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Probing Small-Scale Structure in Galaxies with Strong Gravitational Lensing Arthur Congdon Rutgers University
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Strong Gravitational Lensing S L O Lensing is sensitive to all mass, be it luminous or dark, smooth or lumpy quasar, z ~ 1 5 galaxy, z ~ 0.2 1 Lens equation:where
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Lensing by a Singular Isothermal Sphere (SIS) Reduced deflection angle: Lens equation: Source directly behind lens produces Einstein ring with angular “radius” E
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SIS Lensing Courtesy of S. Rappaport
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SIS with Shear Lens equation: 1, 2 or 4 images can be produced
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SIS with Shear Courtesy of S. Rappaport
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CASTLES ~ www.cfa.harvard.edu/castles Lensing by Galaxies: Hubble Space Telescope Images “Double” “Quad” “Ring”
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Quasars as Lensed Sources Radio emission comes from extended jets Optical, UV and X-ray emission comes mainly from the central accretion disk
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Via Lactea CDM simulation (Diemand et al. 2007)
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Via Lactea CDM simulation (Diemand et al. 2007) Hierarchical structure formation: small objects form first, then aggregate into larger objects
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Via Lactea CDM simulation (Diemand et al. 2007) Hierarchical structure formation: small objects form first, then aggregate into larger objects Large halos contain the remnants of their many progenitors substructure
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Clusters look like this good! cluster of galaxies, ~10 15 M sun single galaxy, ~10 12 M sun (Moore et al. 1999) vs.
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cluster of galaxies, ~10 15 M sun single galaxy, ~10 12 M sun vs. Galaxies don’t - bad? (Moore et al. 1999)
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Strigari et al (2007) Missing Satellites Problem
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Multipole Models of Four-Image Gravitational Lenses with Anomalous Flux Ratios MNRAS 364:1459 (2005)
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Four-Image Lenses Source plane Image plane
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Universal Relations for Folds and Cusps Flux relation for a fold pair (Keeton et al. 2005): Flux relation for a cusp triplet (Keeton et al. 2003): Valid for all smooth mass models D eviations small-scale structure
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Flux Ratio Anomalies Many lenses require small-scale structure (Mao & Schneider 1998; Keeton, Gaudi & Petters 2003, 2005) Could be CDM substructure (Metcalf & Madau 2001; Chiba 2002) Fitting the lenses requires (Dalal & Kochanek 2002) Broadly consistent with CDM Is substructure the only viable explanation?
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“Minimum Wiggle” Model Allow many multipoles, up to mode k max Models underconstrained large solution space Minimize departures from elliptical symmetry. B2045+265
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Multipole Formalism Convergence: Lens potential: 2-D Poisson equation: Fourier expansion:
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Observational Constraints Lens equation: Image positions give 2n constraints Magnification: Flux ratios give n-1 constraints Combine 3n-1 constraints into a single matrix equation:
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Use SVD to solve for parameters when k max >4: Minimize departure from elliptical symmetry (i.e., minimize wiggles): Adding shear leads to nonlinear equations Solving for Unknowns
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Solution for B2045+265
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Isodensity contours (solid) and critical curves (dashed)
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What Have We Learned from Multipoles? Multipole models with shear cannot explain anomalous flux ratios Isodensity contours remain wiggly, regardless of truncation order Wiggles are most prominent near image positions; implies small-scale structure Ruled out a broad class of alternatives to CDM substructure
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Analytic Relations for Magnifications and Time Delays in Gravitational Lenses with Fold and Cusp Configurations Submitted to J. Math. Phys.
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Lens Time Delays Q0957+561 Kundić et al. (1997) Robust probe of dark matter substructure?
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Local Coordinates Caustic (source plane)Critical curve (image plane) foldcusp
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Perturbation Theory for Fold Lenses Lens Potential: Lens Equation: For small displacements: (u 1, u 2 ) ( u 1, u 2 )
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Expand image positions in ε: Solve for coefficients to find: Image separation:
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Time-Delay Relation for Fold Pairs Scaled Time Delay: Use perturbation theory to get differential time delay: Time-delay anomalies may provide a more sensitive probe of small-scale structure than flux-ratio anomalies
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Comparison to “Exact” Numerical Solution Analytic scaling is astrophysically relevant E
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Using Differential Time Delays to Identify Gravitational Lenses with Small-Scale Structure In preparation for submission to ApJ
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Dependence of Time Delay on Lens Potential and Position along Caustic Use h as proxy for time delay Model lens galaxy as SIE with shear Higher-order multipoles are not so important here
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Variation of h along Caustic
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Time Delays for a Realistic Lens Population Perform Monte Carlo simulations: –use galaxies with distribution of ellipticity, octopole moment and shear –use random source positions to create mock four- image lenses –use Gravlens software (Keeton 2001) to obtain image positions and time delays –create time delay histogram for each image pair
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Matching Mock and Observed Lenses
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Histograms for Scaled Time Delay: Folds PG 1115+080 SDSS J1004+4112
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Histograms for Scaled Time Delay: Cusps RX J0911+0551 RX J1131-1231
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Histograms for Time Delay Ratios: Folds B1608+656 HE 0230-2130
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What Have We Learned from Time Delay Analytics and Numerics? Time delay of the close pair in a fold lens scales with the cube of image separation Time delay is sensitive to ellipticity and shear, but not higher-order multipoles For a given image separation and lens potential, the time delay remains constant if the source is not near a cusp Monte Carlo simulations reveal strong time-delay anomalies in RX J0911+0551 and RX J1131-1231
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Acknowledgments I would like to thank my collaborators, Chuck Keeton and Erik Nordgren
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