2025ApJ...990...51K

We obtain spatially resolved kinematics with the Keck Cosmic Web Imager (KCWI) integral-field spectrograph for a sample of 14 massive ($11\lt {\mathrm{log}}_{10}{M}_{* }/{M}_{\odot }\lt 12$) lensing early-type galaxies at z ∼ 0.15-0.35 from the Sloan Lens ACS (SLACS) Survey. We integrate kinematic maps within the galaxy effective radius and examine the rotational and dispersion velocities, showing that 11/14 can be classified as slow rotators. The data set is unprecedented for galaxy-scale strong lenses in terms of signal-to-noise ratio (S/N), sampling, and calibration, vastly superseding previous studies. We find the primary contributions to systematic uncertainties to be the stellar template library and wavelength range of the spectral fit. Systematics are at the 1%-1.4% level, and positive covariance is <1% between sample galaxies and between spatial bins. This enables cosmographic inference with lensing time delays with a <2% uncertainty on H₀. We examine the effects of integration of the data cubes within circular apertures of different sizes and compare with Sloan Digital Sky Survey (SDSS) single-aperture velocity dispersions. We conclude that the velocity dispersions extracted from SDSS spectra for these 14 SLACS galaxies, which have low S/N (∼9 Å^–1) relative to the SLACS candidate parent sample, are subject to systematic errors (and covariance) due to stellar template library selection at the level of 3% (2%), which need to be added to the random errors. Comparison between our KCWI stellar velocity dispersions, our own analysis of SDSS spectra, and previously published measurements based on SDSS spectra shows mean differences within a few percent. However, these differences are not significant given the uncertainties of the SDSS-based stellar velocity dispersions. We find that the correlations between scaling relations using quantities inferred from dynamical, lensing, and stellar population models agree with previous SLACS analysis with no statistically significant change. A follow-up paper will present Jeans modeling in the context of broader studies of galaxy evolution and cosmology.