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

Abstract

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_-3.2 h^-1₇₂ 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 (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_solar, 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.