Galaxy formation in the reionization epoch as hinted by Wide Field Camera 3 observations of the Hubble Ultra Deep Field

We present a large sample of candidate galaxies at z approximate to 7 - 10, selected in the Hubble Ultra Deep Field using the new observations of the Wide Field Camera 3 that was recently installed on the Hubble Space Telescope. Our sample is composed of 20 z(850)-dropouts (four new discoveries), 15 Y-105-dropouts (nine new discoveries) and 20 J(125)-dropouts (all new discoveries). The surface densities of the z(850)-dropouts are close to what was predicted by earlier studies, however, those of the Y-105- and J(125)-dropouts are quite unexpected. While no Y-105- or J(125)-dropouts have been found at AB <= 28.0 mag, their surface densities seem to increase sharply at fainter levels. While some of these candidates seem to be close to foreground galaxies and thus could possibly be gravitationally lensed, the overall surface densities after excluding such cases are still much higher than what would be expected if the luminosity function does not evolve from z similar to 7 to 10. Motivated by such steep increases, we tentatively propose a set of Schechter function parameters to describe the luminosity functions at z approximate to 8 and 10. As compared to their counterpart at z approximate to 7, here L* decreases by a factor of similar to 6.5 and Phi* increases by a factor of 17-90. Although such parameters are not yet demanded by the existing observations, they are allowed and seem to agree with the data better than other alternatives. If these luminosity functions are still valid beyond our current detection limit, this would imply a sudden emergence of a large number of low-luminosity galaxies when looking back in time to z approximate to 10, which, while seemingly exotic, would naturally fit in the picture of the cosmic hydrogen reionization. These early galaxies could easily account for the ionizing photon budget required by the reionization, and they would imply that the global star formation rate density might start from a very high value at z approximate to 10, rapidly reach the minimum at z approximate to 7, and start to rise again towards z approximate to 6. In this scenario, the majority of the stellar mass that the universe assembled through the reionization epoch seems still undetected by current observations at z approximate to 6.