The angular correlations of galaxies in the COSMOS field

We present measurements of the two-point galaxy angular correlation function w(theta) in the COSMOS field. Independent determinations of w( theta) as a function of magnitude limit are presented for both the Hubble Space Telescope ( HST) ACS catalog and also the ground-based data from Subaru and the Canada-France-Hawaii Telescope. Despite having significantly different masks, these three determinations agree well. At bright magnitudes ( I-AB < 22), our data generally match very well with existing measurements and with mock catalogs based on the semianalytic galaxy formation calculations of Kitzbichler & White ( 2007) from the Millennium Simulation. The exception is that our result is at the upper end of the expected cosmic variance scatter for theta > 10', which we attribute to a particularly rich structure known to exist at z similar or equal to 0.8. For fainter samples, however, the level of clustering is somewhat higher than reported by some previous studies; in all three catalogs we find w(theta=1')similar or equal to 0.014 at a median I-AB magnitude of 24. At these very faintest magnitudes, our measurements agree well with the latest determinations from the Canada-France Legacy Survey. This level of clustering is approximately double what is predicted by the semianalytic catalogs ( at all angles). The semianalytic results allow an estimate of cosmic variance, which is too small to account for the discrepancy. We therefore conclude that the mean amplitude of clustering at this level is higher than previously estimated.