Lyman break galaxies at z>6

Extending the study of star-forming galaxies to z>6 is extremely difficult due to the faintness of the sources and the challenging nature of deep near-infrared observations. Nevertheless, current observations are now just good enough that we can begin drawing some conclusions about the nature of galaxies at z≳7. At present, deep near-infrared observations with NICMOS (reaching ≳27 AB mag at 5σ) cover more than 20arcmin² of area with deep optical coverage and allow us to identify four strong z≃7-8 candidates. Comparing this sample with dropout samples at later times (z≃4-6), we are able to study evolution in the rest-frame UV LF over the range z≃8 to z≃4. We find strong evidence for significant evolution in the characteristic luminosity with time (brightening by ∼2 mag, from z≃8 to z≃4). The observed evolution appears to be the direct result of hierarchical growth in the galaxy population.