Radial density profiles of time-delay lensing galaxies

We present nonparametric radial mass profiles for 10 QSO strong-lensing galaxies. Five of the galaxies have profiles close to rho( r) proportional to 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 - although the modeling code did not know this - are either in clusters, or have very high mass-to-light ratios, suggesting dark matter-dominant lenses ( one is actually a pair of merging galaxies). The same models give H-0(-1)=15.2(-1.7)(+2.5) Gyr (H-0 = 64(-9)(+8) 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 H-0 and rho( r) within estimated uncertainties. Our result is contrary to some recent claims that lensing time delays imply either a low H-0 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 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.