A star-forming galaxy at z=5.78 in the Chandra Deep Field South

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 the Hubble Space Telescope /Advanced Camera for Surveys (object 3 in the work of Stanway, Bunker & McMahon 2003). The large colour of (i '-z ')(AB) = 1.6 indicated a spectral break consistent with the Lyman alpha forest absorption shortward of Lyman alpha at z approximate to 6. The galaxy is very compact (marginally resolved with ACS with a half-light radius of 0.08 arcsec, so r (hl) < 0.5 h (-1) (70) kpc). We have obtained a deep (5.5 h) spectrum of this z '(AB) = 24.7 galaxy with the DEIMOS optical spectrograph on the Keck Telescope, and here we report the discovery of a single emission line centred on 8245 Angstrom detected at 20sigma with a flux of f approximate to 2 x 10(-17) erg cm(-2) 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 HST data. The velocity width (Deltav (FWHM) = 260 km s(-1) ) and rest-frame equivalent width (W (Lyalpha) (rest) = 20 Angstrom) 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 vlambda1240- Angstrom emission line, and the absence of this source from the deep Chandra X-ray images. The star formation rate inferred from the rest-frame UV continuum is 34 h (-2) (70) M-circle dot yr(-1) (Omega(M) = 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 technique of Stanway et al. to select star-forming galaxies at z approximate to 6.