities like ALMA
