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A.J. Bunker, E.R. Stanway (Institute of Astronomy, Cambridge), R.S. Ellis (Caltech Astronomy), R.G. McMahon (Institute of Astronomy, Cambridge), P.J. McCarthy (OCIW)
We report the discovery of a luminous z=5.78 star-forming galaxy in the Chandra Deep Field South. This galaxy was selected as an ``i-drop'' from the GOODS public survey imaging with {\em HST}/ACS (object 3 in Stanway, Bunker & McMahon 2003, astro-ph/0302212). The large colour of (i'-z')\rm AB=1.6 indicated a spectral break consistent with the Lyman-\alpha forest absorption short-ward of Lyman-\alpha at z\approx 6. The galaxy is very compact (marginally resolved with ACS with a half-light radius of 0.08\,arcsec, so r\rm hl<0.5\,h-170\,kpc). We have obtained a deep (5.5-hour) spectrum of this z'\rm AB=24.7 galaxy with the DEIMOS optical spectrograph on Keck, and here we report the discovery of a single emission line centered on (8245±1)\,Å\ detected at 20\,\sigma with a flux of f\approx 2\times 10-17\,{\rm ergs\,cm}-2\,{\rm s}-1. The line is clearly resolved with detectable structure at our resolution of better than 55\,km\,s-1, and the only plausible interpretation consistent with the ACS photometry is that we are seeing Lyman-\alpha emission from a z=5.78 galaxy. This is the highest redshift galaxy to be discovered and studied using {\em HST} data. The velocity width (\Delta v\rm FWHM=260\,{\rm km\,s}-1) and rest-frame equivalent width (W\rm rest\rm Ly\alpha=20\,Å) indicate that this line is most probably powered by star formation, as an AGN would typically have larger values. The starburst interpretation is supported by our non-detection of the high-ionization N{\scriptsize~V}\,\lambda\,1240\,Å\ emission line, and the absence of this source from the deep {\em Chandra} X-ray images. The star formation rate inferred from the rest-frame UV continuum is 33.8\,h70-2\,M\odot\,{\rm yr}-1 (\OmegaM=0.3, \Omega\Lambda=0.7). This is the most luminous starburst known at z>5. Our spectroscopic redshift for this object confirms the validity of the i'-drop selection technique of Stanway, Bunker & McMahon (2003) to select star-forming galaxies at z\approx 6.
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The author(s) of this abstract have provided an email address for comments about the abstract: bunker@ast.cam.ac.uk
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.