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We examine the diagnostic power of rest-frame ultraviolet (UV) nebularemission lines, and compare them to more commonly used rest-frame opticalemission lines, using the test case of a single star-forming knot of the brightlensed galaxy RCSGA 032727-132609 at redshift z~1.7. This galaxy has completecoverage of all the major rest-frame UV and optical emission lines fromMagellan/MagE and Keck/NIRSPEC. Using the full suite of diagnostic lines, weinfer the physical properties: nebular electron temperature (T_e), electrondensity (n_e), oxygen abundance (log(O/H)), ionisation parameter (log(q)) andinterstellar medium (ISM) pressure (log(P/k)). We examine the effectiveness ofthe different UV, optical and joint UV-optical spectra in constraining thephysical conditions. Using UV lines alone we can reliably estimate log(q), butthe same is difficult for log(O/H). UV lines yield a higher (~1.5 dex) log(P/k)than the optical lines, as the former probes a further inner nebular regionthan the latter. For this comparison, we extend the existing Bayesian inferencecode IZI, adding to it the capability to infer ISM pressure simultaneously withmetallicity and ionisation parameter. This work anticipates future rest-frameUV spectral datasets from the James Webb Space Telescope (JWST) at highredshift and from the Extremely Large Telescope (ELT) at moderate redshift.

Rest-frame UV and optical emission line diagnostics of ionised gas properties: a test case in a star-forming knot of a lensed galaxy at z~1.7