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Constraints on Star Forming Galaxies at z> 6.5 from HAWK-I Y-band Imaging of GOODS-South

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.