High Resolution Source Reconstruction of Lensed High Redshift Galaxies




Sharma, Soniya

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Spatially resolved studies of high redshift galaxies, particularly in the peak of galaxy formation epoch 1<z<3, hold the key for understanding the physics of galaxy formation and evolution. Natural magnification provided by gravitational lensing provides a rare opportunity to obtain magnified views of these galaxies at an enhanced spatial resolution. However, lensing has not reached its full potential with traditional source reconstruction approaches because of an under-appreciated problem: source-plane point spread function (PSF). We present a forward modelling approach based on a robust lens model to deconvolve the effects of source-plane PSF and achieve a resolution of ~170 pc in the galaxy-source plane of a z~ 2 lensed galaxy, which would have been otherwise unachievable through traditional image inversion methods. The forward modelling technique takes full advantage of the lensing amplification by assimilating all the available information from different multiple images of the lensed system. This is crucial in order to confidently analyse the dynamics of the lensed target especially in the low signal to noise (SNR) regions of the image plane. Final merged reconstruction allows a significant improvement (by a factor > 5) in SNR of emission line maps in the source plane. Moreover, different components were detected in the velocity gradient that were not seen in previous studies of this object, plausibly suggesting an ongoing merger in this system. We extend the idea of the forward approach to develop an automated source reconstruction algorithm integrated with a popular lens modelling software, Lenstool. This technique utilizes constraints from the extended surface-brightness profile of the lensed source for a given lens model and reconstructs its intrinsic distribution on a pixelated grid using a bayesian Monte-Carlo Markov chain optimization algorithm in Lenstool. The pixelated source modelling algorithm is validated through the demonstration of different different test simulations. Results from my forward approach are compared against the corresponding traditional reconstructions to measure the effects of PSF smearing on physical sizes of star-forming clumps at high-redshifts. As a first case study, we applied the algorithm to reconstruct the morphology of the same lensed system at z ~2. I obtain a remarkable improvement over traditional ray-tracing, as our technique recovers low surface brightness clumps in the source morphology as a result of PSF deconvolution in the source-plane. There is a factor of 5 -10 increase in the SNR of the detected clumps in the source plane using the pixelated approach as compared to those obtained with the traditional reconstruction, especially in the most magnified regions of the source. The results from this case study further motivate the use of pixelized forward modelling technique in future detailed studies of physical properties of lensed galaxies at high resolution using instruments on James Webb Space Telescope (JWST).






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