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The Star Formation Rate of the Universe at z~6 from the Hubble Ultra Deep Field | | A. J. Bunker
; E. R. Stanway
; R. S. Ellis
; R. G. McMahon
; | | Date: |
10 Mar 2004 | | Subject: | astro-ph | | Affiliation: | 1 and 2), E. R. Stanway , R. S. Ellis , R. G. McMahon ( University of Exeter, Institute of Astronomy, Cambridge, Caltech Astronomy | | Abstract: | 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 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 8-sigma limiting magnitude of z’(AB)=28.5 (equivalent to 1.5/h{70}^2 M_sun/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 11arcmin^2 portion of the UDF field. The characteristic luminosity (L*) is consistent with values observed at z~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~6 was at least x6 LESS than at z~3 (Stanway, Bunker & McMahon 2003). This declining comoving star formation rate (0.005 h{70}M_sun/yr/Mpc^3 at z~6 for a Salpeter IMF) poses an interesting challenge for models which suggest that L>0.1L* 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. | | Source: | arXiv, astro-ph/0403223 | | Services: | Forum | Review | PDF | Favorites | |
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