SDSS-IV MaNGA: Modeling the Spectral Line-spread Function to Subpercent Accuracy

Abstract

The Sloan Digital Sky Survey IV Mapping Nearby Galaxies at APO (MaNGA) program has been operating from 2014 to 2020, and has now observed a sample of 9269 galaxies in the low redshift universe ( z  ∼ 0.05) with integral-field spectroscopy. With rest-optical ( λλ 0.36–1.0 μ m) spectral resolution R  ∼ 2000 the instrumental spectral line-spread function (LSF) typically has 1 σ width of about 70 km s −1 , which poses a challenge for the study of the typically 20–30 km s −1  velocity dispersion of the ionized gas in present-day disk galaxies. In this contribution, we present a major revision of the MaNGA data pipeline architecture, focusing particularly on a variety of factors impacting the effective LSF (e.g., under-sampling, spectral rectification, and data cube construction). Through comparison with external assessments of the MaNGA data provided by substantially higher-resolution R  ∼ 10,000 instruments, we demonstrate that the revised MPL-10 pipeline measures the instrumental LSF sufficiently accurately (≤0.6% systematic, 2% random around the wavelength of H α ) that it enables reliable measurements of astrophysical velocity dispersions σ H α  ∼ 20 km s −1 for spaxels with emission lines detected at signal-to-noise ratio > 50. Velocity dispersions derived from [O II ], H β , [O III ], [N II ], and [S II ] are consistent with those derived from H α to within about 2% at σ H α  > 30 km s −1 . Although the impact of these changes to the estimated LSF will be minimal at velocity dispersions greater than about 100 km s −1 , scientific results from previous data releases that are based on dispersions far below the instrumental resolution should be reevaluated.