Resolving lensing anomalies with wavelike dark matter

Abstract

The quest to unravel the nature of dark matter (DM) currently has two leading contenders: heavy particles (ϱDM) and ultralight axions (ψDM). The latter, which behaves on macroscopic scales as waves, exhibits ubiquitous density fluctuations throughout galaxies and galaxy clusters owing to quantum interference. The central focus of my thesis is to explore, for the first time, whether the density fluctuations in ψDM can resolve long-standing anomalies found among multiple images of background galaxies created by gravitational lensing. These anomalies manifest themselves as discrepancies in the brightnesses (disagreements of ∼30%) and positions (on the order of ∼ 1 − 10 mas) of lensed images when compared to predictions made by smooth (without satellite sub-halos) ϱDM lens models; the inclusion of sub-halos often fails to account for brightness anomalies, and has not yet been explored to explain position anomalies. Building on my MPhil demonstrating that ψDM density fluctuations are capable of significantly perturbing the brightnesses and positions of lensed images, I first explore the general level of position and brightness anomalies produced by ψDM, finding that the predicted level of anomalies are in good statistical agreement with those left over by ϱDM lens models, providing encouragement to examine individual cases in detail. I then assess the ability of ψDM to exactly reproduce the quadruply-lensed images in two systems that, at the time of my thesis, present the most stringent tests of ψDM. The first is; an optical quasar straddled by two radio jets (HS 0810+2554) and the second, the first gravitationally lensed type Ia supernova (iPTF16geu). Imposing random density fluctuations on the best-fit ϱDM lens model, I show that ψDM lens models are able to perturb the position and brightness of individual lensed images by the levels required to reproduce the observations. Finally, I subject ψDM to its most stringent test: can it exactly reproduce the observations? I find that an exact match to an individual lensed image is often but not always possible, owing to: (i) the ability of the best-fit smooth ϱDM lens model to accurately capture the global 2D-profile of ψDM halos; and (ii) the ability to create a sufficiently large enough number of ψDM lens model realisations to fully capture the possible range of perturbations. Future work will have to keep these limitations in mind when designing metrics for ψDM lens models to assess their ability in reproducing the observations.

My work paves the way for future tests of ψDM in the context of gravitational lensing. The most immediate applications are to other systems involving a single lensing galaxy, in particular to other multiply-lensed SN Ia that have recently been found. Future work should also address lensing anomalies found in galaxy clusters, for which I show how density fluctuations can be imposed on ϱDM lens models to mimic ψDM lens models. The density fluctuations of ψDM in galaxy clusters are very different in nature than for single galaxies, permitting tests of ψDM in an entirely different setting and shining light on whether ψDM is the correct description of dark matter