The star formation rate of the Universe atz≈ 6 from the Hubble ultra-Deep Field

Abstract

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 8 σ limiting magnitude of z′AB= 28.5 (equivalent to 1.5 h−270 M⊙ yr−1 at z= 6.1, or 0.1 L⋆UV for 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 [0.005 h70 M⊙ yr−1 Mpc−3 at z≈ 6 at LUV > 0.1 L⋆ for a Salpeter initial mass function (IMF)] poses an interesting challenge for models which suggest that LUV > 0.1 L⋆ 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.