The Star Formation Rate of the Universe at z ≈ 6 from the Hubble Ultra Deep Field

We determine the abundance of i-band drop-outs in the recently-released HST/ACS Hubble Ultra Deep Field (UDF). Since the majority of these sources are likely to be z ≈ 6 galaxies whose flux decrement between the F775W i-band and F850LP zband 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 σ limiting magnitude of z AB = 28.5 (equivalent to 1.5 h 70 M⊙ yr −1 at z = 6.1, or 0.1L 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 field. 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 ×6 less than at z ≈ 3 (Stanway, Bunker & McMahon 2003). This declining comoving star formation rate (0.005 h70 M⊙ yr −1 Mpc at z ≈ 6) poses an interesting challenge for models which suggest that the bulk of star forming galaxies that reionized the universe lie at redshifts just beyond z ≃ 6.