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Register a new userOn the Effects of Line-of-Sight Structures on Lensing Flux-ratio Anomalies in a LCDM Universe
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The flux-ratio anomalies observed in multiply-lensed quasar images are most plausibly explained as the result of perturbing structures superposed on the underlying smooth matter distribution of the primary lens. The cold dark matter cosmological model predicts that a large number of substructures should survive inside larger halos but, surprisingly, this population alone has been shown to be insufficient to explain the observed distribution of the flux ratios of quasars multiple images. Other halos (and their own subhalos) projected along the line of sight to the primary lens have been considered as additional source of perturbation. In this work, we use ray tracing through the Millennium II simulation to investigate the importance of projection effects due to halos and subhalos of mass m>1E8 Msun/h and extend our analysis to lower masses, m>1E6 Msun/h, using Monte-Carlo halo distributions. We find that the magnitude of the violation depends strongly on the density profile and concentration of the intervening halos, but clustering plays only a minor role. For a typical lensing geometry (lens at redshift 0.6 and source at redshift 2), background haloes (behind the main lens) are more likely to cause a violation than foreground halos. We conclude that line-of-sight structures can be as important as intrinsic substructures in causing flux-ratio anomalies. The combined effect of perturbing structures within the lens and along the line of sight in the LCDM universe results in a cusp-violation probability of 20-30%. This alleviates the discrepancy between models and current data, but a larger observational sample is required for a stronger test of the theory.
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Measurements of The Hubble-Lemaitre constant from early- and local-universe observations show a significant discrepancy. In an attempt to understand the origin of this mismatch, independent techniques to measure H0 are required. One such technique, strong lensing time delays, is set to become a leading contender amongst the myriad methods due to forthcoming large strong lens samples. It is therefore critical to understand the systematic effects inherent in this method. In this paper, we quantify the influence of additional structures along the line-of-sight by adopting realistic light cones derived from the CosmoDC2 semi-analytical extra-galactic catalogue. Using multiple lens plane ray-tracing to create a set of simulated strong lensing systems, we have investigated the impact of line-of-sight structures on time-delay measurements and in turn, on the inferred value of H0. We have also tested the reliability of existing procedures for correcting for line-of-sight effects. We find that if the integrated contribution of the line-of-sight structures is close to a uniform mass sheet, the bias in H0 can be adequately corrected by including a constant external convergence $kappa_{ext}$ in the lens model. However, for realistic line-of-sight structures comprising many galaxies at different redshifts, this simple correction overestimates the bias by an amount that depends linearly on the median external convergence. We, therefore, conclude that lens modelling must incorporate multiple lens planes to account for line-of-sight structures for accurate and precise inference of H0.
Multiply-imaged quasars and AGNs observed in the mid-infrared (MIR) range are commonly assumed to be unaffected by the microlensing produced by the stars in their lensing galaxy. In this paper, we investigate the validity domain of this assumption. Indeed, that premise disregards microlensing of the accretion disc in the MIR range, and does not account for recent progress in our knowledge of the dusty torus. To simulate microlensing, we first built a simplified image of the quasar composed of an accretion disc, and of a larger ring-like torus. The mock quasars are then microlensed using an inverse ray-shooting code. We simulated the wavelength and size dependence of microlensing for different lensed image types and fraction of compact objects projected in the lens. This allows us to derive magnification probabilities as a function of wavelength, as well as to calculate the microlensing-induced deformation of the spectral energy distribution of the lensed images. We find that microlensing variations as large as 0.1 mag are very common at 11 microns (observer-frame). The main signal comes from microlensing of the accretion disc, which may be significant even when the fraction of flux from the disc is as small as 5 % of the total flux. We also show that the torus of sources with Lbol <~ 10^45 erg/s is expected to be noticeably microlensed. Microlensing may thus be used to get insight into the rest near-infrared inner structure of AGNs. Finally, we investigate whether microlensing in the mid-infrared can alter the so-called Rcusp relation that links the fluxes of the lensed images triplet produced when the source lies close to a cusp macro-caustic. This relation is commonly used to identify massive (dark-matter) substructures in lensing galaxies. We find that significant deviations from Rcusp may be expected, which means that microlensing can explain part of the flux ratio problem.
Cluster lensing has become an important tool in the search for high redshift galaxies through its ability to magnify sources. In order to determine the intrinsic properties of these galaxies, lensing mass models must be constructed to determine the magnification of the images. These models are traditionally two-dimensional, focusing on the mass within the cluster and either ignoring or approximating any contribution from line-of-sight galaxies. In this paper, we present the first full set of three-dimensional mass models of the six Hubble Frontier Fields and use them to test for systematic biases in magnifications due to using the traditional 2D approach. We find that omitting foreground or background galaxies causes image position offsets between 0.1-0.4, a non-negligible fraction of the typical 0.3-0.7 residuals of current state-of-the-art models. We also find that median image magnifications can shift by up to 6%, though it is dependent on the field. This can be alleviated in some cases by approximating the mass in the lensing plane, but a 5% magnification bias still exists in other cases; image position offsets are also improved, but are still present at 0.10.
We present an analysis of the line-of-sight structure toward a sample of ten strong lensing cluster cores. Structure is traced by groups that are identified spectroscopically in the redshift range, 0.1 $leq$ z $leq$ 0.9, and we measure the projected angular and comoving separations between each group and the primary strong lensing clusters in each corresponding line of sight. From these data we measure the distribution of projected angular separations between the primary strong lensing clusters and uncorrelated large scale structure as traced by groups. We then compare the observed distribution of angular separations for our strong lensing selected lines of sight against the distribution of groups that is predicted for clusters lying along random lines of sight. There is clear evidence for an excess of structure along the line of sight at small angular separations ($theta leq 6$) along the strong lensing selected lines of sight, indicating that uncorrelated structure is a significant systematic that contributes to producing galaxy clusters with large cross sections for strong lensing. The prevalence of line-of-sight structure is one of several biases in strong lensing clusters that can potentially be folded into cosmological measurements using galaxy cluster samples. These results also have implications for current and future studies -- such as the Hubble Space Telescope Frontier Fields -- that make use of massive galaxy cluster lenses as precision cosmological telescopes; it is essential that the contribution of line-of-sight structure be carefully accounted for in the strong lens modeling of the cluster lenses.
Anomalies in the flux-ratios of the images of quadruply-lensed quasars have been used to constrain the nature of dark matter. Assuming these lensing perturbations are caused by dark matter haloes, it is possible to constrain the mass of a hypothetical Warm Dark Matter (WDM) particle to be $m_chi > 5.2$ keV. However, the assumption that perturbations are only caused by DM haloes might not be correct as other structures, such as filaments and pancakes, exist and make up a significant fraction of the mass in the universe, ranging between 5$%$ -- 50$%$ depending on the dark matter model. Using novel fragmentation-free simulations of 1 and 3keV WDM cosmologies we study these non-halo structures and estimate their impact on flux-ratio observations. We find that these structures display sharp density gradients with short correlation lengths, and can contribute more to the lensing signal than all haloes up to the half-mode mass combined, thus reducing the differences expected among WDM models. We estimate that this becomes especially important for any flux-ratio based constraint sensitive to haloes of mass $M sim 10^8 M_odot$. We conclude that accounting for all types structures in strong-lensing observations is required to improve the accuracy of current and future constraints.
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