The Contribution of High Redshift Galaxies to Cosmic Reionization: New
  Results from Deep WFC3 Imaging of the Hubble Ultra Deep Field
  We have searched for star-forming galaxies at z~7 by applying the Lyman-break
technique to newly-released 1.1micron Y-band images from WFC3 on HST. By
comparing these images of the Hubble Ultra Deep Field with the ACS z'-band
(0.85micron), we identify objects with red colours, (z'-Y)_AB>1.3), consistent
with the Ly-alpha forest absorption at z~6.7-8.8. We identify 12 of these
z'-drops down to a limiting magnitude Y_AB<28.5 (equivalent to a star formation
rate of 1.3M_sun/yr at z=7.1), which are undetected in the other ACS filters.
We use the WFC3 J-band image to eliminate contaminant low mass Galactic stars,
which typically have redder colours than z~7 galaxies. One of our z'-drops is a
probably a T-dwarf star. The z~7 z'-drops have much bluer spectral slopes than
Lyman-break galaxies at lower redshift. Our brightest z'-drop is not present in
the NICMOS J-band image of the same field taken 5 years before, and is a
possible transient object. From the 10 remaining z~7 candidates we determine a
lower limit on the star formation rate density of 0.0017M_sun/yr/Mpc^3 for a
Salpeter initial mass function, which rises to 0.0025-0.0034M_sun/yr/Mpc^3
after correction for luminosity bias. The star formation rate density is a
factor of ~10 less than that at z=3-4, and is about half the value at z~6.
While based on a single deep field, our results suggest that this star
formation rate density would produce insufficient Lyman continuum photons to
reionize the Universe unless the escape fraction of these photons is extremely
high (f_esc>0.5), and the clumping factor of the Universe is low. Even then, we
need to invoke a large contribution from galaxies below our detection limit.
The apparent shortfall in ionizing photons might be alleviated if stellar
populations at high redshift are low metallicity or have a top-heavy IMF.
