Developing gravitational lensing techniques to study the properties of dark matter in galaxies
Final Report Abstract
Understanding the dark matter distribution in the inner regions of galaxies constitutes one of the biggest challenges of extragalactic astrophysics. Gravitational lensing by galaxies provides important observational constraints for our models because it is sensitive to the total mass in the lens, and does not depend on whether this mass is luminous or not. This project studies the density profile of galaxies through two different channels : strong-lensing of quasars by galaxies and quasar micro-lensing. In the context of our participation to the COSMOGRAIL project, we have photometrically monitored the lensed quasar J1131−1231, and measured time-delays with a few percent accuracy. The lightcurves of the lensed images are among the most detailed obtained to-date. They reveal a rich microlensing signal that cannot be reproduced with standard models that assume microlensing of the continuum flux from the accretion disc, precluding to our planned inference of the local dark matter fraction in the lens. We found out that the likely reason for this unexpected signal is the contribution of the broad lines, emitting at the same wavelength as the continuum, and responding with a time-lag to its variations. We proposed to use this effect to perform ”microlensing-aided reverberation mapping”, a new technique which allows the measurement of the size of the broad line region based on the photometric lightcurves of gravitationally lensed quasars. On the other hand, we have also carried out a systematic study of the amplitude of microlensing in spectra of 17 lensed quasars. We uncovered sensible microlensing-induced deformations of the broad lines in 85% of the systems studied, opening new windows in the study of the structure and geometry of the yet poorly known region emitting those lines in the vicinity of the quasar supermassive black hole. In collaboration with a large international team, we have combined time delays for J1131−1231 with state-of-the-art lens models constrained by high signal to noise Hubble Space Telescope imaging, to measure the Hubble constant H0 with 6% precision. At the light of those results, we have re-examined how the use of a power-law density profile affects H0 measurement, and how it impacts the determination of the dark matter content of galaxies. We have explicitly shown that the mass-sheet transformation (MST), enables one to transform a realistic galaxy density profile composed of a luminous+dark matter component, into an exact power law, changing none of the observable except the time-delays. This implies that for systems with measured time-delays, it is possible to derive the density profile of the lensing galaxy with high accuracy (for fixed cosmological parameters). We also disclosed a new family of transformations which generalizes the MST. This so called ”Source Position Transformation”, leaves image positions and flux ratios ”invariant”, at the accuracy provided by best existing observational data. This adds freedom in lens modeling that has to be accounted for when estimating models uncertainties. We have studied how dark matter sub-halos in galaxies perturb the flux and position of strongly lensed images. Based on a systematic modeling of lensed quasars, we have disclosed positional anomalies in two systems. On the other hand, we have confronted anomalies of flux observed at radio wavelengths to expectation for a population of substructures drawn from state-of-the-art high resolution dark matter simulations. Our exhaustive work has enlightened the sensibility of such an investigation on various working assumptions, and shown that existing anomalies globally agree with ΛCDM . We have also estimated if mid-infrared (MIR) flux ratios were un-sensitive to microlensing as commonly assumed. Our simulations of realistic MIR quasar emission have revealed that microlensing of MIR flux is not negligible and may mimic, to some extend, perturbation caused by dark matter sub-halos.
Publications
- 2012, A&A, 538, 99, COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses. X. Modeling based on high-precision astrometry of a sample of 25 lensed quasars: consequences for ellipticity, shear, and astrometric anomalies
D. Sluse, V. Chantry, P. Magain, F. Courbin, G. Meylan
(See online at https://doi.org/10.1051/0004-6361/201015844) - 2012, A&A, 544, 62, Microlensing of the broad line region in 17 lensed quasars
D. Sluse, D. Hutsemékers, F. Courbin, G. Meylan, J. Wambsganss
(See online at https://doi.org/10.1051/0004-6361/201219125) - 2013, A&A, 553, A53, Mid-infrared microlensing of accretion disc and dusty torus in quasars: effects on flux ratio anomalies
D. Sluse, M. Kishimoto, T. Anguita, O. Wucknitz, J. Wambsganss
(See online at https://doi.org/10.1051/0004-6361/201220843) - 2013, A&A, 559, A37 Mass-sheet degeneracy, power-law models and external convergence: Impact on the determination of the Hubble constant from gravitational lensing
P. Schneider, D. Sluse
(See online at https://doi.org/10.1051/0004-6361/201321882) - 2014, A&A, 564, A103 Source-position transformation - an approximate invariance in strong gravitational lensing
P. Schneider, D. Sluse
(See online at https://doi.org/10.1051/0004-6361/201322106) - 2014, A&A, 565, A60, Imprints of the quasar structure in time-delay light curves: Microlensing-aided reverberation mapping
D. Sluse, M. Tewes
(See online at https://doi.org/10.1051/0004-6361/201424776)
