VERY BLUE UV-CONTINUUM SLOPE β OF LOW LUMINOSITY z ∼ 7 GALAXIES FROM WFC3/IR: EVIDENCE FOR EXTREMELY LOW METALLICITIES?

We use the ultra-deep WFC3/IR data over the Hubble Ultra Deep Field 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 similar to 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 similar to -2 derived at similar luminosities at z similar to 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 similar to 4, just 800 Myr later, and even at z similar to 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 similar to -3), extremely low metallicities and mechanisms to reduce the red nebular emission seem to be required. For example, a large escape fraction (i.e., f(esc) greater than or similar to 0.3) could minimize the contribution from this red nebular emission. If this is correct and the escape fraction in faint z similar to 7 galaxies is greater than or similar to 0.3, it may help to explain how galaxies reionize the universe.