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-α emitting blob in the GOODS South field. The blob has a total Lyα luminosity of ∼ 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 discovered Lyα blobs, this blob shows no obvious continuum counterpart in any of the broad-bands. In particular, no optical counterparts are found in the deep HST/ACS imaging available. For previously published blobs, AGN (Active Galactic Nuclei) or " superwind " models have been found to provide the best match with the data. We here argue that the most probable origin of the extended Lyα emission from the blob in the GOODS South field is cold accretion onto a dark matter halo.