  In the context of observations of the rest-frame ultraviolet and optical
emission from distant galaxies, we explore the emission-line properties of
photoionization models of active and inactive galaxies. Our aim is to identify
new line-ratio diagnostics to discriminate between gas photoionization by
active galactic nuclei (AGN) and star formation. We use a standard
photoionization code to compute the emission from AGN narrow-line regions and
compare this with calculations of the nebular emission from star-forming
galaxies achieved using the same code. We confirm the appropriateness of widely
used optical spectral diagnostics of nuclear activity versus star formation and
explore new diagnostics at ultraviolet wavelengths. We find that combinations
of a collisionally excited metal line or line multiplet, such as CIV 1548,1551,
OIII]1661,1666, NIII]1750, [SiIII]1883+[SiIII]1892 and [CIII]1907+CIII]1909,
with the HeII 1640 recombination line are individually good discriminants of
the nature of the ionizing source. Diagrams involving at least 3 of these lines
allow an even more stringent distinction between active and inactive galaxies,
as well as valuable constraints on interstellar gas parameters and the shape of
the ionizing radiation. Several line ratios involving Ne-based emission lines,
such as [NeIV]2424, [NeIII]3343 and [NeV]3426, are also good diagnostics of
nuclear activity. Our results provide a comprehensive framework to identify the
sources of photoionization and physical conditions of the ionized gas from the
ultraviolet and optical nebular emission from galaxies. This will be
particularly useful to interpret observations of high-redshift galaxies with
future facilities, such as the James Webb Space Telescope and extremely large
ground-based telescopes.
