TDCOSMO III: Dark matter substructure meets dark energy — the effects of (sub)halos on strong-lensing measurements of $H_0$. (arXiv:2007.01308v1 [astro-ph.CO])

<a href="http://arxiv.org/find/astro-ph/1/au:+Gilman_D/0/1/0/all/0/1">Daniel Gilman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Birrer_S/0/1/0/all/0/1">Simon Birrer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Treu_T/0/1/0/all/0/1">Tommaso Treu</a>

Time delay cosmography uses the arrival time delays between images in strong

gravitational lenses to measure cosmological parameters, in particular the

Hubble constant $H_0$. The lens models used in time delay cosmography omit dark

matter subhalos and line-of-sight halos because their effects are assumed to be

negligible. We explicitly quantify this assumption by analyzing realistic mock

lens systems that include full populations of dark matter subhalos and

line-of-sight halos, applying the same modeling assumptions used in the

literature to infer $H_0$. We base the mock lenses on six quadruply-imaged

quasars that have delivered measurements of the Hubble constant, and quantify

the additional uncertainties and/or bias on a lens-by-lens basis. We show that

omitting dark substructure does not bias inferences of $H_0$. However,

perturbations from substructure contribute an additional source of random

uncertainty in the inferred value of $H_0$ that scales as the square root of

the lensing volume divided by the longest time delay. This additional source of

uncertainty, for which we provide a fitting function, ranges from $0.6 –

2.4%$. It may need to be incorporated in the error budget as the precision of

cosmographic inferences from single lenses improves, and sets a precision limit

on inferences from single lenses.

Time delay cosmography uses the arrival time delays between images in strong

gravitational lenses to measure cosmological parameters, in particular the

Hubble constant $H_0$. The lens models used in time delay cosmography omit dark

matter subhalos and line-of-sight halos because their effects are assumed to be

negligible. We explicitly quantify this assumption by analyzing realistic mock

lens systems that include full populations of dark matter subhalos and

line-of-sight halos, applying the same modeling assumptions used in the

literature to infer $H_0$. We base the mock lenses on six quadruply-imaged

quasars that have delivered measurements of the Hubble constant, and quantify

the additional uncertainties and/or bias on a lens-by-lens basis. We show that

omitting dark substructure does not bias inferences of $H_0$. However,

perturbations from substructure contribute an additional source of random

uncertainty in the inferred value of $H_0$ that scales as the square root of

the lensing volume divided by the longest time delay. This additional source of

uncertainty, for which we provide a fitting function, ranges from $0.6 –

2.4%$. It may need to be incorporated in the error budget as the precision of

cosmographic inferences from single lenses improves, and sets a precision limit

on inferences from single lenses.

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