A systematic search for very massive galaxies at z > 4

Motivated by the claimed discovery of a very massive galaxy (HUDF-JD2; M similar or equal to 5 x 10(11) M-.) at extreme redshift (z = 6.5) within the Hubble Ultra Deep Field (HUDF), we have completed a systematic search for comparably massive galaxies with z > 4 among the 2688 galaxies in our K-S < 23.5 ( AB) catalogue within the CDFS/GOODS-South field. This search was conducted using redshift estimates based on the recently completed, uniquely-deep 11-band (B, V, i, z, J, H, K-S, 3.6 mu m, 4.5 mu m, 5.8 mu m, 8.0 mu m) imaging in this 125-arcmin(2) field, similar or equal to 25 times larger than the NICMOS HUDF. To ensure completeness, our approach places no special emphasis on the standard V-drop, i-drop or z-drop criteria commonly used to pre-select candidate high-redshift galaxies. Initial spectral fitting, based on published catalogue SEXTRACTOR photometry, led us to conclude that at least 2669 of the galaxies in our sample lie at z < 4. This list includes several galaxies for which redshifts z > 4 have been previously proposed. We carried out a detailed investigation of the 19 remaining z > 4 candidates, performing aperture photometry on all images, and including marginal detections and formal non-detections in the fitting process. This led to the rejection of a further 13 galaxies to lower redshift. Moreover, subjecting HUDF-JD2 to the same analysis, we find that it almost certainly lies at 2 < z < 3, rather than the extreme redshift favoured by Mobasher et al. The six remaining candidates appear to be credible examples of galaxies in the redshift range z = 4 - 6, with plausible stellar ages. However, refitting with allowance for extreme values of extinction, we find that, even for these objects, statistically acceptable solutions can be found at z < 3. In fact, only two galaxies retain formally preferred high-redshift solutions. Moreover, the recently released Spitzer MIPS imaging in GOODS-South has revealed that five out of our six final z > 4 candidates are detected at 24 mu m. This was also the case for HUDF-JD2, and provides further circumstantial evidence in favour of the moderate-redshift dusty solutions. We conclude that there is no convincing evidence for any galaxy with M > 3 x 10(11) M-. and z > 4 within the 125-arcmin(2) GOODS-South field. We briefly discuss the implications of this null result, and revised expectations for the much larger (0.8 deg(2)), and deeper near-infrared UKIDSS Ultra Deep Survey now underway with the WFCAM on the UKIRT.