Radial Density Profiles of Time‐Delay Lensing Galaxies

We present non-parametric radial mass profiles for ten QSO strong lensing galaxies. Five of the galaxies have profiles close to ρ(r) ∝ r−2, while the rest are closer to r−1, consistent with an NFW profile. The former are all relatively isolated early-types and dominated by their stellar light. The latter —though the modeling code did not know this — are either in clusters, or have very high massto-light, suggesting dark-matter dominant lenses (one is a actually pair of merging galaxies). The same models give H −1 0 = 15.2 +2.5 −1.7 Gyr (H0 = 64 +8 −9 km s−1 Mpc−1), consistent with a previous determination. When tested on simulated lenses taken from a cosmological hydrodynamical simulation, our modeling pipeline recovers both H0 and ρ(r) within estimated uncertainties. Our result is contrary to some recent claims that lensing time delays imply either a low H0 or galaxy profiles much steeper than r−2. We diagnose these claims as resulting from an invalid modeling approximation: that small deviations from a power-law profile have a small effect on lensing time-delays. In fact, as we show using using both perturbation theory and numerical computation from a galaxy-formation simulation, a first-order perturbation of an isothermal lens can produce a zeroth-order change in the time delays. Subject headings: gravitational lensing, galaxies: halos 1.