The UV Properties of Star Forming Galaxies I: HST WFC3 Observations of
  Very-high Redshift Galaxies
  The acquisition of deep Near-IR imaging with Wide Field Camera 3 on the
Hubble Space Telescope has provided the opportunity to study the very-high
redshift Universe. For galaxies up to $z\approx 7.7$ sufficient wavelength
coverage exists to probe the rest-frame ultraviolet (UV) continuum without
contamination from either Lyman-$\alpha$ emission or the Lyman-$\alpha$ break.
In this work we use Near-IR imaging to measure the rest-frame UV continuum
colours of galaxies at $4.7<z<7.7$. We are have carefully defined a
colour-colour selection to minimise any inherent bias in the measured UV
continuum slope for the drop-out samples. For the highest-redshift sample
($6.7<z<7.7$), selected as $z_{f850lp}$-band dropouts, we find mean UV
continuum colours approximately equal to zero (AB), consistent with a
dust-free, solar metallicity, star forming population (or a moderately dusty
population of low metallicity). At lower-redshift we find that the mean UV
continuum colours of galaxies (over the same luminosity range) are redder, and
that galaxies with higher luminosities are also slightly redder on average. One
interpretation of this is that lower-redshift and more luminous galaxies are
dustier, however this interpretation is complicated by the effects of the star
formation history and metallicity and potentially the initial mass function on
the UV continuum colours.
