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The Sloan Lens ACS Survey. XII. Extending Strong Lensing to Lower Masses

We present observational results from a new Hubble Space Telescope (HST) Snapshot program to extend the methods of the Sloan Lens ACS (SLACS) Survey to lower lens-galaxy masses. We discover 40 new galaxy-scale strong lenses, which we supplement with 58 previously discovered SLACS lenses. In addition, we determine the posterior PDFs of the Einstein radius for 33 galaxies (18 new and 15 from legacy SLACS data) based on single lensed images. We find a less-than-unity slope of $0.64\pm0.06$ for the $\log_{10} σ_*$-$\log_{10} σ_{\rm SIE}$ relation, which corresponds to a 6-$σ$ evidence that the total mass-density profile of early-type galaxies varies systematically in the sense of being shallower at higher lens-galaxy velocity dispersions. The trend is only significant when single-image systems are considered, highlighting the importance of including both "lenses" and "non-lenses" for an unbiased treatment of the lens population when extending to lower mass ranges. By scaling simple stellar population models to the HST I-band data, we identify a strong trend of increasing dark-matter fraction at higher velocity dispersions, which can be alternatively interpreted as a trend in the stellar initial mass function (IMF) normalization. Consistent with previous findings and the suggestion of a non-universal IMF, we find that a Salpeter IMF is ruled out for galaxies with velocity dispersion less than $180$ km/s. Considered together, our mass-profile and dark-matter-fraction trends with increasing galaxy mass could both be explained by an increasing relative contribution on kiloparsec scales from a dark-matter halo with a spatial profile more extended than that of the stellar component.