Contaminating Weak Lensing Cosmology with Active Galactic Nuclei

  • Nialh Mccallum

Student thesis: Master of Science by Research


With the arrival of the SKA the dawn of a new era of weak gravitational lensing observations is imminent, with scope to probe cosmic shear fields at radio wavelengths to a precision competitive with optical results. Cosmic shear measurements are highly susceptible to bias, with one possible contributor to this bias in the radio regime being AGN type galaxies whose complex shapes are not well constrained by conventional models.Extraction methods are being created for the purpose of removing AGN galaxies from the radio source samples used in radio weak lensing. However, under certain circumstances, such as the case where the PSF is larger than the galaxy size, the multicomponent structure of the AGN galaxy may be obscured, resulting in the extraction techniques failing to recognise it as an AGN galaxy and misidentifying it as a SF galaxy. This will lead to a misidentification bias dependent on the ability of the extraction method i.e. dependent on the fraction of AGN type galaxies successfully removed. It is this misidentification bias that has been examined in this work.It has been determined for the additive bias that the requirements of the SKA, of c < 0.0011, c < 0.00076, and c < 0.00035 for the SKA1-early, SKA1, and SKA2 stages respectively are consistently met for all fractions of AGN galaxies removed from the sample, at all SNR levels. This shows that the additive component of the misidentification bias will not be a problem in radio weak lensing surveys.For the multiplicative bias the SKA requirements that need to be met are m < 0.011, m < 0.0058, and m < 0.0012 for the SKA1-early, SKA1, and SKA2 stages respectively. It has been found that successful removal of 70% of the AGN galaxies from a radio sample is sufficient to meet the SKA1-early requirement, 87% to meet the SKA1 requirement, and 97% to meet the SKA2 requirement. It has been shown that the multiplicative component of the misidentification bias is of great significance in radio weak lensing surveys. The AGN galaxy extraction codes developed must be able to successfully remove 70%, 87%, and 97% of the AGN galaxies from a radio sample in order to meet the bias requirements of the SKA1-early, SKA1, and SKA2 stages of the SKA respectively.As such the contamination of weak lensing cosmology by AGN is certainly an important effect to consider when proceeding with weak lensing observations in the radio regime.
Date of Award3 Jan 2017
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMichael Brown (Supervisor) & Ian Harrison (Supervisor)


  • Radio Wavelengths
  • Cosmic Shear
  • Bias
  • Cosmology
  • Active Galactic Nuclei
  • Weak Gravitational Lensing
  • Misidentification

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