GRAVITATIONAL LENSING
Natarajan has made seminal contributions to the current understanding of the properties and spatial distribution of dark matter using gravitational lensing.
Current on-going projects include mapping dark matter in the outskirts of clusters of galaxies and constraining evolving dark energy models using cluster strong lensing.
Dark matter dominates the universe, is yet detected only indirectly and its true nature remains elusive. Structure formation driven by collision-less dark matter remains one of the main pillars of modern cosmology. Natarajan proposed a brand-new framework that enables mapping the detailed distribution of dark matter on small scales within galaxy clusters using gravitational lensing. She proposed a model for clusters comprising a super-position of large and small-scale self-similar mass distributions that could be constrained using a combination of strong and weak lensing observations. The methodology, which by now is the standard way to model substructure in lensing systems on cluster and galaxy-scales was presented in Lensing by galaxy halos in clusters of galaxies. In early work, she also proposed and applied the Jeans equation to dynamically model cluster lenses using the lensing derived mass distribution in Probing the dynamics of cluster-lenses. This approach has since been applied to individual galaxy lenses as well. She also proposed a novel method to constrain the shapes of dark matter halos by stacking in Two-Dimensional Galaxy-Galaxy Lensing: A Direct Measure of the Flattening and Alignment of Light and Mass in Galaxies which has since been adopted in the analysis of SDSS galaxy data.
The mass function of substructure, she showed enables the direct comparison of the observed lensing distortions with theoretical predictions in The abundance of substructure in clusters of galaxies.
She pioneered the calculation of intrinsic shape correlations between galaxies, an effect that contaminates observational lensing studies in Spin-induced Galaxy Alignments and Their Implications for Weak-Lensing Measurements; Discriminating Weak Lensing from Intrinsic Spin Correlations Using the Curl-Gradient Decomposition. She and her collaborators devised optimal methods that could extract this noise from the signal effectively.
Building upon her early work, she has devised several new precision metrics for use to confront the predictions of simulations of the cold dark matter model with the deepest available observational data . She has developed the precision tests of small-scale physics that permit stress testing the standard cold dark matter model that are permitted by the increasing fidelity of the data. Her work has demonstrated that clusters serve as powerful astrophysical laboratories to study the interplay of baryons and dark matter The Mass-to-Light Ratio of Early-Type Galaxies: Constraints from Gravitational Lensing in the Rich Cluster AC 114 and the physical processes that shape galaxy evolution in clusters like Evidence for tidal stripping of dark matter halos in massive cluster lenses.
She has recently applied these tests to high resolution data from the Hubble Frontier Fields Project and the state of the art Illustris suite of simulations. The tantalizing results of this latest study have not only provided the highest spatial resolution dark matter map to date but also point to potential tension between observations and theory Mapping substructure in the HST Frontier Fields cluster lenses and in cosmological simulations.
In a new paper, Meneghetti et al. just published in Science our team of international researchers analyzed Hubble Space Telescope images from several massive galaxy clusters and found that the smaller dollops of dark matter associated with cluster galaxies were significantly more concentrated than predicted by theorists. We report a factor of 10 discrepancy — between observations and theoretical predictions – of the galaxy-galaxy strong lensing probability. The implications of this finding are very exciting – we may be a missing an ingredient in our current understanding of dark matter and there’s a feature of the real universe that we are simply not capturing in our current theoretical models. Or this could also be signaling a gap in our current understanding of the nature of dark matter and its properties.
Natarajan’s work has been instrumental in establishing lensing as a powerful cosmological probe Cosmological constraints from strong gravitational lensing in clusters of galaxies).
Methodologies developed by her serve as the key science drivers for several on-going and past survey projects on current and planned space observatories. She was one of the principal architects of the Hubble Frontier Fields Initiative and she was instrumental is getting the lens modeling community to collaborate and produce publicly available magnification maps. She is an acknowledged expert in the cluster lensing and co-wrote the authoritative invited review of the field Cluster Lenses.