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

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 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.

Early Results from GLASS–JWST. XXI. Rapid Asembly of a Galaxy at z = 6.23 Revealed by Its C/O Abundance

“…Considering a range of density n e = 1-10 3 cm −3 corresponds to only ±0.03 dex relative to our assumed 200 cm −3 (which is motivated by measurements at z  2; e.g., Sanders et al 2016). Although the high n e = 8200 cm −3 permitted by our data would increase the derived C/O by 0.08 dex, we also note that C III]-based densities are typically higher than found from the more widely used optical diagnostics (e.g., Mingozzi et al 2022). Reddening correction is somewhat difficult to assess, as a Milky Way-like attenuation law (e.g., that adopted from Cardelli et al 1989) indicates that C III] is more attenuated due to the strong 2175 Å "bump" feature, while laws with weak or no bump (e.g., Calzetti et al 2000;Reddy et al 2015) instead have larger attenuation of O III].…”

The Ionizing Spectra of Extremely Metal-poor O Stars: Constraints from the Only H ii Region in Leo P

“…Unfortunately, our KCWI spectra do not cover the most common nebular density diagnostic lines in the optical ([O II] λλ3727, 3729 or [S II] λλ6717, 6731). We do detect the higher-ionization, density-sensitive [Ar IV] λλ4711, 4740 lines, but because this diagnostic saturates at low n e , our [Ar IV] λ4711/[Ar IV] λ4740 measurement of 1.38 ± 0.64 only yields a rough upper limit of n e  10 2−3 cm −3 (e.g., Mingozzi et al 2022). Therefore, we adopt the electron density n 45 The density is securely in the low-density limit, meaning that the temperature we derive from our observed [O III] lines is not sensitive to the precise n e that we adopt.…”

CLASSY. VI. The Density, Structure, and Size of Absorption-line Outflows in Starburst Galaxies

“…We have re-sampled the spectra into bins of 0.18 Å (spectral resolution ∼6000-10,000 from the blue to red end; Xu et al 2022a). These galaxies' redshift are derived from fitting the optical emission lines discussed in Mingozzi et al (2022).…”