Discovery of a z ˜ 6 Galaxy in the Chandra Deep Field South

Details Discovery of a z ˜ 6 Galaxy in the Chandra Deep Field South Discovery of a z ˜ 6 Galaxy in the Chandra Deep Field South

May 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003aas...202.1110b&link_type=abstract

American Astronomical Society Meeting 202, #11.10; Bulletin of the American Astronomical Society, Vol. 35, p.716

Astronomy and Astrophysics

Astronomy

1

Scientific paper

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 HST/ACS (object 3 in Stanway, Bunker & McMahon 2003, astro-ph/0302212). The large colour of (i'-z')AB=1.6 indicated a spectral break consistent with the Lyman-α forest absorption short-ward 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 rhl<0.5 h-170 kpc). We have obtained a deep (5.5-hour) spectrum of this z'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 σ with a flux of f≈ 2x 10-17 ergs 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 (Δ vFWHM=260 km s-1) and rest-frame equivalent width (Wrest Lyα =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 Nriptsize V λ 1240 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 33.8 h70-2 Msun 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 selection technique of Stanway, Bunker & McMahon (2003) to select star-forming galaxies at z≈ 6.

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