CLUSTERING OF i_(775) DROPOUT GALAXIES AT z ～ 6 IN GOODS AND THE UDf

摘要

We measured the angular clustering at z ～ 6 from a large sample of i_(775) dropout galaxies (293 with z_(850) ≤ 27.5 from GOODS and 95 with z_(850) ≤ 29.0 from the UDF). Our largest and most complete subsample (having L approx > 0.5L_(z=6)~*) shows the presence of clustering at 94% significance. For this sample we derive a (comoving) correlation length of r_0 = 4.5_(-3.2)~(+2.1)h_(72)~(-1) Mpc and bias b = 4.l_(-2.6)~(+1.5), 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 i_(775) dropouts are hosted by dark matter halos having masses of ～10~(11) solar mass, similar to that of V_(606) dropouts at z ～ 5. We evaluate a recent claim that at z approx > 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.