Constraints on the z ~ 10 LF from our combined data set, evaluated in 0.5 mag bins. The upper limits correspond to 90% (dashed) and 1σ (solid) Poissonian limits including the additional conservative estimate of 50% cosmic variance per pointing. It is clear that the LF evolves strongly from z ~ 8 to z ~ 10, as our upper limits are a factor of ~2–5 below the measured z ~ 8 LF of Bouwens et al. (orange data; 2011b). The expected z ~ 10 LF as extrapolated from fits to lower redshift LBG LFs is shown as a dashed red line. Using this LF, we would expect to detect six sources in the full data set (~3.4 in the HUDF and ~1.2 in each of the HUDF09 parallels; see Figure  6). For comparison also the z ~ 4 and z ~ 6 LFs are plotted (Bouwens et al. 2007), showing the dramatic buildup of UV luminosity across ~1 Gyr of cosmic time. The light gray vectors along the lower axis indicate the range of luminosities over which the different data sets dominate the z ~ 10 LF constraints.