Kavli Affiliate: Philip J. Marshall
| First 5 Authors: Tian Li, Thomas E. Collett, Philip J. Marshall, Sydney Erickson, Wolfgang Enzi
| Summary:
The time delay between multiple images of strongly lensed quasars has been
used to infer the Hubble constant. The primary systematic uncertainty for
time-delay cosmography is the mass-sheet transform (MST), which preserves the
lensing observables while altering the inferred $H_0$. The TDCOSMO
collaboration used velocity dispersion measurements of lensed quasars and
lensed galaxies to infer that mass sheets are present, which decrease the
inferred $H_0$ by 8$%$. Here, we test the assumption that the density profiles
of galaxy-galaxy and galaxy-quasar lenses are the same. We use a composite
star-plus-dark-matter mass profile for the parent deflector population and
model the selection function for galaxy-galaxy and galaxy-quasar lenses. We
find that a power-law density profile with an MST is a good approximation to a
two-component mass profile around the Einstein radius, but we find that
galaxy-galaxy lenses have systematically higher mass-sheet components than
galaxy-quasar lenses. For individual systems, $lambda_mathrm{int}$ correlates
with the ratio of the half-light radius and Einstein radius of the lens. By
propagating these results through the TDCOSMO methodology, we find that $H_0$
is lowered by a further $sim$3%. Using the velocity dispersions from
citet{slacs9} and our fiducial model for selection biases, we infer $H_0 =
66pm4 mathrm{(stat)} pm 1 mathrm{(model sys)} pm 2
mathrm{(measurement sys)} mathrm{km} mathrm{s}^{-1}
mathrm{Mpc}^{-1}$ for the TDCOSMO plus SLACS dataset. The first residual
systematic error is due to plausible alternative choices in modeling the
selection function, and the second is an estimate of the remaining systematic
error in the measurement of velocity dispersions for SLACS lenses. Accurate
time-delay cosmography requires precise velocity dispersion measurements and
accurate calibration of selection biases.
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