Very Large Array broad-band monitoring of the flux-ratio anomalous lens system B2045+265 as a probe for dark matter

Abstract

In this thesis, we investigate the flux-ratio anomaly of the cusp gravitational lens sys-tem CLASS B2045+265 with the aim of gaining insight into dark matter distributions within the galactic halo. CLASS 2045+265 has a radio-loud background galaxy that is being gravitationally lensed by a radio-loud foreground galaxy to form four lensed images. The flux-ratios of the four lensed images are known from previous observa-tions to be inconsistent with the predictions from a simple singular isothermal ellip-soidal model for the foreground mass distribution. Here, we investigate the possible causes of the flux-ratio anomaly within CLASS B2045+265, which include a popu-lation of low mass dark matter haloes, as predicted by various dark matter models, variability from the background active galactic nucleus (AGN), or a propagation ef-fect as the light passes through the foreground lensing galaxy. For this, we have used monitoring data taken with the Karl G. Jansky Very Large Array at frequencies between 12 and 18 GHz. Firstly, we introduce a pipeline for use with the Common Astronomy Software Applications (CASA) package, which allowed us to calibrate large quantities of radio interferometric data. We have also modified a python script to automate the process of self-calibration and model fitting using the Difference Mapping (DIFMAP) package. We use the forementioned pipelines to perform a spectral analysis of CLASS B2045+265 over a 63-day monitoring period between 2022 March and May. The spectral indices of the three brightest lensed images A, B and C between 12 and 18 GHz are found to be α A = 0.945 ± 0.003, α B = 0.954 ± 0.006 and αC = 0.962 ± 0.005, respectively, which are consistent at the 2σ-level. We find the flux densities of these components to be varying over time, but the change is not significant enough to contribute to the anomalous flux ratios. We find a 0.97% variation in the Rcusp parameter (0.5137 ± 0.0003) between day 0 and day 63 which quantifies the deviation from perfect symmetry in the ratios of a cusp lens system where a system with perfect symmetry has Rcusp = 0 . Based on our an analysis of the radio spectra, we do not observe any variations as a function of frequency, which allows us to rule out any frequency dependent effects, such as free-free ab-sorption. Therefore, we believe the flux-ratio anomaly present in CLASS B2045+265 is most likely caused by a perturbation to the mass model by means of a combination between complex mass structures and sub-haloes present within the system. How-ever, given the large mass-fraction in sub-haloes that we find is needed to explain the extreme Rcusp parameter for this system, it is most likely that these sub-haloes are along the line-of-sight towards the distant AGN, as opposed to being purely within the lensing galaxy. Further analysis with high angular resolution telescopes or next gen (ngVLA, SKA, etc.) will be needed to determine whether this is the cause of the flux-ratio anomaly in CLASS B2045+265.