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Licentiate Thesis; Gravitational lensing and radio interferometry as a probe of the small-scale structure of dark matter
  Thesis defense

Monday 11 January 2016
from 10:00 to 12:00
at FA31
Speaker : Saghar Asadi, Department of Astronomy, Stockholm University
Abstract : Gravitational lensing_ has been widely used in astronomy for about a century. The fact that gravity treats dark and luminous mass the same way gives GL a unique advantage over many other observational methods, to provide an independent test for the presence of dark structures of various size and mass. Strong lensing by a foreground object can produce multiple images, known as _macroimage_, of a background light source. Measurements of strong lens systems provide information about the mass distribution of the lens as well as the structure of emission from the source. On the other hand, the concordance model of cosmology has yet to describe the observed Universe at all scales simultaneously. One way to probe the small-scale structures of dark matter is to study a particular type of lens system where the foreground galaxy produces multiple images of the background light source, and the substructure within the foreground dark matter halo that surrounds the galaxy presents itself as surface brightness perturbations in one of the macroimages. Detecting small-scale surface brightness perturbations requires high-resolution imaging of lensed extended sources. Radio interferometry is a technique that connects an array of radio antennae to essentially work as a single dish with the diameter as large as the maximum distance within the array. Arrays such as VLBI networks or ALMA, or combinations of these, make it possible to probe radio quasars and sub-mm galaxies with sub-milliarcsecond angular resolutions. In this thesis, we simulate observations of multiply-lensed sources (radio quasars and dusty star-forming galaxies) at z = 2 with different (present and near-future) radio interferometers. These simulations are then used to explore the extent to which various forms of halo substructures can be constrained. In __Paper I__, we derive the minimum mass that subhalos with different internal density profiles need to have in order to produce detectable lensing effects on multiply-imaged quasars using three different radio arrays. Using the derived minimum detectable mass of different forms of subhalos, we estimate constraints that observations of a number of such lens systems can place on the contribution of each form of substructure in the mass of the parent halo. __Paper II__ focuses on simulations of SMGs using ALMA and the prospects for detecting standard CDM subhalos. In this paper, we show that standard CDM subhalos in the sub-galactic mass range are detectable using high-frequency observations with ALMA. In addition to probing the mass of the substructure, such observations are able to discern the difference between the standard CDM subhalos and other dark compact objects. This can place constraints on the contribution of CDM subhalos to the mass of their host halo - a quantity directly comparable to cosmological simulations.

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