Abstract

We analyze the CIII]-lambda 1908 angstrom emission properties in a sample of 3899 star-forming galaxies (SFGs) at 2 < z < 3.8 drawn from the VIMOS Ultra-Deep Survey (VUDS). We find a median rest-frame equivalent width EW(CIII]) = 2.0 +/- 0.2 to 2.2 +/- 0.2 angstrom for the whole SFG population at 2 < z < 3 and 3 < z < 4, respectively. About 24% of SFGs are showing EW(CIII]) > 3 angstrom, including similar to 20% with modest emission 3 < EW(CIII]) < 10 angstrom and similar to 4% with strong emission EW(CIII])> 10 angstrom. A small but significant fraction of 1.2% of SFGs presents strong CIII] emission 20 < EW(CIII]) < 40 angstrom; the four strongest emitters (EW(CIII]) > 40 angstrom up to similar to 95 angstrom) 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-alpha, this line alone cannot be considered an efficient substitute for measuring a galaxy redshift in the absence of Ly alpha 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(Ly alpha), with galaxies showing strong CIII] and no Ly alpha, and vice versa. The spectra of SFGs with 10 < EW(CIII]) < 40 angstrom present strong emission lines that include CIV-lambda 1549, HeII-lambda 1640, and OIII-lambda 1664, but also weaker emission features of highly ionized elements such as SiIV-lambda 1403, NIV-lambda 1485, NIII-lambda 1750, or SiIII-lambda 1888, indicating the presence of a hard radiation field. We present a broad range of observational evidence supporting the presence of AGN in the strong CIII] emitting population. As EW(CIII]) is rising, we identify increasingly powerful outflows with velocities up to 1014 km s(-1); this is beyond what stellar winds are commonly producing. The strongest CIII] emitters are preferentially located below the main sequence of star-forming galaxies; the median star formation rate is reduced by a factor of two. In addition, we find that the median stellar age of the strongest emitters is similar to 0.8 Gyr, which is about three times that of galaxies with EW(CIII]) < 10 angstrom. X-ray stacked imaging of the strong CIII] emitters sample show a marginal 2 sigma detection that is consistent with low-luminosity AGN log(L-X(2-10 keV)) similar to 42.9 erg s(-1). Previously presented spectral line analysis and classification support that the strongest emitters require the presence of an AGN. We conclude that this complementary set of evidence is indicative of significant AGN feedback acting in SFGs at 2 < z < 3.8, and it strongly contributes to star formation quenching. We find that quenching timescales of similar to 0.25-0.5 x 10(9) years are necessary for this AGN feedback to turn part of the star-forming galaxy population with M-star > 10(10) M-circle dot at z similar to 3 into the population of quiescent galaxies observed at redshift z similar to 1-2.