  We present the results of our search for high-redshift Lyman-break galaxies
over the GOODS-South field. We use HST-ACS data in B, V, i' & z', VLT-ISAAC J
and Ks, Spitzer-IRAC 3.6, 4.5, 5.8 and 8.0 micron data in conjunction with the
new HAWK-I Y-band science verification data to search for dropout galaxies in
the redshift range 6<z<9. We survey ~119 arcmin^2 to Y_AB=25.7 (5sigma), of
which 37.5 arcmin^2 reaches Y_AB=25.9. Candidate z' and Y drop-outs were
selected on the basis of a colour cut of (Y-J)_{AB}>0.75 mag and
(z'-Y)_{AB}>1.0 mag respectively. We find no robust Y-drops (z~9) brighter than
J_{AB}<25.4. In our search for z'-band dropouts (z~6.5-7.5), we identify four
possible candidates, two with z'-drop colours and clear Spitzer-IRAC detections
and two less likely candidates. We also identify two previously-known Galactic
T-dwarf stellar contaminants with these colours, and two likely transient
objects seen in the Y-band data. The implications if all or none of our
candidates are real on the Ultra-Violet galaxy luminosity functions at z>6.5
are explored. We find our number of z'-drop candidates to be insufficient based
on the expected number of z' drops in a simple no-evolution scenario from the
z=3 Lyman-break galaxy luminosity function but we are consistent with the
observed luminosity function at z~6 (if all our candidates are real). However,
if one or both of our best z'-drop candidates are not z>6.5 galaxies, this
would demand evolution of the luminosity function at early epochs. We show that
the future surveys to be conducted with the ESO VISTA telescope over the next
five years will be able to measure the bulk of the luminosity function for both
z' and Y drop-outs.
