The star formation rate of the Universe at z≈6 from the Hubble Ultra-Deep Field

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 approximate to 6 galaxies whose flux decrement between the F775W i'-band and F850LP z'-band arises from Lyman-alpha 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 8 limiting magnitude of z'(AB) = 28.5 (equivalent to 1.5 h(70)(-2) M-circle dot yr(-1) at z = 6.1, or 0.1 L*(UV) for the z approximate to 3 U-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-arcmin(2) portion of the UDF. The characteristic luminosity (L*) is consistent with values observed at z approximate to 3. The faint end slope (alpha) is less well constrained, but is consistent with only modest evolution. The main change appears to be in the number density (). 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 [0.005 h(70) M-circle dot yr(-1) Mpc(-3) at z approximate to 6 at L-UV > 0.1 L* for a Salpeter initial mass function (IMF)] poses an interesting challenge for models which suggest that L-UV > 0.1 L* star-forming galaxies at z similar or equal to 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.