2008MNRAS.385.2175M

Massive high-redshift galaxies form in overdense regions where the probability of forming other galaxies is also strongly enhanced. Given an observed flux of a galaxy, the inferred mass of its host halo tends to be larger as its inferred redshift increases. As the mass and redshift of a galaxy halo increase, the expected clustering of other galaxies around it gets stronger. It is therefore possible to verify the high-redshift identity of a galaxy (prior to an unambiguous spectral identification) from the clustering of other galaxies around it. We illustrate this method for the massive galaxy suggested by Mobasher et al. to be at redshift z ∼ 6.5. If this galaxy were to exist at z ∼ 6.5, there should have been a mean of ∼10 galaxies larger than a hundredth of its mass and having z-band magnitudes less than ∼22 detected as i-dropouts in the Hubble Ultra Deep Field (HUDF). We calculate an approximate probability distribution for neighbour galaxies and determine that there is less than a ∼0.3 per cent chance of detecting no massive neighbour galaxies. The lack of other massive z ∼ 6.5 galaxies in the HUDF image argues that the Mobasher et al. galaxy is instead a low-redshift interloper. We generalize our results to other galaxy masses and redshifts.