The dynamics of self-interacting dark matter in galaxy clusters

This thesis presents three different but connected projects related to the study of the nature of dark matter (DM) using galaxy clusters. In particular, in the first two projects I use cosmological simulations to investigate how DM particles that interact through forces other than gravity affect galaxy clusters as a whole as well as the galaxies that reside inside them. First, I compared the mass loss of galaxies accreted unto simulated clusters ran with both cold dark matter (CDM) and self-interacting dark matter (SIDM) physics. Due to the additional interactions between the DM haloes of the galaxies and of the clusters, we expect there to be additional mass loss in SIDM galaxies on top of the tidal mass loss due to the gravitational field from the cluster. Indeed, I find that on average not only do SIDM galaxies lose more mass, they are also more susceptible to total disruption. Second, I investigated the effects of SIDM on major mergers of galaxy clusters. In such events, the gas is offset from the collisionless galaxies due to ram pressure. If the SIDM cross-section is non-zero, the DM can be offset from the galaxies as well. By comparing the offsets of the gas, DM, and stars in simulations ran with different SIDM cross-sections, I found that the DM offset increases with cross-section as expected from analytical models. The third project was undertaken for the upcoming balloon-borne telescope SuperBIT, whose main science goal will be to map out the DM in and surrounding galaxy clusters. To keep up with SuperBIT's (and any possible successor's) relatively high data rate, we have developed a toolkit of hardware and software that would allow us to physically downlink data mid-flight. I wrote software predicting the trajectories of the system, given the location and time of the release. The system was successfully tested from beginning to end during the SuperBIT 2019 test flight. In essence, all three projects are based around simulations to predict the trajectories of some form of matter falling into some other form of matter, i.e. DM into clusters, or parachutes into the Earth's atmosphere. The intention was to bring the three projects together and use the SuperBIT hardware that I have helped develop to measure the behaviour of DM and calibrate it against the cosmological simulations. Unfortunately SuperBIT's first science flight was delayed due to the COVID-19 pandemic, and I did not get to measure the DM effects on real astronomical data. I intend to do so in the future

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

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

Search for Λb0→[K+π−]DpK−\Lambda_{b}^{0} \rightarrow [K^{+}\pi^{-}]_{D}pK^{-} and the study of CP violation

CP violation has been observed in many processes, but has yet to be found in one key area: The decay of baryons. This project aims to search for the beauty baryon Lambda via its suppressed decay: $\Lambda_{b}^{0} \rightarrow [K^{+}\pi^{-}]_{D}pK^{-}$ and to study CP violation in this mode

The effects of self-interacting dark matter on the stripping of galaxies that fall into clusters

We use the Cluster-EAGLE (C-EAGLE) hydrodynamical simulations to investigate the effects of self-interacting dark matter (SIDM) on galaxies as they fall into clusters. We find that SIDM galaxies follow similar orbits to their cold dark matter (CDM) counterparts, but end up with ∼25 per cent less mass by the present day. One in three SIDM galaxies is entirely disrupted, compared to one in five CDM galaxies. However, the excess stripping will be harder to observe than suggested by previous DM-only simulations because the most stripped galaxies form cores and also lose stars: The most discriminating objects become unobservable. The best test will be to measure the stellar-to-halo mass relation (SHMR) for galaxies with stellar mass 1010−1011M⊙⁠. This is 8 times higher in a cluster than in the field for a CDM universe, but 13 times higher for an SIDM universe. Given intrinsic scatter in the SHMR, these models could be distinguished with noise-free galaxy–galaxy strong lensing of ∼32 cluster galaxies

A 500-year experiment

Charles Cockell and colleagues describe an experiment that started in 2014 and will finish in 2514. It will document how long desiccated microbes can survive, with implications for life in the planetary crust and in space

Strong gravitational lensing's `external shear' is not shear

International audienceThe distribution of mass in galaxy-scale strong gravitational lenses is often modelled as an elliptical power law plus `external shear', which notionally accounts for neighbouring galaxies and cosmic shear. We show that it does not. Except in a handful of rare systems, the best-fit values of external shear do not correlate with independent measurements of shear: from weak lensing in 45 Hubble Space Telescope images, or in 50 mock images of lenses with complex distributions of mass. Instead, the best-fit shear is aligned with the major or minor axis of 88% of lens galaxies; and the amplitude of the external shear increases if that galaxy is disky. We conclude that `external shear' attached to a power law model is not physically meaningful, but a fudge to compensate for lack of model complexity. Since it biases other model parameters that are interpreted as physically meaningful in several science analyses (e.g. measuring galaxy evolution, dark matter physics or cosmological parameters), we recommend that future studies of galaxy-scale strong lensing should employ more flexible mass models

Optical Night Sky Brightness Measurements from the Stratosphere

This paper presents optical night sky brightness measurements from the stratosphere using CCD images taken with the Super-pressure Balloon-borne Imaging Telescope (SuperBIT). The data used for estimating the backgrounds were obtained during three commissioning flights in 2016, 2018, and 2019 at altitudes ranging from 28 to 34 km above sea level. For a valid comparison of the brightness measurements from the stratosphere with measurements from mountain-top ground-based observatories (taken at zenith on the darkest moonless night at high Galactic and high ecliptic latitudes), the stratospheric brightness levels were zodiacal light and diffuse Galactic light subtracted, and the airglow brightness was projected to zenith. The stratospheric brightness was measured around 5.5 hr, 3 hr, and 2 hr before the local sunrise time in 2016, 2018, and 2019, respectively. The B, V, R, and I brightness levels in 2016 were 2.7, 1.0, 1.1, and 0.6 mag arcsec−2 darker than the darkest ground-based measurements. The B, V, and R brightness levels in 2018 were 1.3, 1.0, and 1.3 mag arcsec−2 darker than the darkest ground-based measurements. The U and I brightness levels in 2019 were 0.1 mag arcsec−2 brighter than the darkest ground-based measurements, whereas the B and V brightness levels were 0.8 and 0.6 mag arcsec−2 darker than the darkest ground-based measurements. The lower sky brightness levels, stable photometry, and lower atmospheric absorption make stratospheric observations from a balloon-borne platform a unique tool for astronomy. We plan to continue this work in a future midlatitude long duration balloon flight with SuperBIT

Robust diffraction-limited near-infrared-to-near-ultraviolet wide-field imaging from stratospheric balloon-borne platforms—Super-pressure Balloon-borne Imaging Telescope performance

At a fraction of the total cost of an equivalent orbital mission, scientific balloon-borne platforms, operating above 99.7% of the Earth’s atmosphere, offer attractive, competitive, and effective observational capabilities—namely, space-like seeing, transmission, and backgrounds—which are well suited for modern astronomy and cosmology. The Super-pressure Balloon-borne Imaging Telescope (SUPERBIT) is a diffraction-limited, wide-field, 0.5 m telescope capable of exploiting these observing conditions in order to provide exquisite imaging throughout the near-infrared to near-ultraviolet. It utilizes a robust active stabilization system that has consistently demonstrated a 48 mas 1σ sky-fixed pointing stability over multiple 1 h observations at float. This is achieved by actively tracking compound pendulations via a three-axis gimballed platform, which provides sky-fixed telescope stability at < 500 mas and corrects for field rotation, while employing high-bandwidth tip/tilt optics to remove residual disturbances across the science imaging focal plane. SUPERBIT’s performance during the 2019 commissioning flight benefited from a customized high-fidelity science-capable telescope designed with an exceptional thermo- and opto-mechanical stability as well as a tightly constrained static and dynamic coupling between high-rate sensors and telescope optics. At the currently demonstrated level of flight performance, SUPERBIT capabilities now surpass the science requirements for a wide variety of experiments in cosmology, astrophysics, and stellar dynamics

Data Downloaded via Parachute from a NASA Super-Pressure Balloon

In April 2023, the superBIT telescope was lifted to the Earth’s stratosphere by a helium-filled super-pressure balloon to acquire astronomical imaging from above (99.5% of) the Earth’s atmosphere. It was launched from New Zealand and then, for 40 days, circumnavigated the globe five times at a latitude 40 to 50 degrees south. Attached to the telescope were four “drs” (Data Recovery System) capsules containing 5 TB solid state data storage, plus a gnss receiver, Iridium transmitter, and parachute. Data from the telescope were copied to these, and two were dropped over Argentina. They drifted 61 km horizontally while they descended 32 km, but we predicted their descent vectors within 2.4 km: in this location, the discrepancy appears irreducible below ∼2 km because of high speed, gusty winds and local topography. The capsules then reported their own locations within a few metres. We recovered the capsules and successfully retrieved all of superBIT’s data despite the telescope itself being later destroyed on landing