  We present a sample of 16 robust z~7 z-drop galaxies detected by the newly
installed WFC3/IR camera on the Hubble Space Telescope. Our analysis is based
on the first epoch data of the HUDF09 program covering the Hubble Ultra Deep
Field with 60 orbits of Y, J, and H observations. These remarkable data cover
4.7 arcmin^2 and are the deepest NIR images ever taken, reaching to ~29 mag AB
(5 sigma). The 16 z~6.5-7.5 galaxies have been identified based on the Lyman
Break technique utilizing (z-Y) vs. (Y-J) colors. They have magnitudes J =
26.0-29.0 (AB), an average apparent half-light radius of ~0.16 arcsec (<~1
kpc), and show very blue colors (some even beta<~-2.5), in particular at low
luminosities. The WFC3/IR data confirms previous NICMOS detections indicating
that the dropout selection at z~7 is very reliable. Our data allow a first
determination of the faint end slope of the z~7 luminosity function, reaching
down to M_UV ~ -18, a full magnitude fainter than previous measurements. When
fixing phi*=1.4e-3/Mpc^3/mag to the value previously measured at z~6, we find a
best-fit value of alpha=-1.77+-0.20, with a characteristic luminosity of
M_*=-19.91+-0.09. This steep slope is similar to what is seen at z~2-6 and
indicates that low luminosity galaxies could potentially provide adequate flux
to reionize the universe. The remarkable depth and resolution of these new
images provide insights into the coming power of JWST.
