The star formation rate of the Universe atz≈ 6 from theHubble Ultra-Deep Field.

We determine the abundance ofi′-band dropouts in the recently releasedHST/ACSHubble Ultra-Deep Field(UDF). Because the majority of these sources are likely to begalaxies whose flux decrement between the F775Wi′-band and F850LPz′-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 anlimiting magnitude of(equivalent toat, orfor theU-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) atwithover the deepest 11-arcmin² portion of the UDF. The characteristic luminosityis consistent with values observed at. The faint end slopeis 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 atwas approximately six timeslessthan at. This declining comoving star formation rate[atatfor a Salpeter initial mass function (IMF)] poses an interesting challenge for models which suggest thatstar-forming galaxies atreionized 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. [ABSTRACT FROM AUTHOR]