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])

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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