WEAK GRAVITATIONAL LENSING STUDIES USING RADIO INFORMATION

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Abstract

Weak gravitational lensing has developed to be one of the most powerful tools for studying the (dark) matter distribution in the Universe. Most weak lensing studies thus far were con- ducted in the optical and near infrared. Measuring weak lensing in the radio though, provided it is feasible, can be very advantageous. One can exploit the well known and deterministic beam pattern of a radio telescope and the polarisation information in radio data to reduce shape biases and intrinsic alignment effects respectively. Combining the information from an optical and a radio survey can also help remove systematics from both datasets. This has motivated this study that uses archival radio and optical data to treat telescope systematics and measure an unbiased weak lensing signal using shape information derived from radio observations.Using simulations I have shown that an unbiased convergence cross power spectrum can be measured in the presence of the large scale (θ>1◦) systematics detected in FIRST and SDSS. The method however amplifies the uncertainties by a factor ∼2.5 compared to the errors due to cosmic variance and noise due to galaxy intrinsic shape alone. Using the shape information from the two surveys I measure a Cκκ spectrum signal that is inconsistent with zero at the 2.7σ. The placed constraints are consistent with the expected signal in the concordance cosmological model assuming recent estimates of the cosmological parameters from the Planck satellite and literature values for the median redshifts of SDSS and FIRST.Through simulations I also show that I can successfully remove position based small scale systematics (θ<200′′) detected in FIRST. Correlating the positions of the SDSS data release 10, Brightest cluster galaxy (BCG) and the FIRST sources that match a position of a galaxy in SDSS with the shapes of the FIRST selected sources, I measure a tangential shear signal that is inconsistent with zero at 10σ, 3.8σ and 9σ respectively. Using an NFW and an SIS mass profile I extract the Virial mass for the three lensing samples. The masses for the SDSS and BCG groups are in good agreement (1σ) with values quoted in the literature. No previous work had been performed on the third sample.SuperCLASS is an experiment that aims to show the feasibility of measuring weak lens- ing in the radio on a range of angular scales. I was tasked with editing and imaging the first e-MERLIN observations of the SuperCLASS field around the Abell cluster A0981. The ob- servations yield an RMS noise of ∼40μJy/beam. A performed source extraction revealed 153 sources at a signal-to ratio of >5. Using the deconvolved information for the resolved sources I calculate a FWHM median size and flux density of 0.5′′ and 300μJy respectively. Comparing the source number density and RMS noise of the study with those of FIRST, I extrapolate to predict that the number density of sources at >5σ will be ∼5arcmin−2, assuming the target noise threshold for the survey is reached.

Keyword(s)

Cosmic Shear; Cosmological Parameters; Cross Correlation; Telescope Systematics; Weak Gravitational Lensing

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