2006ApJ...648L...5O

We measured the angular clustering at z∼6 from a large sample of i₇₇₅ dropout galaxies (293 with z₈₅₀≤27.5 from GOODS and 95 with z₈₅₀≤29.0 from the UDF). Our largest and most complete subsample (having L≳0.5L^*_z=6) shows the presence of clustering at 94% significance. For this sample we derive a (comoving) correlation length of r₀=4.5^+2.1_–3.2h^–1₇₂Mpc and bias b=4.1^+1.5_–2.6, using an accurate model for the redshift distribution. No clustering could be detected in the much deeper but significantly smaller UDF, yielding b<4.4 (1 σ). We compare our findings to Lyman break galaxies at z∼3-5 at a fixed luminosity. Our best estimate of the bias parameter implies that i₇₇₅ dropouts are hosted by dark matter halos having masses of ∼10¹¹ M_☉, similar to that of V₆₀₆ dropouts at z∼5. We evaluate a recent claim that at z≳5 star formation might have occurred more efficiently compared to that at z=3-4. This may provide an explanation for the very mild evolution observed in the UV luminosity density between z=6 and 3. Although our results are consistent with such a scenario, the errors are too large to find conclusive evidence for this.