CLASSY. VIII. Exploring the Source of Ionization with UV Interstellar Medium Diagnostics in Local High-z Analogs*

Abstract

In the current JWST era, rest-frame UV spectra play a crucial role in enhancing our understanding of the interstellar medium (ISM) and stellar properties of the first galaxies in the epoch of reionization (z > 6). Here, we compare well-known and reliable optical diagrams sensitive to the main ionization source (i.e., star formation, SF; active galactic nuclei, AGN; and shocks) to UV counterparts proposed in the literature—the so-called “UV–BPT diagrams”—using the HST COS Legacy Archive Spectroscopic SurveY (CLASSY), which is the largest high-quality, high-resolution, and broad-wavelength range atlas of far-UV spectra for 45 local star-forming galaxies. In particular, we explore where CLASSY UV line ratios are located in the different UV diagnostic plots, taking into account state-of-the-art photoionization and shock models, and, for the first time, the measured ISM and stellar properties (e.g., gas-phase metallicity, ionization parameter, carbon abundance, and stellar age). We find that the combination of C iii] λ λ1907,9 He ii λ1640 and O iii] λ1666 can be a powerful tool to separate between SF, shocks, and AGN at subsolar metallicities. We also confirm that alternative diagrams without O iii] λ1666 still allow us to define an SF-locus, with some caveats. Diagrams including C iv λ λ1548,51 should be taken with caution given the complexity of this doublet profile. Finally, we present a discussion detailing the ISM conditions required to detect UV emission lines, visible only in low gas-phase metallicity (12 + log(O/H) ≲ 8.3) and high ionization parameter (log(U) ≳ −2.5) environments. Overall, CLASSY and our UV toolkit will be crucial in interpreting the spectra of the earliest galaxies that JWST is currently revealing.

The MUSE Ultra Deep Field (MUDF). V. Characterizing the Mass–Metallicity Relation for Low-mass Galaxies at z ∼ 1–2

“…This diagnostic identifies 20 sources as AGN that we exclude from the analysis because the abundance methods are calibrated for stellar-ionized gas. In addition, we take advantage of the rest-frame UV coverage at higher redshifts to select sources where the He II λ1640, C III] λ1907, 1909, and O III] λ1666 emission lines are detected at S/N 4 to construct the C3He2-O3He2 ionization diagram discussed in Mingozzi et al (2024) and Feltre et al (2016). There are 11 sources meeting these criteria, and all are consistent with star-forming galaxies.…”

Characterizing the Average Interstellar Medium Conditions of Galaxies at z ∼ 5.6–9 with Ultraviolet and Optical Nebular Lines

“…Several diagnostic diagrams using UV lines have been proposed in recent works (Feltre et al 2016;Jaskot & Ravindranath 2016;Nakajima et al 2018;Byler et al 2020;Hirschmann et al 2022) to distinguish between star formation, AGNs, and shock-driven photoionization. In this work, we adopt the C III] λλ1907, 1909/He II λ1640 versus O III] λ1666/He II λ1640 (hereafter, C3He2-O3He2) diagram, which is suggested to show the best classification power among those diagnostics (Mingozzi et al 2023). In Figure 7, we present our measurements along with those from local dwarf galaxies (Berg et al 2016(Berg et al , 2019(Berg et al , 2022Senchyna et al 2017) and He II emitters at z ∼ 2−4 (Nanayakkara et al 2019;Saxena et al 2020).…”

“…We overplot the separators between star formation, AGNs, and shocks derived by comparing the ionization models (Feltre et al 2016;Gutkin et al 2016;Alarie & Morisset 2019) with the well-studied local dwarf galaxies (Mingozzi et al 2023) in the left panel of Figure 7. The position of the composite spectrum in the C3He2-O3He2 diagram indicates that highredshift galaxies are dominated by ionization from star formation with a modest significance of ∼1σ.…”

“…Thus, the composite region introduced in Hirschmann et al (2019) may be more applicable to massive galaxies, where lower-mass galaxies (like local dwarfs and high-redshift galaxies) are more likely to show The line ratios measured from this work are marked as the red circle. A series of local galaxies compiled from the literature are marked as purple (Berg et al 2016), cyan (Berg et al 2019), green (Senchyna et al 2017), and blue (Berg et al 2022;Olivier et al 2022;Mingozzi et al 2023) open stars. We highlight the local galaxy SBS 0335-052, which has similar metallicity and is close to our composite spectrum in the diagram, as the blue solid star.…”

CLASSY. IX. The Chemical Evolution of the Ne, S, Cl, and Ar Elements

“…We do not include nitrogen here because it is the focus of an upcoming paper (K. Z. Arellano-Córdova et al 2024, in preparation; see also, Stephenson et al 2023). These elements are ejected into the interstellar medium by core-collapse supernovae (CCSNe; Henry & Worthey 1999;Kobayashi et al 2020a;Prantzos et al 2020).…”