A systematic search for very massive galaxies at z > 4

Abstract

Motivated by the claimed discovery of a very massive galaxy (HUDF-JD2; M~5x10^11 Msun) at extreme redshift (z = 6.5) within the Hubble Ultra Deep Field (HUDF) (Mobasher et al. 2005), we have completed a systematic search for comparably massive galaxies with z > 4 among the 2688 galaxies in our K < 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 imaging in this 125 square arcmin field, ~25 times larger than the NICMOS HUDF. 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. 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 lies at z ~ 2.2, rather than the extreme redshift favoured by Mobasher et al. (2005). The 6 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. Moreover, the recently-released Spitzer MIPS imaging in GOODS-South has revealed that 5 of our 6 final z > 4 candidates are detected at 24 microns. We conclude that there is no convincing evidence for any galaxy with M > 3 x 10^11 Msun, and z > 4 within the GOODS-South field (abridged).