Evidence of Primordial Clustering around the QSO SDSS J1030+0524 at z = 6.28

We present tentative evidence of primordial clustering, manifested as an excess of color-selected objects in the field of the QSO SDSS J1030+0524 at redshift z = 6.28. We have selected objects red in i775 - z850 on the basis of Hubble Space Telescope Advanced Camera for Surveys imaging of a field centered on the QSO. Compared to data at comparable depth obtained by the Great Observatories Origins Deep Survey, we find an excess of objects with i775 - z850 ≥ 1.5 in the QSO field. The significance of the detection is estimated to be ~97% on the basis of the counts alone and increases to 99.4% if one takes into account the color distribution. If confirmed, this would represent the highest redshift example of galaxy clustering and would have implications on models for the growth of structure. Bias-driven clustering of first luminous objects forming in the highest peaks of the primordial density field is expected in most models of early structure formation. The redshift of one of the candidates has been found to be z = 5.970 by our spectroscopy with the Keck I Low Resolution Imaging Spectrometer, confirming the validity of our color selection.