2004MNRAS.355..374B

We determine the abundance of i'-band dropouts in the recently released HST/ACS Hubble Ultra-Deep Field (UDF). Because the majority of these sources are likely to be z~ 6 galaxies whose flux decrement between the F775W i'-band and F850LP z'-band arises from Lyman-α absorption, the number of detected candidates provides a valuable upper limit to the unextincted star formation rate at this redshift. We demonstrate that the increased depth of UDF enables us to reach an 8σ limiting magnitude of z'_AB= 28.5 (equivalent to 1.5h^–2₇₀M_☉yr^–1 at z= 6.1, or 0.1L^*_UVfor the z~ 3U-drop population), permitting us to address earlier ambiguities arising from the unobserved form of the luminosity function. We identify 54 galaxies (and only one star) at z'_AB< 28.5 with (i'-z')_AB> 1.3 over the deepest 11-arcmin² portion of the UDF. The characteristic luminosity (L^*) is consistent with values observed at z~ 3. The faint end slope (α) is less well constrained, but is consistent with only modest evolution. The main change appears to be in the number density (Φ*). Specifically, and regardless of possible contamination from cool stars and lower-redshift sources, the UDF data support our previous result that the star formation rate at z~ 6 was approximately six times less than at z~ 3. This declining comoving star formation rate [0.005h₇₀M_☉/yr/Mpc3 at z~ 6 at L_UV> 0.1L^* for a Salpeter initial mass function (IMF)] poses an interesting challenge for models which suggest that L_UV> 0.1L^* star-forming galaxies at z≃ 6 reionized the Universe. The short-fall in ionizing photons might be alleviated by galaxies fainter than our limit, or a radically different IMF. Alternatively, the bulk of reionization might have occurred at z≫ 6.