Precision Spectroscopy for 1.9 Million Galaxies from SDSS-IV: Improved Spectral Measurements and Catalogs for eBOSS

The Sloan Digital Sky Survey IV DR17 Extended Baryon Oscillation Spectroscopic Survey (eBOSS) consists of 2,233,939 high-quality optical galaxy spectra obtained through 2" fibers, providing a rich spectroscopic resource for studying galaxy evolution across a broad redshift range. eBOSS was designed primarily for large-scale structure and BAO measurements and, as such, focused on galaxy redshifts, leaving much of the information contained in the spectra unexplored. In addition to the trove of spectra, the large number of repeat observations (197,521 duplicate spectra) enables evaluation of the survey’s spectrophotometric quality. To unlock this potential, we introduce the eBOSS Data Analysis Pipeline (eBOSS-DAP), adapted from the MaNGA-DAP, which delivers uniform measurements of emission-line fluxes and equivalent widths, stellar and gas kinematics, continuum spectral indices, and stellar population fits. Using the eBOSS-DAP, we successfully analyze 1,899,553 high-quality galaxy spectra below a redshift of $z < 1.12$ to produce an extensive spectroscopic catalog for the eBOSS galaxy sample. We characterize the calibration performance, quantify the reliability of the derived measurements, and release a suite of data products that fully exploit the power of the eBOSS dataset. These catalogs open the door to a new generation of studies in galaxy evolution and cosmology.

💡 Research Summary

This paper presents a comprehensive re‑analysis of the Sloan Digital Sky Survey IV (SDSS‑IV) Data Release 17 (DR17) Extended Baryon Oscillation Spectroscopic Survey (eBOSS) galaxy spectra, delivering uniform measurements of emission‑line properties, stellar and gas kinematics, continuum spectral indices, and stellar‑population parameters for nearly two million galaxies. While eBOSS was originally designed for large‑scale structure and baryon acoustic oscillation (BAO) studies, its spectra contain a wealth of information about galaxy evolution that has remained largely untapped. The authors address this gap by adapting the Mapping Nearby Galaxies at Apache Point Observatory Data Analysis Pipeline (MaNGA‑DAP) to the specific characteristics of eBOSS data, creating the eBOSS‑DAP (Data Analysis Pipeline).

Data selection began with the SDSS‑DR17 “spAll‑v5_13_2” summary file, selecting objects classified as GALAXY, with PLATEQUALITY = GOOD, redshift 0.0005 < z < 1.12, and ZWARNING = 0. This yielded 2 233 939 spectra, of which 1 901 834 passed the quality cuts. After removing duplicates (197 521 repeat observations, corresponding to 1 704 313 unique galaxies), the final analysis sample comprises 1 899 553 high‑quality spectra. The sample spans six target classes—LRG, ELG, QSO (galaxies originally targeted as quasars), TDSS/SPIDERS, LOWZ LRG, and others—covering a redshift peak at z ≈ 0.55 and a median signal‑to‑noise per pixel of ~1.9, considerably lower than the SDSS‑I main galaxy sample.

The eBOSS‑DAP inherits the core algorithms of MaNGA‑DAP but modifies them for the lower S/N, broader wavelength coverage (3610–10140 Å, R ≈ 1560–2650), and the 2″ fiber aperture of eBOSS. The pipeline proceeds through: (1) flux‑calibrated spectrum ingestion (using the SDSS idlspec2D v5_13_2 reductions), (2) stellar continuum fitting with a library of theoretical simple stellar population (SSP) models spanning a wide range of ages, metallicities, and dust attenuations, employing pPXF for full‑spectrum fitting; (3) simultaneous non‑linear fitting of a comprehensive emission‑line list (≈ 50 lines, including Balmer series,