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

Abstract

Abstract 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 λ 1483,87, C iv λλ 1548,51, He ii λ 1640, O iii ] λ λ 1661,6, Si iii ] λλ 1883,92, C iii ] λ 1907,9). 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 λ 1907,9) ∼ 10 4 cm −3 ) than those inferred from their optical counterparts (e.g., n e ([S ii ] λ λ 6717,31) ∼ 10 2 cm −3 ; n e ([Ar iv ] λ λ 4714,41) ∼ 10 3 cm −3 ). T e derived from the hybrid ratio [O iii ] λ 1666/ λ 5007 proves to be reliable, implying differences in determining 12+log(O/H) compared to the optical counterpart O iii ] λ 4363/[O iii ] λ 5007 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; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>sSFR</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≳</mml:mo> <mml:mo>−</mml:mo> <mml:mn>8</mml:mn> </mml:math> 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.