GALAXIES AT z ～ 6: THE UV LUMINOSITY FUNCTION AND LUMINOSITY DENSITY FROM 506 HUDF, HUDF PARALLEL ACS FIELD, AND GOODS i-DROPOUTS

摘要

We have detected 506 i-dropouts (z ～ 6 galaxies) in deep, wide-area HST ACS fields: HUDF, enhanced GOODS, and HUDF parallel ACS fields (HUDF-Ps). The contamination levels are approx< 8% (i.e., approx> 92% are atz ～ 6). With these samples, we present the most comprehensive, quantitative analyses of z ～ 6 galaxies yet and provide optimal measures of the UV luminosity function (LF) and luminosity density at z ～ 6, and their evolution to z ～ 3. We redetermine the size and color evolution from z ～ 6 to z ～ 3. Field-to-field variations (cosmic variance), completeness, flux, and contamination corrections are modeled systematically and quantitatively. After corrections, we derive a rest-frame continuum UV(～ 1350 A) LF atz ～ 6 that extends to M_(1350, AB)～ —17.5(0.04L_(z=3)~*). There is strong evidence for evolution of the LF between z ～ 6 and z ～ 3, most likely through a brightening (0.6 ± 0.2 mag) of M~* (at 99.7% confidence), although the degree depends on the faint-end slope. As expected from hierarchical models, the most luminous galaxies are deficient at z ～ 6. Density evolution (φ~*) is ruled out at >99.99% confidence. Despite large changes in the LF, the luminosity density at z ～ 6 is similar to (0.82 ± 0.21 times) that at z ～ 3. Changes in the mean UV color of galaxies from z ～ 6 to z ～ 3 suggest an evolution in dust content, indicating that the true evolution is substantially larger: at z ～ 6 the star formation rate density is just ～3 0% of the z ～ 3 value. Our UV LF is consistent with z ～ 6 galaxies providing the necessary UV flux to reionize the universe.