M. Mingozzi, B. L. James, K. Z. Arellano-Córdova, D. A. Berg, P. Senchyna, J. Chisholm, J. Brinchmann, A. Aloisi, R. O. Amorín, S. Charlot, A. Feltre, M. Hayes, T. Heckman, A. Henry, S. Hernandez, N. Kumari, C. Leitherer, M. Llerena, C. L. Martin, T. Nanayakkara, S. Ravindranath, E. D. Skillman, Y. Sugahara, A. Wofford, X. Xu

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] λλ1483,87, C iv λλ1548,51, He iiλ1640, O iii]λλ1661,6, 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]λλ1907,9) ∼ 10⁴ cm⁻³) than those inferred from their optical counterparts (e.g., n_e([S ii]λλ6717,31) ∼ 10² cm⁻³; n_e([Ar iv]λλ4714,41) ∼ 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.

Keywords
Dwarf galaxies; Ultraviolet astronomy; Galaxy chemical evolution; Galaxy spectroscopy; High-redshift galaxies; Emission line galaxies

The Astrophysical Journal
Volume 939, Number 2
2022 November

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