The VIMOS Ultra Deep Survey: Nature, ISM properties, and ionizing
  spectra of CIII]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: To interpret the observed UV spectra of distant galaxies in
terms of the dominant radiation field and the physical condition of the
interstellar medium. Methods: We construct a large grid of photoionization
models and derive new spectral UV line diagnostics using equivalent widths
(EWs) of CIII]1909, CIV1549 and the line ratios of CIII], CIV, and HeII1640
recombination lines. We apply these diagnostics to a sample of 450
CIII]-emitting galaxies at z=2-4 previously identified in the VIMOS Ultra Deep
Survey. Results: We show that the average star-forming galaxy (EW(CIII])~2A) is
well described by stellar photoionization from single and binary stars. The
inferred metallicity and ionization parameter is typically Z=0.3-0.5Zsun and
logU=-2.7 to -3, in agreement with earlier works at similar redshifts. The
models also indicate an average age of 50-200Myr since the beginning of the
current star-formation, and an ionizing photon production rate, xi_ion, of
log(xi_ion/[Hz/erg])~25.3-25.4. Among the sources with EW(CIII])=10-20A, ~30%
are likely dominated by AGNs. Their derived metallicity is low, Z=0.02-0.2Zsun,
and the ionization parameter higher (logU=-1.7). To explain the average UV
observations of the strongest but rarest CIII] emitters (EW(CIII])>20A), we
find that stellar photoionization is clearly insufficient. A radiation field
consisting of a mix of a young stellar population (log(xi_ion/[Hz/erg])~25.7)
plus an AGN component is required. Furthermore an enhanced C/O abundance ratio
is needed for metallicities Z=0.1-0.2Zsun and logU=-1.7 to -1.5. Conclusions:
The UV diagnostics we propose should serve as an important basis for the
interpretation of observations of high-redshift galaxies. [abridged]
