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

We present tentative evidence for primordial clustering, manifested as anexcess of color-selected objects in the field of the QSO SDSS J1030+0524 atredshift z=6.28. We have selected objects red in i_{775}-z_{850} on the basisof Hubble Space Telescope Advanced Camera for Surveys imaging of a fieldcentered on the QSO. Compared to data at comparable depth obtained by the GOODSsurvey, we find an excess of objects with (i_{775}-z_{850}) \geq 1.5 in the QSOfield. The significance of the detection is estimated to be ~97% on the basisof the counts alone and increases to 99.4% if one takes into account the colordistribution. If confirmed this would represent the highest redshift example ofgalaxy clustering and would have implications on models for the growth ofstructure. Bias-driven clustering of first luminous objects forming in thehighest peaks of the primordial density field is expected in most models ofearly structure formation. The redshift of one of the candidates has been foundto be z=5.970 by our spectroscopy with Keck I/LRIS, confirming the validity ofour color selection.Comment: 5 pages, 3 figures; accepted by ApJ