The VIMOS Ultra Deep Survey:Nature, ISM properties, and ionizing spectra of CIII]lambda 1909 emitters at z=2-4

Context. Ultraviolet (UV) emission-line spectra are used to spectroscopically confirm high-z galaxies and increasingly also to determine their physical properties.Aims. We construct photoionization models to interpret the observed UV spectra of distant galaxies in terms of the dominant radiation field and the physical condition of the interstellar medium (ISM). These models are applied to new spectroscopic observations from the VIMOS Ultra Deep Survey (VUDS).Methods. We construct a large grid of photoionization models, which use several incident radiation fields (stellar populations, active galactic nuclei (AGNs), mix of stars and AGNs, blackbodies, and others), and cover a wide range of metallicities and ionization parameters. From these models we derive new spectral UV line diagnostics using equivalent widths (EWs) of [CIII]lambda 1909 doublet, CIV lambda 1549 doublet and the line ratios of [CIII], CIV, and He II lambda 1640 recombination lines. We apply these diagnostics to a sample of 450 [CIII]-emitting galaxies at redshifts z = 2-4 previously identified in VUDS.Results. We demonstrate that our photoionization models successfully reproduce observations of nearby and high-redshift sources with known radiation field and/or metallicity. For star-forming galaxies our models predict that [CIII] EW peaks at sub-solar metallicities, whereas CIV EW peaks at even lower metallicity. Using the UV diagnostics, we show that the average star-forming galaxy (EW([CIII]) similar to 2 angstrom) based on the composite of the 450 UV-selected galaxies' spectra The inferred metallicity and ionization parameter is typically Z = 0.3-0.5 Z(circle dot) and logU = 2.7 to 3, in agreement with earlier works at similar redshifts. The models also indicate an average age of 50-200 Myr since the beginning of the current star-formation, and an ionizing photon production rate, xi(ion), of log xi(ion)/erg(-1) Hz = 25.3-25.4. Among the sources with EW([CIII]) > = 10 angs