Free-form Lens Reconstruction of Hubble Frontier Fields Galaxy Clusters

University of Minnesota Ph.D. dissertation. September 2018. Major: Physics. Advisor: Liliya Williams. 1 computer file (PDF); x, 96 pages.Galaxy clusters, because of their massive size, act as powerful lenses for background objects. The Hubble Frontier Fields project was a multiyear international colloboration to examine six galaxy clusters acting as gravitational lenses with the aide of the Hubble Space Telescope. Lens modelling teams used shared data to reconstruct the cluster mass distributions using a variety of methods. We used our free-form method Grale to solve for the mass distribution of each cluster. The only inputs used were related to the observed images, and unlike most other methods, no information about visible light of the cluster galaxies was part of the input. The lensing models produced by each modelling team were used to study magnified high redshift galaxies, and construct their luminosity functions. These scientific advances prepare the community ahead of the James Webb Space Telescope launch. Upon reconstructing the cluster distributions, our goal was to see if light traces mass and investigate Grale uncertainties. We focused on the first two Hubble Frontier Fields clusters, Abell 2744 and MACS J0416. No significant offsets were found between brightest cluster member galaxies and local mass peaks for either cluster on scales of ≈ 10 − 15kpc. We calculated the correlation function between cluster core member galaxies and mass distribution for each cluster. Our results confirmed the standard biasing scenario of galaxy formation, meaning the clustering of galaxies is heavily influenced by the underlying dark matter distribution. We found light traces mass within HFF clusters, Abell 2744 and MACS J0416. We directly compared two Abell 2744 Grale reconstructions to gauge the robustness of calculated uncertainties, and confirmed that Grale uncertainties were robust to changes in input data and slight modifications in the Grale code parameters. Moreover, both maps produced relatively low Lens-plane RMS values, comparable to those of other methods. We explained our method for calculating Lens-plane RMS, while also providing multiple alter- native definitions, because of a lack of consensus on the subject in the published literature

Quantifying substructures in {\it Hubble Frontier Field} clusters: comparison with ΛCDM\Lambda CDM simulations

The Hubble Frontier Fields (HFF) are six clusters of galaxies, all showing indications of recent mergers, which have recently been observed for lensed images. As such they are the natural laboratories to study the merging history of galaxy clusters. In this work, we explore the 2D power spectrum of the mass distribution $P_{\rm M}(k)$ as a measure of substructure. We compare $P_{\rm M}(k)$ of these clusters (obtained using strong gravitational lensing) to that of $\Lambda$CDM simulated clusters of similar mass. To compute lensing $P_{\rm M}(k)$, we produced free-form lensing mass reconstructions of HFF clusters, without any light traces mass (LTM) assumption. The inferred power at small scales tends to be larger if (i)~the cluster is at lower redshift, and/or (ii)~there are deeper observations and hence more lensed images. In contrast, lens reconstructions assuming LTM show higher power at small scales even with fewer lensed images; it appears the small scale power in the LTM reconstructions is dominated by light information, rather than the lensing data. The average lensing derived $P_{\rm M}(k)$ shows lower power at small scales as compared to that of simulated clusters at redshift zero, both dark-matter only and hydrodynamical. The possible reasons are: (i)~the available strong lensing data are limited in their effective spatial resolution on the mass distribution, (ii)~HFF clusters have yet to build the small scale power they would have at $z\sim 0$, or (iii)~simulations are somehow overestimating the small scale power.Comment: 13 pages, 10 figures, 1 table; Accepted for publication in MNRA

Testing light-traces-mass in Hubble Frontier Fields Cluster MACS-J0416.1-2403

We reconstruct the projected mass distribution of a massive merging Hubble Frontier Fields cluster MACSJ0416 using the genetic algorithm based free-form technique called Grale. The reconstructions are constrained by 149 lensed images identified by Jauzac et al. using HFF data. No information about cluster galaxies or light is used, which makes our reconstruction unique in this regard. Using visual inspection of the maps, as well as galaxy-mass correlation functions we conclude that overall light does follow mass. Furthermore, the fact that brighter galaxies are more strongly clustered with mass is an important confirmation of the standard biasing scenario in galaxy clusters. On the smallest scales, approximately less than a few arcseconds, the resolution afforded by 149 images is still not sufficient to confirm or rule out galaxy-mass offsets of the kind observed in ACO 3827. We also compare the mass maps of MACSJ0416 obtained by three different groups: Grale, and two parametric Lenstool reconstructions from the CATS and Sharon/Johnson teams. Overall, the three agree well; one interesting discrepancy between Grale and Lenstool galaxy-mass correlation functions occurs on scales of tens of kpc and may suggest that cluster galaxies are more biased tracers of mass than parametric methods generally assume