Constraints on star-forming galaxies at z >= 6.5 from HAWK-I Y-band imaging of GOODS-South

Abstract

We present the results of our search for high-redshift Lyman-break galaxies over the GOODS-South field. We use Hubble Space Telescope (HST)-ACS data in B, V, i' & z', Very Large Telescope (VLT)-ISAAC J and Ks, Spitzer-Infrared Array Camera (IRAC) 3.6, 4.5, 5.8 and 8.0 μm 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 ~119arcmin² to Y_AB = 25.7 (5σ), of which 37.5arcmin² reaches Y_AB = 25.9. Candidate z' and Y dropouts were selected on the basis of a colour cut of (Y - J)_AB > 0.75mag and (z' - Y)_AB > 1.0mag, 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 is real on the ultraviolet 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, in the sense that the number density of ultraviolet luminous star-forming galaxies at z > 7 is less than at z ~ 6. We show that the future surveys to be conducted with the European Southern Observatory VISTA telescope over the next 5yr will be able to measure the bulk of the luminosity function for both z' and Y dropouts and thus provide the strongest constraints on the level of star-formation within the epoch of reionization.