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We present a large sample of candidate galaxies at z~7--10, selected in theHUDF using the new observations made by the HST/WFC3. Our sample is composed of20 z-dropouts, 15 Y-dropouts, and 20 J-dropouts. The surface densities of thez-dropouts are close to what predicted by earlier studies, however, those ofthe Y- and J-dropouts are quite unexpected. While no Y- or J-dropouts have beenfound at AB < 28.0 mag, their surface densities seem to increase sharply atfainter levels. While some of these candidates seem to be close to foregroundgalaxies and thus could possibly be gravitationally lensed, the overall surfacedensities after excluding such cases are still much higher than what would beexpected if the luminosity function does not evolve from z~7 to 10. Motivatedby such steep increases, we tentatively propose a set of Schechter functionparameters to describe the LFs at z~8 and 10. As compared to their counterpartat z~7, here L* decreases by ~ 6.5x and Phi* increases by 17--90x. Althoughsuch parameters are not yet demanded by the existing observations, they areallowed and seem to agree with the data better than other alternatives. Ifthese LFs are still valid beyond our current detection limit, this would implya sudden emergence of a large number of low-luminosity galaxies when lookingback in time to z~10, which, while seemingly exotic, would naturally fit in thepicture of the cosmic hydrogen reionization. These early galaxies could easilyaccount for the ionizing photon budget required by the reionization, and theywould imply that the global star formation rate density might start from a veryhigh value at z~10, rapidly reach the minimum at z~7, and start to rise againtowards z~6. In this scenario, the majority of the stellar mass that theuniverse assembled through the reionization epoch seems still undetected bycurrent observations at z~6. [Abridged]

Galaxy Formation In The Reionization Epoch As Hinted By Wide Field Camera 3 Observations Of The Hubble Ultra Deep Field