A Lyman-α blob in the GOODS South field:: evidence for cold accretion onto a dark matter halo

We report on the discovery of a z = 3.16 Lyman-alpha emitting blob in the Great Observatories Origins Deep Survey (GOODS) South field. The discovery was made with the VLT, through narrow-band imaging. The blob has a total Ly alpha luminosity of similar to 10(43) erg s(-1) and a diameter larger than 60 kpc. The available multi-wavelength data in the GOODS field consists of 13 bands from X-rays (Chandra) to infrared (Spitzer). Unlike other known Lya blobs, this blob shows no obvious continuum counter-parts in any of the broad-bands. In particular, no optical counter-parts are found in deep HST/ACS imaging. For previously published blobs, AGN (Active Galactic Nuclei) or "superwind" models have been found to provide the best match to the data. We here argue that the most probable origin of the extended Lya emission from this blob is cold accretion onto a dark matter halo.