Astronomy and Astrophysics – Astrophysics
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005assl..329p..74s&link_type=abstract
Starbursts: From 30 Doradus to Lyman Break Galaxies, Held in Cambridge, UK, 6-10 September 2004. Edited by R. de Grijs and R.M.
Astronomy and Astrophysics
Astrophysics
Scientific paper
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 Lyα 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 the UDF enables us to reach an 8 σ limiting magnitude of z'AB=28.5 mag (equivalent to 1.5 h-270 M&sun; 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 mag, with (i'-z')AB>1.3 over the deepest 11 arcmin2 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 by 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&sun; yr-1 Mpc-3 at z≈ 6 at LUV>0.1 L* for a Salpeter IMF) poses an interesting challenge for models that 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 by a radically different IMF. Alternatively, the bulk of the reionization might have occurred at z⋙6.
Bunker Ellis R. A.
McMahon Richard
Stanway Elizabeth
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