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arXiv 1507.07937astro-ph.GAastro-ph.CO
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Illustris模拟中的透镜星系：幂律模型和来自时间延迟的哈勃常数偏差

Lens galaxies in the Illustris simulation: power-law models and the bias of the Hubble constant from time-delays

Dandan Xu, Dominique Sluse, Peter Schneider, Volker Springel, Mark Vogelsberger, Dylan Nelson, Lars Hernquist

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18 pages, 15 figures, 2 tables. Monthly Notices of the Royal Astronomical Society, Volume 456, Issue 1, p.739-755
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摘要：

幂律密度模型，即，$ρ(r) \propto r^{-γ'}$通常用于强引力透镜研究，包括用于推断哈勃常数$H_0$的所谓时间延迟技术。然而，由于强透镜特征形成的径向尺度对应于从重子物质为主到暗物质为主的过渡，因此没有已知的原因为什么星系的密度应该遵循幂律。幂律的假设人为地破坏了质量片简并，这是引力透镜中一种众所周知的不变性变换，它会影响哈勃常数和时间延迟的乘积，因此会导致从时间延迟技术确定$H_0$的偏差。在本文中，我们使用的Illustris流体动力学模拟来估计这种偏差的幅度，并了解它是如何与星系的观测特性。研究了大量的速度色散$σ_{SIE}$ >160 km/s的星系样本，发现幂律假设对$z=1$的偏差可以达到20%-50%，色散为$10\%-30\%$（rms）.然而，我们发现，通过选择具有接近等温的幂律模型斜率的星系，可以将$H_0$的偏差降低到<5%，并将散射降低到<10%。这可能会被用来在即将到来的大调查时代形成$H_0$测量的偏差较小的统计样本。

英文摘要：

A power-law density model, i.e., $ρ(r) \propto r^{-γ'}$ has been commonly employed in strong gravitational lensing studies, including the so-called time-delay technique used to infer the Hubble constant $H_0$. However, since the radial scale at which strong lensing features are formed corresponds to the transition from the dominance of baryonic matter to dark matter, there is no known reason why galaxies should follow a power law in density. The assumption of a power law artificially breaks the mass-sheet degeneracy, a well-known invariance transformation in gravitational lensing which affects the product of Hubble constant and time delay and can therefore cause a bias in the determination of $H_0$ from the time-delay technique. In this paper, we use the Illustris hydrodynamical simulations to estimate the amplitude of this bias, and to understand how it is related to observational properties of galaxies. Investigating a large sample of Illustris galaxies that have velocity dispersion $σ_{SIE}$>160 km/s at redshifts below $z=1$, we find that the bias on $H_0$ introduced by the power-law assumption can reach 20%-50%, with a scatter of $10\%-30\%$ (rms). However, we find that by selecting galaxies with an inferred power-law model slope close to isothermal, it is possible to reduce the bias on $H_0$ to <5%, and the scatter to <10%. This could potentially be used to form less biased statistical samples for $H_0$ measurements in the upcoming large survey era.

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