The COS Legacy Archive Spectroscopy Survey (CLASSY) Treasury Atlas*

<jats:title>Abstract</jats:title> <jats:p>Far-ultraviolet (FUV; ∼1200–2000 Å) 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 the James Webb Space Telescope 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 Cosmic Origins Spectrograph Legacy Spectroscopic Survey (CLASSY) treasury and its first high-level science product, the CLASSY atlas. CLASSY builds on the Hubble Space Telescope (HST) archive to construct the first high-quality (S/N<jats:sub>1500 Å</jats:sub> ≳ 5/resel), high-resolution (<jats:italic>R</jats:italic> ∼ 15,000) FUV spectral database of 45 nearby (0.002 < <jats:italic>z</jats:italic> < 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 < log <jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub>(<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) < 10.1), star formation rate (−2.0 < log SFR (<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O<jats:sub>32</jats:sub> < 38.0), reddening (0.02 < <jats:italic>E</jats:italic>(<jats:italic>B</jats:italic> − <jats:italic>V</jats:italic>) < 0.67), and nebular density (10 < <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> (cm<jats:sup>−3</jats:sup>) < 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a sample that is consistent with the <jats:italic>z</jats:italic> ∼ 0 mass–metallicity relationship, but is offset to higher star formation rates by roughly 2 dex, similar to <jats:italic>z</jats:italic> ≳ 2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.</jats:p>