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Abstract: The COS Legacy Archive Spectroscopic SurveY (CLASSY) is designed to provide the community with a spectral atlas of 45 nearby star-forming galaxies that were chosen to cover similar properties to those seen at high z (z > 6). The prime high-level science product of CLASSY is accurately coadded UV spectra, ranging from ∼1000 to 2000 Å, derived from a combination of archival and new data obtained with HST’s Cosmic Origins Spectrograph (COS). This paper details the multistage technical processes of creating this prime data product and the methodologies involved in extracting, reducing, aligning, and coadding far-ultraviolet and near-ultraviolet (NUV) spectra. We provide guidelines on how to successfully utilize COS observations of extended sources, despite COS being optimized for point sources, and best-practice recommendations for the coaddition of UV spectra in general. Moreover, we discuss the effects of our reduction and coaddition techniques in the scientific application of the CLASSY data. In particular, we find that accurately accounting for flux calibration offsets can affect the derived properties of the stellar populations, while customized extractions of NUV spectra for extended sources are essential for correctly diagnosing the metallicity of galaxies via C iii] nebular emission. Despite changes in spectral resolution of up to ∼25% between individual data sets (due to changes in the COS line-spread function), no adverse affects were observed on the difference in velocity width and outflow velocities of isolated absorption lines when measured in the final combined data products, owing in part to our signal-to-noise regime of S/N < 20.

Strong nebular emission lines are an important diagnostic tool for tracing the evolution of star-forming galaxies across cosmic time. However, different observational setups can affect these lines, and the derivation of the physical nebular properties. We analyze 12 local star-forming galaxies from the COS Legacy Spectroscopy SurveY (CLASSY) to assess the impact of using different aperture combinations on the determination of the physical conditions and gas-phase metallicity. We compare optical spectra observed with the Sloan Digital Sky Survey Data Release aperture, which has a 3″ diameter similar to COS, IFU, and long-slit spectra, including new LBT/MODS observations of five CLASSY galaxies. We calculate the reddening, electron densities and temperatures, metallicities, star formation rates, and equivalent widths (EWs). We find that measurements of the electron densities and temperatures, and metallicity remained roughly constant with aperture size, indicating that the gas conditions are relatively uniform for this sample. However, using IFU observations of three galaxies, we find that the E(B - V) values derived from the Balmer ratios decrease (by up to 53%) with increasing aperture size. The values change most significantly in the center of the galaxies, and level out near the COS aperture diameter of 2.″5. We examine the relative contributions from the gas and stars using the Hα and [O iii] λ5007 EWs as a function of aperture light fraction, but find little to no variations within a given galaxy. These results imply that the optical spectra provide nebular properties appropriate for the far-UV CLASSY spectra, even when narrow 1.″0 long-slit observations are used.

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/N1500 Å ≳ 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 < log M ⋆(M ⊙) < 10.1), star formation rate (−2.0 < log SFR (M ⊙ yr−1) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O32 < 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 the z ∼ 0 mass-metallicity relationship, but is offset to higher star formation rates 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.

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.

The rest-frame UV emission from massive stars contains a wealth ofinformation about the physical nature and conditions of star formation ingalaxies. Using studies of the rest-frame UV, the past decade has witnessed thebeginning of knowledge about the existence and properties of galaxies duringthe first few billion years after the Big Bang. This period of historycorresponds to the formation of the first stars, the rapid formation of galaxystellar populations, the reionization of the IGM, the production anddissemination of heavy elements, and the formation of the first black holes.Massive stars in these galaxies drive all of these events, and their lightdominates the spectral energy distributions of galaxies. As we look to the2020s, fundamental questions remain about the nature of these stellarpopulations and their evolution, from just before the peak of the cosmic starformation density (z~3), up to the epoch of reionization (z > 6). This nextdecade will provide transformative gains both in our ability to identifystar-forming galaxies and accreting supermassive black holes at these earlyepochs with imaging surveys in the rest-frame UV (e.g., LSST, WFIRST).Ground-based, rest-frame UV spectroscopy on >20 m-class telescopes (e.g.,GMT/TMT) offers the ability to investigate the astrophysical conditions ingalaxies at the earliest cosmic times. This includes studies of the evolutionin galaxy stellar populations, gas ionization (temperature, pressure),metallicity, and interstellar (and circumgalactic) gas kinematics and coveringfractions. In this white paper, we describe the scientific prospects and therequirements for research in this area.

The epoch of reionization (6 < z < 10) marks the period in our universe whenthe first large galaxies grew to fruition, and began to affect the universearound them. Massive stars, and potentially accreting supermassive black holes,filled the universe with ionizing radiation, burning off the haze of neutralgas that had filled the intergalactic medium (IGM) since recombination(z~1000). The evolution of this process constrains key properties of theseearliest luminous sources, thus observationally constraining reionization is akey science goal for the next decade. The measurement of Lyman-alpha emissionfrom photometrically-identified galaxies is a highly constraining probe ofreionization, as a neutral IGM will resonantly scatter these photons, reducingdetectability. While significant work has been done with 8-10m telescopes,these observations require extremely large telescopes (ELTs); the flux limitsavailable from today's 10m class telescopes are sufficient for only thebrightest known galaxies (m < 26). Ultra-deep surveys with the Giant MagellanTelescope (GMT) and Thirty Meter Telescope (TMT) will be capable of detectingLyman-alpha emission from galaxies 2-3 magnitudes fainter than today's deepestsurveys. Wide-field fiber spectroscopy on the GMT combined with narrow-fieldAO-assisted slit spectroscopy on the TMT will be able to probe the expectedsize of ionized bubbles throughout the epoch of reionization, following updegree scale deep imaging surveys with the Wide Field Infrared Space Telescope.These data will provide the first resolved Lyman-alpha-based maps of theionized intergalactic medium throughout the epoch of reionization, constrainingmodels of both the temporal and spatial evolution of this phase change.