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We present late-time Hubble Space Telescope imaging of the fields of sixSwift GRBs lying at 5.0<z<9.5. Our data includes very deep observations of thefield of the most distant spectroscopically confirmed burst, GRB 090423, atz=8.2. Using the precise positions afforded by their afterglows we can placestringent limits on the luminosities of their host galaxies. In one case, thatof GRB 060522 at z=5.11, there is a marginal excess of flux close to the GRBposition which may be a detection of a host at a magnitude J(AB)=28.5. None ofthe others are significantly detected meaning that all the hosts lie belowL\star at their respective redshifts, with star formation rates SFR<4Mo/yr inall cases. Indeed, stacking the five fields with WFC3-IR data we conclude amean SFR<0.17Mo/yr per galaxy. These results support the proposition that thebulk of star formation, and hence integrated UV luminosity, at high redshiftsarises in galaxies below the detection limits of deep-field observations.Making the reasonable assumption that GRB rate is proportional to UV luminosityat early times allows us to compare our limits with expectations based ongalaxy luminosity functions derived from the Hubble Ultra-Deep Field (HUDF) andother deep fields. We infer that a luminosity function which is evolvingrapidly towards steeper faint-end slope (alpha) and decreasing characteristicluminosity (L\star), as suggested by some other studies, is consistent with ourobservations, whereas a non-evolving LF shape is ruled out at >90% confidence.Although it is not yet possible to make stronger statements, in the future,with larger samples and a fuller understanding of the conditions required forGRB production, studies like this hold great potential for probing the natureof star formation, the shape of the galaxy luminosity function, and the supplyof ionizing photons in the early universe.

Star formation in the early universe: beyond the tip of the iceberg