Very blue UV-continuum slopes of low luminosity z~7 galaxies from
  WFC3/IR: Evidence for extremely low metallicities?
  We use the ultra-deep WFC3/IR data over the HUDF and the Early Release
Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband
spectral properties of candidate star-forming galaxies at z~7. We determine the
UV-continuum slope beta in these galaxies, and compare the slopes with galaxies
at later times to measure the evolution in beta. For luminous L*(z=3) galaxies,
we measure a mean UV-continuum slope beta of -2.0+/-0.2, which is comparable to
the beta~-2 derived at similar luminosities at z~5-6. However, for the lower
luminosity 0.1L*(z=3) galaxies, we measure a mean beta of -3.0+/-0.2. This is
substantially bluer than is found for similar luminosity galaxies at z~4, just
800 Myr later, and even at z~5-6. In principle, the observed beta of -3.0 can
be matched by a very young, dust-free stellar population, but when nebular
emission is included the expected beta becomes >~-2.7. To produce these very
blue beta's (i.e., beta~-3), extremely low metallicities and mechanisms to
reduce the red nebular emission are likely required. For example, a large
escape fraction (i.e., f_{esc}>~0.3) could minimize the contribution from this
red nebular emission. If this is correct and the escape fraction in faint z~7
galaxies is >~0.3, it may help to explain how galaxies reionize the universe.
