The VIMOS Ultra-Deep Survey: evidence for AGN feedback in galaxies with CIII]-λ1908 Å emission 10.8 to 12.5 Gyr ago

We analyze the CIII]-λ1908 A emission properties in a sample of 3899 star-forming galaxies (SFGs) at 2  3 A, including ∼20% with modest emission 3 10 A. A small but significant fraction of 1.2% of SFGs presents strong CIII] emission 20 40 A up to ∼95 A) are associated with broad-line AGN. While this makes CIII] the second most frequent emission line in the UV rest-frame spectra of SFGs after Lyman-α, this line alone cannot be considered an efficient substitute for measuring a galaxy redshift in the absence of Lyα emission, unless the spectral resolution is R >  3000 to distinguish among different possible doublets. We find a large dispersion in the weak correlation between EW(CIII]) and EW(Lyα), with galaxies showing strong CIII] and no Lyα, and vice versa. The spectra of SFGs with 10  1010 M⊙ at z ∼ 3 into the population of quiescent galaxies observed at redshift z ∼ 1−2.