William Dawson

Lawrence Livermore National Laboratory | LLNL · Physics Division

popclass is a python package that provides a flexible, probabilistic framework for classifying the lens of a gravitational microlensing event. popclass allows a user to match characteristics of a microlensing signal to a simulation of the Galaxy to calculate lens type probabilities for an event. Constraints on any microlensing signal characteristic...

There are expected to be millions of isolated black holes in the Galaxy resulting from the death of massive stars. Measuring the abundance and properties of this remnant population would shed light on the end stages of stellar evolution and the evolution paths of black hole systems. Detecting isolated black holes is currently only possible via grav...

We introduce a population model to analyze the mixing between hypothesised power-law and ∼35M⊙ Gaussian-bump black hole populations in the latest gravitational wave catalog, GWTC-3, estimating their colocation and separation. We find a relatively low level of mixing, 3.1−3.1+5.0%, between the power-law and Gaussian populations, compared to the perc...

We introduce a population model to analyze the mixing between hypothesised power-law and $\sim 35 M_\odot$ Gaussian bump black hole populations in the latest gravitational wave catalog, GWTC-3, estimating their co-location and separation. We find a relatively low level of mixing, $3.1^{+5.0}_{-3.1}\%$, between the power-law and Gaussian populations...

Primordial black holes (PBHs), theorized to have originated in the early Universe, are speculated to be a viable form of dark matter. If they exist, they should be detectable through photometric and astrometric signals resulting from gravitational microlensing of stars in the Milky Way. Population Synthesis for Compact-object Lensing Events, or Pop...

Primordial black holes (PBHs) could explain some fraction of dark matter and shed light on many areas of early-Universe physics. Despite over half a century of research interest, a PBH population has so far eluded detection. The most competitive constraints on the fraction of dark matter comprised of PBHs ( f DM ) in the (10 ⁻⁹ –10) M ⊙ mass ranges...

We analyse 1602 microlensing events found in the VISTA Variables in the Via Lactea (VVV) near-infrared (NIR) survey data. We obtain spatially resolved, efficiency-corrected time-scale distributions across the Galactic bulge (|ℓ| < 10°, |b| < 5°), using a Bayesian hierarchical model. Spatially resolved peaks and means of the time-scale distributions...

From the formation mechanisms of stars and compact objects to nuclear physics, modern astronomy frequently leverages surveys to understand populations of objects to answer fundamental questions. The population of dark and isolated compact objects in the Galaxy contains critical information related to many of these topics, but is only practically ac...

Primordial black holes (PBHs), theorized to have originated in the early universe, are speculated to be a viable form of dark matter. If they exist, they should be detectable through photometric and astrometric signals resulting from gravitational microlensing of stars in the Milky Way. Population Synthesis for Compact-object Lensing Events, or Pop...

Cosmological and astrophysical observations currently provide the the only robust, positive evidence for dark matter. Cosmic probes of dark matter, which seek to determine the fundamental properties of dark matter through observations of the cosmos, have emerged as a promising means to reveal the nature of dark matter. This report summarizes the cu...

Modern surveys of gravitational microlensing events have progressed to detecting thousands per year, and surveys are capable of probing Galactic structure, stellar evolution, lens populations, black hole physics, and the nature of dark matter. One of the key avenues for doing this is the microlensing Einstein radius crossing time ( t E ) distributi...

The investigation of merging galaxy clusters that exhibit radio relics is strengthening our understanding of the formation and evolution of galaxy clusters, the nature of dark matter, the intracluster medium, and astrophysical particle acceleration. Each merging cluster provides only a single view of the cluster formation process, and the variety o...

We present constraints on the physical properties (including stellar mass, age, and star formation rate) of 207 6 ≲ z ≲ 8 galaxy candidates from the Reionization Lensing Cluster Survey (RELICS) and Spitzer-RELICS surveys. We measure photometry using T-PHOT and perform spectral energy distribution fitting using EA z Y and BAGPIPES. Of the 207 candid...

The investigation of radio relic merging galaxy clusters is strengthening our understanding of the formation and evolution of galaxy clusters, the nature of dark matter, the intracluster medium, and astrophysical particle acceleration. Each merging cluster provides only a single view of the cluster formation process and the variety of merging clust...

Modern surveys of gravitational microlensing events have progressed to detecting thousands per year. Surveys are capable of probing Galactic structure, stellar evolution, lens populations, black hole physics, and the nature of dark matter. One of the key avenues for doing this is studying the microlensing Einstein radius crossing time distribution...

We present constraints on the physical properties (including stellar mass, age, and star formation rate) of 207 $6\lesssim z \lesssim8$ galaxy candidates from the Reionization Lensing Cluster Survey (RELICS) and companion Spitzer-RELICS surveys. We measure photometry using T-PHOT and perform spectral energy distribution fitting using EA$z$Y and BAG...

Extensive surveys with the Hubble Space Telescope over the past decade, targeting some of the most massive clusters in the sky, have uncovered dozens of galaxy cluster strong lenses. The massive cluster strong-lens scale is typically θ E ∼ 10″ to ∼30″–35″, with only a handful of clusters known with Einstein radii θ E ∼ 40″ or above (for z source =...

Microlensing surveys have discovered thousands of events, with almost all events discovered within the Galactic bulge or toward the Magellanic Clouds. The Zwicky Transient Facility (ZTF), while not designed to be a microlensing campaign, is an optical time-domain survey that observes the entire northern sky, including the Galactic plane, every few...

Microlensing surveys have discovered thousands of events with almost all events discovered within the Galactic bulge or toward the Magellanic clouds. The Zwicky Transient Facility (ZTF), while not designed to be a microlensing campaign, is an optical time-domain survey that observes the entire northern sky every few nights including the Galactic pl...

Extensive surveys with the \textit{Hubble Space Telescope} (HST) over the past decade, targeting some of most massive clusters in the sky, have uncovered dozens of galaxy-cluster strong lenses. The massive cluster strong-lens scale is typically $\theta_{E}\sim10"$ to $\sim30-35"$, with only a handful of clusters known with Einstein radii $\theta_{E...

Large surveys of galaxy clusters with the Hubble Space Telescope ( HST ) and Spitzer , including the Cluster Lensing And Supernova survey with Hubble and the Frontier Fields, have demonstrated the power of strong gravitational lensing to efficiently deliver large samples of high-redshift galaxies. We extend this strategy through a wider, shallower...

We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the solar system, exploring the transient optical sky, and mapping the Milky Way. LSST will be a large, wide-field...

Astrophysical observations currently provide the only robust, empirical measurements of dark matter. In the coming decade, astrophysical observations will guide other experimental efforts, while simultaneously probing unique regions of dark matter parameter space. This white paper summarizes astrophysical observations that can constrain the fundame...

Strong gravitational lensing by clusters of galaxies probes the mass distribution at the core of each cluster and magnifies the universe behind it. MACS J0417.5-1154 at z = 0.443 is one of the most massive clusters known based on weak lensing, X-ray, and Sunyaev-Zel'dovich analyses. Here we compute a strong lens model of MACS J0417 based on Hubble...

Large surveys of galaxy clusters with the Hubble and Spitzer Space Telescopes, including CLASH and the Frontier Fields, have demonstrated the power of strong gravitational lensing to efficiently deliver large samples of high-redshift galaxies. We extend this strategy through a wider, shallower survey named RELICS, the Reionization Lensing Cluster S...

Astrophysical and cosmological observations currently provide the only robust, empirical measurements of dark matter. Future observations with Large Synoptic Survey Telescope (LSST) will provide necessary guidance for the experimental dark matter program. This white paper represents a community effort to summarize the science case for studying the...

A115 is a merging galaxy cluster at $z\sim0.2$ with a number of remarkable features including a giant ($\sim2.5$ Mpc) radio relic, two asymmetric X-ray peaks with trailing tails, and a peculiar line-of-sight velocity structure. We present a multi-wavelength study of A115 using optical imaging data from Subaru, X-ray data from $Chandra$, and spectro...

Strong gravitational lensing by clusters of galaxies probes the mass distribution at the core of each cluster and magnifies the universe behind it. MACS J0417.5-1154 at z = 0:443 is one of the most massive clusters known, based on weak lensing, X-ray, and Sunyaev-Zel'dovich analyses. Here we compute a strong lens model of MACSJ0417 based on Hubble...

Strong gravitational lensing (SL) is a powerful means to map the distribution of dark matter. In this work, we perform a SL analysis of the prominent X-ray cluster RXJ0152.7-1357 (z=0.83, also known as CL 0152.7-1357) in \textit{Hubble Space Telescope} images, taken in the framework of the Reionization Lensing Cluster Survey (RELICS). On top of a p...

Despite the progress that has been made in understanding radio relics, there are still open questions regarding the underlying particle acceleration mechanisms. In this paper, we present deep 1-4 GHz Very Large Array (VLA) observations of CIZA J2242.8+5301 (z = 0.1921), a double radio relic cluster characterized by small projection on the plane of...

We present a lens model for the cluster SPT-CLJ0615-5746, which is the highest-redshift (z = 0.972) system in the Reionization of Lensing Clusters Survey, making it the highest-redshift cluster for which a full, strong lens model is published. We identify three systems of multiply imaged lensed galaxies, two of which we spectroscopically confirm at...

Despite progress in understanding radio relics, there are still open questions regarding the underlying particle acceleration mechanisms. In this paper we present deep 1--4 GHz VLA observations of CIZA\,J2242.8+5301 ($z=0.1921$), a double radio relic cluster characterized by small projection on the plane of the sky. Our VLA observations reveal, for...

Golovich et al. 2017b presents an optical imaging and spectroscopic survey of 29 radio relic merging galaxy clusters. In this paper, we study this survey to identify substructure and quantify the dynamics of the mergers. Using a combined photometric and spectroscopic approach, we identify the minimum number of substructures in each system to descri...

We present a lens model for the cluster SPT-CLJ0615$-$5746, which is the highest redshift ($z=0.972$) system in the Reionization of Lensing Clusters Survey (RELICS), making it the highest redshift cluster for which a full strong lens model is published. We identify three systems of multiply-imaged lensed galaxies, two of which we spectroscopically...

We present LOFAR $120-168$ MHz images of the merging galaxy cluster Abell 1240 that hosts double radio relics. In combination with the GMRT $595-629$ MHz and VLA $2-4$ GHz data, we characterised the spectral and polarimetric properties of the radio emission. The spectral indices for the relics steepen from their outer edges towards the cluster cent...

We present a strong-lensing analysis of four massive galaxy clusters imaged with the Hubble Space Telescope in the framework of the Reionization Lensing Cluster Survey (RELICS). We use a Light-Traces-Mass modeling technique to uncover sets of multiply imaged galaxies, and constrain the mass distribution and strong-lensing properties of the clusters...

We report L-band VLA observations of 1RXS J0603.3+4214, a cluster that hosts a bright radio relic, known as the Toothbrush, and an elongated giant radio halo. These new observations allow us to study the surface brightness distribution down to one arcsec resolution with very high sensitivity. Our images provide an unprecedented detailed view of the...

Strong gravitational lensing by galaxy clusters magnifies background galaxies, enhancing our ability to discover statistically-significant samples of galaxies at z>6, in order to constrain the high-redshift galaxy luminosity functions. Here, we present the first five lens models out of the Reionization Lensing Cluster Survey (RELICS) Hubble Treasur...

Massive foreground galaxy clusters magnify and distort the light of objects behind them, permitting a view into both the extremely distant and intrinsically faint galaxy populations. We present here the z ~ 6 - 8 candidate high-redshift galaxies from the Reionization Lensing Cluster Survey (RELICS), a Hubble and Spitzer Space Telescope survey of 41...

The second most significant detection of the Planck Sunyaev Zel'dovich survey, PLCKG287.0+32.9 (z=0.385) is an extremely massive galaxy cluster. This cluster belongs to a rare sample of galaxy clusters boasting two similarly bright radio relics and a radio halo. Perhaps even more intriguing is the asymmetry of the radio relics with one located $\si...

ZwCl 2341.1+0000, a merging galaxy cluster with disturbed X-ray morphology and widely separated (~3 Mpc) double radio relics, was thought to be an extremely massive () and complex system, with little known about its merger history. We present JVLA 2?4 GHz observations of the cluster, along with new spectroscopy from our Keck/DEIMOS survey, and appl...

We analyze a rich dataset including Subaru/SuprimeCam, HST/ACS and WFC3, Keck/DEIMOS, Chandra/ACIS-I, and JVLA/C and D array for the merging galaxy cluster ZwCl 0008.8+5215. With a joint Subaru/HST weak gravitational lensing analysis, we identify two dominant subclusters and estimate the masses to be M$_{200}=\text{5.7}^{+\text{2.8}}_{-\text{1.8}}\...

In an influential recent paper, Harvey et al (2015) derive an upper limit to the self-interaction cross section of dark matter ($\sigma_{\rm DM} < 0.47$ cm$^2$/g at 95\% confidence) by averaging the dark matter-galaxy offsets in a sample of merging galaxy clusters. Using much more comprehensive data on the same clusters, we identify several substan...

To investigate the relationship between thermal and non-thermal components in merger galaxy clusters, we present deep JVLA and Chandra observations of the HST Frontier Fields cluster MACS J0717.5+3745. The Chandra image shows a complex merger event, with at least four components belonging to different merging subclusters. NW of the cluster, $\sim 0...

Nature Astronomy 1 , 0005 (2017); published 4 January 2017; corrected 13 January 2017. In the version of this Letter originally published Marcus Brüggen's name was spelled incorrectly.

On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-si...

We infer gravitational lensing shear and convergence fields from galaxy ellipticity catalogs under a spatial process prior for the lensing potential. We demonstrate the performance of our algorithm with simulated Gaussian-distributed cosmological lensing shear maps and a reconstruction of the mass distribution of the merging galaxy cluster Abell 78...

We present an analysis of the merging cluster MACS J1149.5+2223 using archival imaging from Subaru/Suprime-Cam and multi-object spectroscopy from Keck/DEIMOS and Gemini/GMOS. We employ two and three dimensional substructure tests and determine that MACS J1149.5+2223 is composed of two separate mergers between three subclusters occurring $\sim$1 Gyr...

We present results based on X-ray, optical, and radio observations of the massive galaxy cluster CIZA J0107.7+5408. We find that this system is a post core passage, dissociative, binary merger, with the optical galaxy density peaks of each subcluster leading their associated X-ray emission peaks. This separation occurs because the diffuse gas exper...

The Hubble Space Telescope Frontier Fields cluster MACS J1149.6+2223 is one of the most complex merging clusters, believed to consist of four dark matter halos. We present results from deep (365 ks) Chandra observations of the cluster, which reveal the most distant cold front (z = 0.544) discovered to date. In the cluster outskirts, we also detect...

We present deep LOFAR observations between 120 and 181 MHz of the "Toothbrush" (RX J0603.3+4214), a cluster that contains one of the brightest radio relic sources known. Our LOFAR observations exploit a new and novel calibration scheme to probe 10 times deeper than any previous study in this relatively unexplored part of the spectrum. The LOFAR obs...

LOFAR, the Low-Frequency Array, is a powerful new radio telescope operating between 10 and 240 MHz. LOFAR allows detailed sensitive high-resolution studies of the low-frequency radio sky. At the same time LOFAR also provides excellent short baseline coverage to map diffuse extended emission. However, producing high-quality deep images is challengin...

We report on high-resolution JVLA and Chandra observations of the HST Frontier Cluster MACS J0717.5+3745. MACS J0717.5+3745 offers the largest contiguous magnified area of any known cluster, making it a promising target to search for lensed radio and X-ray sources. With the high-resolution 1.0-6.5 GHz JVLA imaging in A and B configuration, we detec...

The galaxy cluster RX J0603.3+4214 at z=0.225 is one of the rarest clusters boasting an extremely large (~2 Mpc) radio-relic. Because of the remarkable morphology of the relic, the cluster is nicknamed "Toothbrush Cluster". Although the cluster's underlying mass distribution is one of the critical pieces of information needed to reconstruct the mer...

Diffuse radio emission in the form of radio haloes and relics has been found in a number of merging galaxy clusters. These structures indicate that shock and turbulence associated with the merger accelerate electrons to relativistic energies. We report the discovery of a radio relic + radio halo system in PSZ1 G108.18-11.53 (z = 0.335). This cluste...

CIZA J2242.8+5301 (z = 0.188, nicknamed ‘Sausage’) is an extremely massive (M200 ∼ 2.0 × 1015 M⊙), merging cluster with shock waves towards its outskirts, which was found to host numerous emission line galaxies. We performed extremely deep Westerbork Synthesis Radio Telescope H i observations of the ‘Sausage’ cluster to investigate the effect of th...

CIZA J2242.8+5301 (‘Sausage’) and 1RXS J0603.3+4213 (‘Toothbrush’) are two low-redshift (z ∼ 0.2), massive (∼2 × 1015 M⊙), post-core passage merging clusters, which host-shock waves traced by diffuse radio emission. To study their star formation properties, we uniformly survey the ‘Sausage’ and ‘Toothbrush’ clusters in broad- and narrow-band filter...

Cluster mergers may play a fundamental role in the formation and evolution of cluster galaxies. Stroe et al. revealed unexpected overdensities of candidate Hα emitters near the ∼1-Mpc-wide shock fronts of the massive (∼2 × 1015 M⊙) ‘Sausage’ merging cluster, CIZA J2242.8+5301. We used the Keck/Deep Imaging Multi-Object Spectrograph and the William...

Extended steep-spectrum radio emission in a galaxy cluster is usually associated with a recent merger. However, given the complex scenario of galaxy cluster mergers, many of the discovered sources hardly fit into the strict boundaries of a precise taxonomy. This is especially true for radio phoenixes that do not have very well defined observational...

We present first results from the third GRavitational lEnsing Accuracy Testing (GREAT3) challenge, the third in a sequence of challenges for testing methods of inferring weak gravitational lensing shear distortions from simulated galaxy images. GREAT3 was divided into experiments to test three specific questions, and included simulated space- and g...

Merging galaxy clusters with radio relics provide rare insights to the merger dynamics as the relics are created by the violent merger process. We demonstrate one of the first uses of the properties of the radio relic to reduce the uncertainties of the dynamical variables and determine the 3D configuration of a cluster merger, ACT-CL J0102-4915, ni...

Point estimators for the shearing of galaxy images induced by gravitational lensing involve a complex inverse problem in the presence of noise, pixelization, and model uncertainties. We present a probabilistic forward modeling approach to gravitational lensing inference that has the potential to mitigate the biased inferences in most common point e...

The statistics of shear peaks have been shown to provide valuable cosmological information beyond the power spectrum, and will be an important constraint of models of cosmology with the large survey areas provided by forthcoming astronomical surveys. Surveys include masked areas due to bright stars, bad pixels etc, which must be accounted for in pr...

X-ray and radio observations of CIZA J2242.8+5301 suggest that it is a major cluster merger. Despite being well studied in the X-ray, and radio, little has been presented on the cluster structure and dynamics inferred from its galaxy population. We carried out a deep (i<25) broad band imaging survey of the system with Subaru SuprimeCam (g & i bands...

CIZA J2242.8+5301 is a merging system with a prominent (~2 Mpc long) radio relic, which together with the morphology of the X-ray emission provides strong evidence for a violent collision along the N-S axis. We present our constraints on the dark matter distribution of this unusual system using Subaru and CFHT imaging data. Measuring a high S/N len...

CIZA J2242.8+5301 (`Sausage') and 1RXS J0603.3+4213 (`Toothbrush') are two low-redshift ($z\sim0.2$), massive ($\sim2\times10^{15}M_\odot$), post-core passage merging clusters, which host shock waves traced by diffuse radio emission. To study their star-formation properties, we uniformly survey the `Sausage' and `Toothbrush' clusters in broad and n...

Context. While searching the NRAO VLA Sky Survey (NVSS) for diffuse radio emission, we have serendipitously discovered extended radio emission close to the Galactic plane. The radio morphology suggests the presence of a previously unknown Galactic supernova remnant. An unclassified {\gamma}-ray source detected by EGRET (3EG J1744-3934) is present i...

Using overlapping fields with space-based Hubble Space Telescope (HST) and ground-based Subaru Telescope imaging we identify a population of blended galaxies that would not be easily distinguished with ground-based monochromatic imaging alone, which we label as 'ambiguous blends'. For the depth targeted with the Large Synoptic Survey Telescope (LSS...

We present the first sample of 882 optically selected galaxy clusters in the Deep Lens Survey (DLS), selected with the Bayesian Cluster Finder. We create mock DLS data to assess completeness and purity rates, and find that both are at least $70\%$ within 0.1$\le z \le$ 1.2 for clusters with $M_{200}\ge 1.2\times 10^{14}M_{\odot}$. We verified the i...