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A. J. Bunker (School of Physics, University of Exeter, UK), E. R. Stanway (Institute of Astronomy, Cambridge, UK), R. S. Ellis (California Institute of Technology), R. G. McMahon (Institute of Astronomy, Cambridge, UK)
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\approx 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 10-sigma limiting magnitude of z'(AB)=28.5 (equivalent to 1.5/h702 Msun/yr at z=6, 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')>1.3 over the deepest 11sq.arcmin portion of the UDF field. The characteristic luminosity (L*) is consistent with values observed at z\approx 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 (Phi*). 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\approx 6 was at least x6 LESS than at z\approx 3 (Stanway, Bunker & McMahon 2003). This declining comoving star formation rate (0.0046h70Msun/yr/Mpc3 at z\approx 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\approx 6.
The author(s) of this abstract have provided an email address for comments about the abstract: bunker@astro.ex.ac.uk
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Bulletin of the American Astronomical Society, 36 #2
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