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 ≈ 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 −2 70  M ⊙  yr −1 at z = 6.1 , or 0.1  L ⋆ UV for the z ≈ 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 ≈ 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  h 70  M ⊙  yr −1  Mpc −3 at z ≈ 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 ≃ 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 .