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~7--10, selected in the
HUDF using the new observations made by the HST/WFC3. Our sample is composed of
20 z-dropouts, 15 Y-dropouts, and 20 J-dropouts. The surface densities of the
z-dropouts are close to what predicted by earlier studies, however, those of
the Y- and J-dropouts are quite unexpected. While no Y- or J-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~7 to 10. Motivated
by such steep increases, we tentatively propose a set of Schechter function
parameters to describe the LFs at z~8 and 10. As compared to their counterpart
at z~7, here L* decreases by ~ 6.5x and Phi* increases by 17--90x. 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 LFs 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~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~10, rapidly reach the minimum at z~7, and start to rise again
towards z~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~6. [Abridged]
