  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 $\log_{10}
{\sigma}_*$-$\log_{10} {\sigma}_{\rm SIE}$ relation, which corresponds to a
6-$\sigma$ 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.

Astrophysics of Galaxies