CLASSY IV. Exploring UV Diagnostics of the Interstellar Medium in Local High-z Analogs at the Dawn of the JWST Era

The Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY (CLASSY) provides the first high-resolution spectral catalog of 45 local high-z analogs in the ultraviolet (UV; 1200-2000 Å) to investigate their stellar and gas properties. Here we present a toolkit of UV interstellar medium (ISM) diagnostics, analyzing the main emission lines of CLASSY spectra (N iv] λ λ1483,87, C iv λλ1548,51, He ii λ1640, O iii]λ λ1661,6, Si iii] λλ1883,92, C iii] λ1907,9). Specifically, our aim is to provide accurate diagnostics for the reddening E(B − V), electron density n _e , electron temperature T _e , metallicity 12+log(O/H), and ionization parameter log(U), taking the different ISM ionization zones into account. We calibrate our UV toolkit using well-known optical diagnostics, analyzing archival optical spectra for all CLASSY targets. We find that UV density diagnostics estimate n _e values that are ∼1-2 dex higher (e.g., n _e (C iii]λ λ1907,9) ∼ 10⁴ cm⁻³) than those inferred from their optical counterparts (e.g., n _e ([S ii]λ λ6717,31) ∼ 10² cm⁻³; n _e ([Ar iv]λ λ4714,41) ∼ 10³ cm⁻³). T _e derived from the hybrid ratio [O iii] λ1666/λ5007 proves to be reliable, implying differences in determining 12+log(O/H) compared to the optical counterpart O iii] λ4363/[O iii] λ5007 within ∼ ±0.3 dex. We also investigate the relation between the stellar and gas E(B − V), finding consistent values at high specific star formation rates (sSFRs; log ( sSFR ) ≳ − 8 yr⁻¹), while at low sSFRs we confirmed an excess of dust attenuation in the gas. Finally, we investigate UV line ratios and equivalent widths to provide correlations with 12+log(O/H) and log(U), but note that there are degeneracies between the two. With this suite of UV-based diagnostics, we illustrate the pivotal role CLASSY plays in understanding the chemical and physical properties of high-z systems that JWST can observe in the rest-frame UV.