Abstract
Rest-frame UV spectra play a key role in the understanding of massive stellar populations, chemical evolution, feedback processes, and reionization. Indeed, in the current JWST era, the UV spectroscopic frontier has been pushed to higher redshifts than ever before, to finally reveal the first galaxies in the distant Universe. In this context, the HST COS Legacy Archive Spectroscopic SurveY (CLASSY) provides the first high-quality, high-resolution and broad-wavelength range catalog of 45 local star-forming galaxies (SFGs) in the rest-frame UV (1150 −2000 Å) to investigate their stellar and gas properties. The sample is representative of SFGs across all redshifts, including extremely metal-poor objects similar to reionization-era systems. Hence, CLASSY provides the ideal UV atlas with which we can tailor a complete UV diagnostic toolkit to explore the interstellar medium (ISM) properties (i.e., density, temperature, gas-phase metallicity, ionization parameter). In this talk I will present such a toolkit, obtained from the analysis of the main emission lines of CLASSY spectra excluding Lyα (i.e., C IV λλ1548,51, He IIλ1640, O III]λλ1661,6, C III] λλ1907,9) and the comparison with well-known optical diagnostics. We also compared our measurements with state-of-the-art photoionization models, to provide the best diagnostics plots to identify the main source of ionization. Overall, CLASSY and our UV toolkit can be crucial to interpret the earliest galaxies revealed with JWST.
