Astronomy and Astrophysics – Astronomy
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aas...204.9103b&link_type=abstract
American Astronomical Society Meeting 204, #91.03; Bulletin of the American Astronomical Society, Vol. 36, p.822
Astronomy and Astrophysics
Astronomy
3
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 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≈ 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 (0.0046h70Msun/yr/Mpc3 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.
Bunker Andrew J.
Ellis Richard S.
McMahon Richard G.
Stanway Elizabeth R.
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