Clustering of i775 Dropout Galaxies at z ~ 6 in GOODS and the UDF

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) shows the presence of clustering at 94% significance. For this sample we derive a (comoving) correlation length of r0 = 4.5 h Mpc and bias b = 4.1, 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.