Constraints on star-forming galaxies atz≥ 6.5 from HAWK-IY-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 ≈119 arcmin2 to YAB= 25.7 (5σ), of which 37.5 arcmin2 reaches YAB= 25.9. Candidate z′ and Y dropouts 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 JAB < 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 5 yr 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.