Mapping Dark Matter in Galaxy Clusters with Gravitational Lensing

This thesis uses gravitational lensing to map the distribution of dark matter around galaxy clusters, and to infer their formation history. Galaxy clusters are the oldest and most massive gravitationally-bound objects in the Universe, exploited in the most discriminating tests of cosmology. It is therefore essential to understand the astrophysics of their formation. Indeed, clusters grow through filamentary connections with surrounding large-scale structures - and to chart their history is to trace the evolution and trajectory of the Universe itself. Gravitational lensing is the apparent distortion in the shapes of distant galaxies due to foreground mass, such as a galaxy cluster. Many software algorithms have been developed to measure gravitational lensing and to reconstruct the distribution of foreground mass. In this thesis, we assess the performance of two mass-mapping techniques, using mock images of the BAHAMAS simulation, where the true distribution of mass is known. We find the methods suitable for different applications: MRLens suppresses noise without bias, while Lenstool suppresses noise further, but at a cost of over-estimating the mass in cluster outskirts (R>1Mpc) by up to a factor 2. We also develop a filter to search for large-scale filaments connected to galaxy clusters. We then use these calibrated techniques, and the largest ever mosaic of Hubble Space Telescope imaging, to study galaxy cluster MS 0451-03 (z=0.54). We map the distribution of its dark matter, and discover six group-scale substructures, linked to the cluster halo by three possible filaments. By comparing lensing results with analyses of X-ray emission and optical spectroscopy, we conclude that the cluster collided with another 2--7 Gyr ago. Its star formation was quenched and its gas was heated; its gas has still not yet relaxed, and the dark matter halos are approaching second apocentre. In the next decade, space-based telescopes will reveal this richness of detail about tens of thousands of galaxy clusters. If these observations are properly calibrated, via studies like this thesis, they will bring a new era of precision cosmology. As a final step towards this future, we present preliminary results from two ongoing projects: using deep learning to further suppress noise in lensing mass reconstruction, and the first successful measurement of gravitational lensing from a balloon-borne telescope at the edge of space

Beyond the Ultra-deep Frontier Fields And Legacy Observations (BUFFALO): a high-resolution strong + weak-lensing view of Abell 370

The HST treasury program BUFFALO provides extended wide-field imaging of the six Hubble Frontier Fields galaxy clusters. Here we present the combined strong and weak-lensing analysis of Abell 370, a massive cluster at z=0.375. From the reconstructed total projected mass distribution in the 6arcmin x 6arcmin BUFFALO field-of-view, we obtain the distribution of massive substructures outside the cluster core and report the presence of a total of seven candidates, each with mass $\sim 5 \times 10^{13}M_{\odot}$. Combining the total mass distribution derived from lensing with multi-wavelength data, we evaluate the physical significance of each candidate substructure, and conclude that 5 out of the 7 substructure candidates seem reliable, and that the mass distribution in Abell 370 is extended along the North-West and South-East directions. While this finding is in general agreement with previous studies, our detailed spatial reconstruction provides new insights into the complex mass distribution at large cluster-centric radius. We explore the impact of the extended mass reconstruction on the model of the cluster core and in particular, we attempt to physically explain the presence of an important external shear component, necessary to obtain a low root-mean-square separation between the model-predicted and observed positions of the multiple images in the cluster core. The substructures can only account for up to half the amplitude of the external shear, suggesting that more effort is needed to fully replace it by more physically motivated mass components. We provide public access to all the lensing data used as well as the different lens models.Comment: 29 pages, 17 figures, 3 table

The Projected Dark and Baryonic Ellipsoidal Structure of 20 CLASH Galaxy Clusters

We reconstruct the two-dimensional (2D) matter distributions in 20 high-mass galaxy clusters selected from the CLASH survey by using the new approach of performing a joint weak gravitational lensing analysis of 2D shear and azimuthally averaged magnification measurements. This combination allows for a complete analysis of the field, effectively breaking the mass-sheet degeneracy. In a Bayesian framework, we simultaneously constrain the mass profile and morphology of each individual cluster, assuming an elliptical Navarro-Frenk-White halo characterized by the mass, concentration, projected axis ratio, and position angle (PA) of the projected major axis. We find that spherical mass estimates of the clusters from azimuthally averaged weak-lensing measurements in previous work are in excellent agreement with our results from a full 2D analysis. Combining all 20 clusters in our sample, we detect the elliptical shape of weak-lensing halos at the 5σ significance level within a scale of 2 Mpc h. The median projected axis ratio is 0.67 ± 0.07 at a virial mass of M = (15.2 ± 2.8) × 10 M, which is in agreement with theoretical predictions from recent numerical simulations of the standard collisionless cold dark matter model. We also study misalignment statistics of the brightest cluster galaxy, X-ray, thermal Sunyaev-Zel'dovich effect, and strong-lensing morphologies with respect to the weak-lensing signal. Among the three baryonic tracers studied here, we find that the X-ray morphology is best aligned with the weak-lensing mass distribution, with a median misalignment angle of |ΔPA| = 21° ± 7°. We also conduct a stacked quadrupole shear analysis of the 20 clusters assuming that the X-ray major axis is aligned with that of the projected mass distribution. This yields a consistent axis ratio of 0.67 ± 0.10, suggesting again a tight alignment between the intracluster gas and dark matter. © 2018. The American Astronomical Society. All rights reserved.This work was made possible by the availability of high-quality weak-lensing data produced by the CLASH survey. We are grateful to the CLASH team who enabled us to carry out this work. We thank the anonymous referee for constructive suggestions and comments. We thank Masamune Oguri for making his simulated Subaru Suprime-Cam observations available to us. K.U. acknowledges support from the Ministry of Science and Technology of Taiwan (grants MOST 103-2112-M-001-030-MY3 and MOST 106-2628-M-001-003-MY3) and from the Academia Sinica Investigator Award. M.S. and S.E. acknowledge financial support from the contracts ASI-INAF I/009/10/0, NARO15 ASI-INAF I/037/12/0, ASI 2015-046-R.0 and ASI-INAF n.2017-14-H.0. Support for D.G. was provided by NASA through Einstein Postdoctoral Fellowship grant number PF5-160138 awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. T.O. acknowledges support from the Ministry of Science and Technology of Taiwan under the grant MOST 106-2119-M-001-031-MY3. M.M., M.S., S.E., and J.S. acknowledge support from the Italian Ministry of Foreign Affairs and International Cooperation, Directorate General for Country Promotion (Project "Crack the lens"). J.S. was supported by NSF/AST-1617022

Lensing in the Blue. II. Estimating the Sensitivity of Stratospheric Balloons to Weak Gravitational Lensing

The Superpressure Balloon-borne Imaging Telescope (SuperBIT) is a diffraction-limited, wide-field, 0.5 m, near-infrared to near-ultraviolet observatory designed to exploit the stratosphere's space-like conditions. SuperBIT's 2023 science flight will deliver deep, blue imaging of galaxy clusters for gravitational lensing analysis. In preparation, we have developed a weak-lensing measurement pipeline with modern algorithms for PSF characterization, shape measurement, and shear calibration. We validate our pipeline and forecast SuperBIT survey properties with simulated galaxy cluster observations in SuperBIT's near-UV and blue bandpasses. We predict imaging depth, galaxy number (source) density, and redshift distribution for observations in SuperBIT's three bluest filters; the effect of lensing sample selections is also considered. We find that, in three hours of on-sky integration, SuperBIT can attain a depth of b = 26 mag and a total source density exceeding 40 galaxies per square arcminute. Even with the application of lensing-analysis catalog selections, we find b-band source densities between 25 and 30 galaxies per square arcminute with a median redshift of z = 1.1. Our analysis confirms SuperBIT's capability for weak gravitational lensing measurements in the blue

Lensing in the Blue II: Estimating the Sensitivity of Stratospheric Balloons to Weak Gravitational Lensing

The Superpressure Balloon-borne Imaging Telescope (SuperBIT) is a diffraction-limited, wide-field, 0.5 m, near-infrared to near-ultraviolet observatory designed to exploit the stratosphere's space-like conditions. SuperBIT's 2023 science flight will deliver deep, blue imaging of galaxy clusters for gravitational lensing analysis. In preparation, we have developed a weak lensing measurement pipeline with modern algorithms for PSF characterization, shape measurement, and shear calibration. We validate our pipeline and forecast SuperBIT survey properties with simulated galaxy cluster observations in SuperBIT's near-UV and blue bandpasses. We predict imaging depth, galaxy number (source) density, and redshift distribution for observations in SuperBIT's three bluest filters; the effect of lensing sample selections is also considered. We find that in three hours of on-sky integration, SuperBIT can attain a depth of b = 26 mag and a total source density exceeding 40 galaxies per square arcminute. Even with the application of lensing-analysis catalog selections, we find b-band source densities between 25 and 30 galaxies per square arcminute with a median redshift of z = 1.1. Our analysis confirms SuperBIT's capability for weak gravitational lensing measurements in the blue.Comment: Submitted to Astronomical Journa

Auto-tuned thermal control on stratospheric balloon experiments

Balloon-borne experiments present unique thermal design challenges, which are a combination of those present for both space and ground experiments. Radiation and conduction are the predominant heat transfer mechanisms with convection effects being minimal and difficult to characterize at 35-40 km. This greatly constrains the thermal design options and makes predicting flight thermal behaviour very difficult. Due to the limited power available on long duration balloon flights, efficient heater control is an important factor in minimizing power consumption. SuperBIT, or the Super-Pressure Balloon-borne Imaging Telescope, aims to study weak gravitational lensing using a 0.5m modified Dall-Kirkham telescope capable of achieving 0.02" stability and capturing deep exposures from visible to near UV wavelengths. To achieve the theoretical stratospheric diffraction-limited resolution of 0.25", mirror deformation gradients must be kept to within 20 nm. The thermal environment must be stable on time scales of an hour and the thermal gradients on the telescope must be minimized. During its 2018 test-flight, SuperBIT will implement two types of thermal parameter solvers: one for post-flight characterization and one for in-flight control. The payload has 85 thermistors as well as pyranometers and far-infrared sensors which will be used post-flight to further understand heat transfer in the stratosphere. This document describes the in-flight thermal control method, which predicts the thermal circuit of components and then auto-tunes the heater PID gains. Preliminary ground testing shows the ability to control the components to within 0.01 K

Testing LCDM Predictions for the Dark-Matter Halo Asphericity Using Cluster Weak Lensing

Cosmological N-body simulations of Λ cold dark matter (ΛCDM) predicted the shape of DM halos to be triaxial, reflecting the collisionless nature of DM, and to be elongated in the preferential infall direction of materials. The predicted halo ellipticity is about 0.4(Jing & Suto (2002) [16]). Here we test this well-defined pre- dictions of the standard ΛCDM paradigm using wide-field weak shear lensing data of Umetsu et al.(2016) [41] obtained for a sample of 16 galaxy clusters targeted in the CLASH survey. We examine the mean halo ellipticity (projected halo aspheric- ity) in our cluster sample by measuring the quadrupole weak-lensing signal for each cluster, following the method of Clampitt & Jain(2015) [7]. First we assume the DM halos align with the BCGs. Our best-fit value for the ellipticity of an ensemble of CLASH clusters in this measurement is 0.26±0.07 corresponding to axis-ratio of 0.73±0.07. We also use the position angle of X-ray gas, assuming the dark matter halos align with the X-ray gas halo. In this measurement, we get the mean ellip- ticity is 0.29 ± 0.09 corresponding to axis-ratio of 0.71±0.10. The different between there two measurement is not significant but this give us a hint that the DM halo may be more likely align with X-ray gas. We will also discuss the estimates of the misalignment between central galaxies and halos from simulations and apply this correction to our result

Testing the Collisionless Nature of Dark Matter with the Radial Acceleration Relation in Galaxy Clusters

The radial acceleration relation (RAR) represents a tight empirical relation between the inferred total and baryonic centripetal accelerations, $g_{\rm{tot}}=GM_{\rm{tot}}(<r)/r^2$ and $g_{\rm{bar}}=GM_{\rm{bar}}(<r)/r^2$, observed in galaxies and galaxy clusters. The tight correlation between these two quantities can provide insight into the nature of dark matter. Here we use BAHAMAS, a state-of-the-art suite of cosmological hydrodynamical simulations, to characterize the RAR in cluster-scale halos for both cold and collisionless dark matter (CDM) and self-interacting dark matter (SIDM) models. SIDM halos generally have reduced central dark matter densities, which reduces the total acceleration in the central region when compared with CDM. We compare the RARs in galaxy clusters simulated with different dark matter models to the RAR inferred from CLASH observations. Our comparison shows that the cluster-scale RAR in the CDM model provides an excellent match to the CLASH RAR obtained by Tian et al. including the high-acceleration regime probed by the brightest cluster galaxies (BCGs). By contrast, models with a larger SIDM cross-section yield increasingly poorer matches to the CLASH RAR. Excluding the BCG regions results in a weaker but still competitive constraint on the SIDM cross-section. Using the RAR data outside the central $r<100$kpc region, an SIDM model with $\sigma/m=0.3$cm$^{2}$g$^{-1}$ is disfavored at the $3.8\sigma$ level with respect to the CDM model. This study demonstrates the power of the cluster-scale RAR for testing the collisionless nature of dark matter.Comment: submitted to ApJ; 13 pages, 7 figures, 3 table