The COS Legacy Archive Spectroscopy SurveY (CLASSY) Treasury Atlas

Far-ultraviolet (FUV; ~1200-2000 angstroms) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of JWST will soon usher in a new era, pushing the UV spectroscopic frontier to higher redshifts than ever before, however, its success hinges on a comprehensive understanding of the massive star populations and gas conditions that power the observed UV spectral features. This requires a level of detail that is only possible with a combination of ample wavelength coverage, signal-to-noise, spectral-resolution, and sample diversity that has not yet been achieved by any FUV spectral database. We present the COS Legacy Spectroscopic SurveY (CLASSY) treasury and its first high level science product, the CLASSY atlas. CLASSY builds on the HST archive to construct the first high-quality (S/N_1500 >~ 5/resel), high-resolution (R~15,000) FUV spectral database of 45 nearby (0.002 < z < 0.182) star-forming galaxies. The CLASSY atlas, available to the public via the CLASSY website, is the result of optimally extracting and coadding 170 archival+new spectra from 312 orbits of HST observations. The CLASSY sample covers a broad range of properties including stellar mass (6.2 < logM_star(M_sol) < 10.1), star formation rate (-2.0 < log SFR (M_sol/yr) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O_32 < 38.0), reddening (0.02 < E(B-V < 0.67), and nebular density (10 < n_e (cm^-3) < 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a sample that is consistent with z~0 mass-metallicity relationship, but is offset to higher SFRs by roughly 2 dex, similar to z >~2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.Comment: Accepted for publication in Ap

Massive Stars in Low-metallicity Galaxies in the CLASSY Survey

Abstract: Rest-frame far-ultraviolet spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive star populations (MSs), chemical evolution, feedback processes, and reionization. JWST is ushering in a new era, pushing the FUV frontier beyond z=10. The success of such endeavors hinges on a comprehensive understanding of the MSs and gas conditions that power the observed spectra. The COS Legacy Archive Spectroscopic SurveY (CLASSY) is a powerful and promising solution providing high-quality, high-resolution FUV spectra of 45 nearby star-forming galaxies. The spectra contain a suite of features that simultaneously characterize the MSs that populate metal-poor galaxies, physical properties of large-scale outflows, and chemical abundance patterns. The CLASSY sample is consistent with the z 0 mass-metallicity relationship and spans 1.5 dex in metallicity. These unique properties make CLASSY the benchmark training set for studies of MSs in star-forming galaxies both across cosmic time and connecting metal-poor to metal-rich populations