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

Abstract

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 α forest absorption shortward of Lyman α at z~ 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^-1₇₀ 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 Å detected at 20σ with a flux of f~ 2 × 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 α emission from a z= 5.78 galaxy. This is the highest redshift galaxy to be discovered and studied using HST data. The velocity width (Δv_FWHM= 260 km s^-1) and rest-frame equivalent width (W^Lyα_rest= 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 Vλ1240-Å 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^-2₇₀ M_solar yr^-1 (Ω_M= 0.3, Ω_Λ= 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 atz~ 6.