Preprint typeset using L ATEX style emulateapj v. 6/22/04 CLUSTERING OF i775 DROPOUT GALAXIES AT z ∼ 6 IN GOODS AND THE UDF 1

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 (co-moving) correlation length of r0 = 4.5 +2.1 −3.2 h −1 72 Mpc 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 (1σ). 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 ∼ 10 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. Subject headings: cosmology: observations – early universe – large-scale structure of universe