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Charles R. Keeton

Publications and source records attributed to Charles R. Keeton.

At least 19 recordsLinked to original sources
arXiv2025

Constraints from the Giant Arc in Abell 370. A New Framework for Understanding Systematic Errors in Cluster Lens Modeling. IV. Constraints from the Giant Arc in Abell 370

We aim to improve cluster lens modeling and source reconstruction by utilizing the full information in giant, caustic-crossing arcs lensed by galaxy clusters. Lens models are generally constrained using image positions and assuming point sources, but spatially extended giant arcs provide more constraints; however, they require a more complex model that accounts for the structure of the extended source. We seek to determine whether improvements to the lens model and reconstructed source merit the difficulty of handling the extra constraints. We choose the spatially extended $z=0.725$ giant arc in the $z=0.375$ Abell 370 galaxy cluster field for our study. We present 1) a series of pixel-based source reconstructions for cluster mass models exploring the range of uncertainties in our fiducial model, 2) a similar analysis done using a prototype \textit{python} de-lensing code for cluster mass models from each of the Hubble Frontier Fields modeling teams, 3) an optimized model with pixel-based source reconstructions, and 4) and an investigation of how our optimized model affects the cluster mass model locally and globally in the highest-magnification regions. We find that our optimized model 1) is able to correct resolution-limited assumptions in cluster model inputs local to the arc, 2) has significantly smaller arc model residuals than results from the standard Hubble Frontier Fields models, and 3) affects the critical curves and therefore the information derived from highest-magnification zones most significantly in regions local to the arc.

astro-ph.CO↗
arXiv2024

Statistical Bias in the Hubble Constant and Mass Power Law Slope for Mock Strong Lenses

Strong gravitational lensing offers constraints on the Hubble constant that are independent of other methods. However, those constraints are subject to uncertainties in lens models. Previous studies suggest that using an elliptical power law + external shear (EPL+XS) for the lensing galaxy can yield results that are precise but inaccurate. We examine such models by generating and fitting mock lenses which produces multiple images of a background quasar-like point source. Despite using the same model for input and output, we find statistical bias in the Hubble constant on the order of 3% to 5%, depending on whether the elliptical lenses have noise or not. The phase space distribution has a `flared' shape that causes the mass power law slope to be underestimated and the Hubble constant to be overestimated. The bias varies with image configuration, which we quantify through annulus length between images with the first and second time delays ($Δr_{1,2}$). The statistical bias is worse for configurations that have narrow annuli (e.g., symmetric cross configurations). Assuming a source at redshift 2.0 and an EPL+XS lens at redshift 0.3, we find that the bias can be reduced, but not eliminated, if we limit the sample to systems with annulus lengths $Δr_{1,2} \gtrsim 0.3$ arcsec. As lens samples grow, it may be helpful to prioritize this range of image configurations for follow-up observation and analysis.

astro-ph.CO↗
arXiv2024

Comparisons Between Resolved Star Formation Rate and Gas Tracers in the Strongly Lensed Galaxy SDSS J0901+1814 at Cosmic Noon

We report new radio observations of SDSS J090122.37+181432.3, a strongly lensed star-forming galaxy at $z=2.26$. We image 1.4 GHz (L-band) and 3 GHz (S-band) continuum using the VLA and 1.2 mm (band 6) continuum with ALMA, in addition to the CO(7-6) and CI(${\rm ^3P_2\rightarrow ^3\!P_1}$) lines, all at $\lesssim1.^{\prime\prime}7$ resolution. Based on the VLA integrated flux densities, we decompose the radio spectrum into its free-free (FF) and non-thermal components. The infrared-radio correlation (IRRC) parameter $q_{\rm TIR}=2.65_{-0.31}^{+0.24}$ is consistent with expectations for star forming galaxies. We obtain radio continuum-derived SFRs that are free of dust extinction, finding $\rm {620}_{-220}^{+280}\,M_\odot\,yr^{-1}$, $\rm {230}_{-160}^{+570}\,M_\odot\,yr^{-1}$, and $\rm {280}_{-120}^{+460}\,M_\odot\,yr^{-1}$ from the FF emission, non-thermal emission, and when accounting for both emission processes, respectively, in agreement with previous results. We estimate the gas mass from the CI(${\rm ^3P_2\rightarrow ^3\!P_1}$) line as $M_{\rm gas}=(1.2\pm0.2)\times10^{11}\,M_\odot$, which is consistent with prior CO(1-0)-derived gas masses. Using our new IR and radio continuum data to map the SFR, we assess the dependence of the Schmidt-Kennicutt relation on choices of SFR and gas tracer for $\sim{\rm kpc}$ scales. The different SFR tracers yield different slopes, with the IR being the steepest, potentially due to highly obscured star formation in J0901. The radio continuum maps have the lowest slopes and overall fidelity for mapping the SFR, despite producing consistent total SFRs. We also find that the Schmidt-Kennicutt relation slope is flattest when using CO(7-6) or CI(${\rm ^3P_2\rightarrow ^3\!P_1}$) to trace gas mass, suggesting that those transitions are not suitable for tracing the bulk molecular gas in galaxies like J0901.

astro-ph.GA↗
arXiv2023

Multiplane gravitational lenses with an abundance of images

We consider gravitational lensing of a background source by a finite system of point-masses. The problem of determining the maximum possible number of lensed images has been completely resolved in the single-plane setting (where the point masses all reside in a single lens plane), but this problem remains open in the multiplane setting. We construct examples of $K$-plane point-mass gravitational lens ensembles that produce $\prod_{i=1}^K (5g_i-5)$ images of a single background source, where $g_i$ is the number of point masses in the $i^\text{th}$ plane. This gives asymptotically (for large $g_i$ with $K$ fixed) $5^K$ times the minimal number of lensed images. Our construction uses Rhie's single-plane examples and a structured parameter-rescaling algorithm to produce preliminary systems of equations with the desired number of solutions. Utilizing the stability principle from differential topology, we then show that the preliminary (nonphysical) examples can be perturbed to produce physically meaningful examples while preserving the number of solutions. We provide numerical simulations illustrating the result of our construction, including the positions of lensed images as well as the structure of the critical curves and caustics. We observe an interesting ``caustic of multiplicity'' phenomenon that occurs in the nonphysical case and has a noticeable effect on the caustic structure in the physically meaningful perturbative case.

math-ph↗
arXiv2022

Asymmetric Light Bending in the Equatorial Kerr Metric

The observation of the bending of light by mass, now known as gravitational lensing, was key in establishing general relativity as one of the pillars of modern physics. In the past couple of decades, there has been increasing interest in using gravitational lensing to test general relativity beyond the weak deflection limit. Black holes and neutron stars produce the strong gravitational fields needed for such tests. For a rotating compact object, the distinction between prograde and retrograde photon trajectories becomes important. In this paper, we explore subtleties that arise in interpreting the bending angle in this context and address the origin of seemingly contradictory results in the literature. We argue that analogies that cannot be precisely quantified present a source of confusion.

gr-qc↗
arXiv2022

A New Method for the Reconstruction of Strongly Lensed Galaxies with Resolved Kinematics

Integral field spectroscopy of high-redshift galaxies has become a powerful tool for understanding their dynamics and evolutionary states. However, in the case of gravitationally lensed systems, it has proved difficult to model both lensing and intrinsic kinematics in a way that takes full advantage of the information available in the spectral domain. In this paper, we introduce a new method for pixel-based source reconstruction that alters standard regularization schemes for two-dimensional data in a way that leverages kinematic information in a physically motivated but flexible fashion, and that is better suited to the three-dimensional nature of integral field data. To evaluate the performance of this method, we compare its results to those of a more traditional two-dimensional non-parametric approach using mock ALMA observations of a typical high-redshift dusty star-forming galaxy. We find that 3D regularization applied to an entire data cube reconstructs a source's intensity and velocity structure more accurately than 2D regularization applied to separate velocity channels. Cubes reconstructed with 3D regularization also have more uniform noise and resolution properties and are less sensitive to the signal-to-noise ratio of individual velocity channels than the results of 2D regularization. Our new approach to modeling integral field observations of lensed systems can be implemented without making restrictive a priori assumptions about intrinsic kinematics, and opens the door to new observing strategies that prioritize spectral resolution over spatial resolution (e.g., for multi-configuration arrays like ALMA).

astro-ph.GA↗
arXiv2021

Deep XMM-Newton Observations of an X-ray Weak, Broad Absorption Line Quasar at $z=6.5$

We report X-ray observations of the most distant known gravitationally lensed quasar, J0439+1634 at $z=6.52$, which is also a broad absorption line (BAL) quasar, using the XMM-Newton Observatory. With a 130 ks exposure, the quasar is significantly detected as a point source at the optical position with a total of 358$^{+19}_{-19}$ net counts using the EPIC instrument. By fitting a power-law plus Galactic absorption model to the observed spectra, we obtain a spectral slope of $Γ=1.45^{+0.10}_{-0.09}$. The derived optical-to-X-ray spectral slope $α_{\rm{ox}}$ is $-2.07^{+0.01}_{-0.01}$, suggesting that the X-ray emission of J0439+1634 is weaker by a factor of 18 than the expectation based on its 2500 Angstrom luminosity and the average $α_{\rm{ox}}$ vs. luminosity relationship. This is the first time that an X-ray weak BAL quasar at $z>6$ has been observed spectroscopically. Its X-ray weakness is consistent with the properties of BAL quasars at lower redshift. By fitting a model including an intrinsic absorption component, we obtain intrinsic column densities of $N_{\rm{H}}=2.8^{+0.7}_{-0.6}\times10^{23}\,\rm{cm}^{-2}$ and $N_{\rm{H}}= 4.3^{+1.8}_{-1.5}\times10^{23}\,\rm{cm}^{-2}$, assuming a fixed $Γ$ of 1.9 and a free $Γ$, respectively. The intrinsic rest-frame 2--10 keV luminosity is derived as $(9.4-15.1)\times10^{43}\,\rm{erg\,s}^{-1}$, after correcting for lensing magnification ($μ=51.3$). The absorbed power-law model fitting indicates that J0439+1634 is the highest redshift obscured quasar with a direct measurement of the absorbing column density. The intrinsic high column density absorption can reduce the X-ray luminosity by a factor of $3-7$, which also indicates that this quasar could be a candidate of intrinsically X-ray weak quasar.

astro-ph.GA↗
arXiv2021

ALMA Observations of the Sub-kpc Structure of the Host Galaxy of a z= 6.5 Lensed Quasar: A Rotationally-Supported Hyper-Starburst System at the Epoch of Reionization

We report ALMA observations of the dust continuum and {\cii} emission of the host galaxy of J0439+1634, a gravitationally lensed quasar at $z=6.5$. Gravitational lensing boosts the source-plane resolution to $\sim0\farcs15$ $(\sim0.8\text{ kpc})$. The lensing model derived from the ALMA data is consistent with the fiducial model in \citet{fan19} based on {\it HST} imaging. The host galaxy of J0439+1634 can be well-fitted by a Sérsic profile consistent with an exponential disk, both in the far-infrared (FIR) continuum and the {\cii} emission. The overall magnification is $4.53\pm0.05$ for the continuum and $3.44\pm0.05$ for the {\cii} line. The host galaxy of J0439+1634 is a compact ultra-luminous infrared galaxy, with a total star formation rate (SFR) of $1.56\times10^{3}M_\odot/\text{year}$ after correcting for lensing and an effective radius of $0.74$ kpc. The resolved regions in J0439+1634 follow the ``{\cii} deficit," where the {\cii}-to-FIR ratio decreases with FIR surface brightness. The reconstructed velocity field of J0439+1634 appears to be rotation-like. The maximum line-of-sight rotation velocity of 130 km/s at a radius of 2 kpc. However, our data cannot be fit by an axisymmetric thin rotating disk, and the inclination of the rotation axis, $i$, remains unconstrained. We estimate the dynamical mass of the host galaxy to be $7.9\sin^{-2}(i)\times10^{9}M_\odot$. J0439+1634 is likely to have a high gas-mass fraction and an oversized SMBH compared to local relations. The SFR of J0439+1634 reaches the maximum possible values, and the SFR surface density is close to the highest value seen in any star-forming galaxy currently known in the universe.

astro-ph.GA↗
arXiv2019

Exploring Effects on Magnifications due to Line-of-Sight Galaxies in the Hubble Frontier Fields

Cluster lensing has become an important tool in the search for high redshift galaxies through its ability to magnify sources. In order to determine the intrinsic properties of these galaxies, lensing mass models must be constructed to determine the magnification of the images. These models are traditionally two-dimensional, focusing on the mass within the cluster and either ignoring or approximating any contribution from line-of-sight galaxies. In this paper, we present the first full set of three-dimensional mass models of the six Hubble Frontier Fields and use them to test for systematic biases in magnifications due to using the traditional 2D approach. We find that omitting foreground or background galaxies causes image position offsets between 0.1-0.4", a non-negligible fraction of the typical 0.3-0.7" residuals of current state-of-the-art models. We also find that median image magnifications can shift by up to 6%, though it is dependent on the field. This can be alleviated in some cases by approximating the mass in the lensing plane, but a 5% magnification bias still exists in other cases; image position offsets are also improved, but are still present at 0.10".

astro-ph.CO↗
arXiv2019

Quantifying the power spectrum of small-scale structure in semi-analytic galaxies

In the cold dark matter (CDM) picture of structure formation, galaxy mass distributions are predicted to have a considerable amount of structure on small scales. Strong gravitational lensing has proven to be a useful tool for studying this small-scale structure. Much of the attention has been given to detecting individual dark matter subhalos through lens modeling, but recent work has suggested that the full population of subhalos could be probed using a power spectrum analysis. In this paper we quantify the power spectrum of small-scale structure in simulated galaxies, with the goal of understanding theoretical predictions and setting the stage for using measurements of the power spectrum to test dark matter models. We use a sample of simulated galaxies generated from the \texttt{Galacticus} semi-analytic model to determine the power spectrum distribution first in the CDM paradigm and then in a warm dark matter scenario. We find that a measurement of the slope and amplitude of the power spectrum on galaxy strong lensing scales ($k\sim 1$ kpc$^{-1}$) could be used to distinguish between CDM and alternate dark matter models, especially if the most massive subhalos can be directly detected via gravitational imaging.

astro-ph.GA↗
arXiv2019

Beyond subhalos: Probing the collective effect of the Universe's small-scale structure with gravitational lensing

Gravitational lensing has emerged as a powerful probe of the matter distribution on subgalactic scales, which itself may contain important clues about the fundamental origins and properties of dark matter. Broadly speaking, two different approaches have been taken in the literature to map the small-scale structure of the Universe using strong lensing, with one focused on measuring the position and mass of a small number of discrete massive subhalos appearing close in projection to lensed images, and the other focused on detecting the collective effect of all the small-scale structure between the lensed source and the observer. In this paper, we follow the latter approach and perform a detailed study of the sensitivity of galaxy-scale gravitational lenses to the ensemble properties of small-scale structure. As in some previous studies, we adopt the language of the substructure power spectrum to characterize the statistical properties of the small-scale density field. We present a comprehensive theory that treats lenses with extended sources as well as those with time-dependent compact sources (such as quasars) in a unified framework for the first time. Our approach uses mode functions to provide both computational advantages and insights about couplings between the lens and source. The goal of this paper is to develop the theory and gain the intuition necessary to understand how the sensitivity to the substructure power spectrum depends on the source and lens properties, with the eventual aim of identifying the most promising targets for such studies.

astro-ph.CO↗
arXiv2019

Far-Infrared Properties of the Bright, Gravitationally Lensed Quasar J0439+1634 at z=6.5

We present IRAM/NOEMA, JCMT/SCUBA-2 and VLA observations of the most distant known gravitationally lensed quasar J0439+1634 at z = 6.5. We detect strong dust emission, [CII] 158 $μ$m, [CI] 369 $μ$m, [OI] 146 $μ$m, CO(6-5), CO(7-6), CO(9-8), CO(10-9), H$_{\rm 2}$O $3_{\rm 1,2}-2_{\rm 2,1}$, and H$_{\rm 2}$O $3_{\rm 2,1}-3_{\rm 1,2}$ lines as well as a weak radio continuum. The strong [CII] line yields a systemic redshift of the host galaxy to be z=6.5188$\pm$0.0001. The magnification makes J0439+1634 the far-infrared (FIR) brightest quasar at z > 6 known, with the brightest [CII] line yet detected at this redshift. The FIR luminosity is (3.4$\pm$0.2)$\times$10$^{13}$ $μ^{-1}$ $L_{\odot}$, where $μ$ $\sim$ 2.6 - 6.6 is the magnification of the host galaxy, estimated based on the lensing configuration from HST imaging. We estimate the dust mass to be (2.2$\pm$0.1)$\times$10$^{9}$ $μ^{-1}$ $M_{\odot}$. The CO Spectral Line Energy Distribution using four CO lines is best fit by a two-component model of the molecular gas excitation. The estimates of molecular gas mass derived from CO lines and atomic carbon mass are consistent, in the range of 3.9 - 8.9 $\times$10$^{10} μ^{-1}$ $M_{\odot}$. The [CII]/[CI], [CII]/CO, and [OI]/[CII] line luminosity ratios suggest a photodissociation region model with more than one component. The ratio of H$_{\rm 2}$O $3_{\rm 2,1}-3_{\rm 1,2}$ line luminosity to $L_{\rm TIR}$ is consistent with values in local and high redshift ultra-/hyper-luminous infrared galaxies. The VLA observations reveal an unresolved radio continuum source, and indicate that J0439+1634 is a radio quiet quasar with R = 0.05 - 0.17.

astro-ph.GA↗
arXiv2019

Resolved Molecular Gas and Star Formation Properties of the Strongly Lensed z=2.26 Galaxy SDSS J0901+1814

We present ~1" resolution (~2 kpc in the source plane) observations of the CO(1-0), CO(3-2), Halpha, and [N II] lines in the strongly-lensed z=2.26 star-forming galaxy SDSS J0901+1814. We use these observations to constrain the lensing potential of a foreground group of galaxies, and our source-plane reconstructions indicate that SDSS J0901+1814 is a nearly face-on (i~30 degrees) massive disk with r_{1/2}>~4 kpc for its molecular gas. Using our new magnification factors (mu_tot~30), we find that SDSS J0901+1814 has a star formation rate (SFR) of 268^{+63}_{-61} M_sun/yr, M_gas=(1.6^{+0.3}_{-0.2})x10^11x(alpha_CO/4.6) M_sun, and M_star=(9.5^{+3.8}_{-2.8})x10^10 M_sun, which places it on the star-forming galaxy "main sequence." We use our matched high-angular resolution gas and SFR tracers (CO and Halpha, respectively) to perform a spatially resolved (pixel-by-pixel) analysis of SDSS J0901+1814 in terms of the Schmidt-Kennicutt relation. After correcting for the large fraction of obscured star formation (SFR_Halpha/SFR_TIR=0.054^{+0.015}_{-0.014}), we find SDSS J0901+1814 is offset from "normal" star-forming galaxies to higher star formation efficiencies independent of assumptions for the CO-to-H_2 conversion factor. Our mean best-fit index for the Schmidt-Kennicutt relation for SDSS J0901+1814, evaluated with different CO lines and smoothing levels, is n=1.54+/-0.13; however, the index may be affected by gravitational lensing, and we find n=1.24+/-0.02 when analyzing the source-plane reconstructions. While the Schmidt-Kennicutt index largely appears unaffected by which of the two CO transitions we use to trace the molecular gas, the source-plane reconstructions and dynamical modeling suggest that the CO(1-0) emission is more spatially extended than the CO(3-2) emission.

astro-ph.GA↗
arXiv2019

Probing the Fundamental Nature of Dark Matter with the Large Synoptic Survey Telescope

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 fundamental physics of dark matter with LSST. We discuss how LSST will inform our understanding of the fundamental properties of dark matter, such as particle mass, self-interaction strength, non-gravitational couplings to the Standard Model, and compact object abundances. Additionally, we discuss the ways that LSST will complement other experiments to strengthen our understanding of the fundamental characteristics of dark matter. More information on the LSST dark matter effort can be found at https://lsstdarkmatter.github.io/ .

astro-ph.CO↗
arXiv2019

Dark Matter Science in the Era of LSST

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 fundamental physics of dark matter in the era of LSST. We describe how astrophysical observations will inform our understanding of the fundamental properties of dark matter, such as particle mass, self-interaction strength, non-gravitational interactions with the Standard Model, and compact object abundances. Additionally, we highlight theoretical work and experimental/observational facilities that will complement LSST to strengthen our understanding of the fundamental characteristics of dark matter.

astro-ph.CO↗
arXiv2018

The Discovery of a Gravitationally Lensed Quasar at z = 6.51

Strong gravitational lensing provides a powerful probe of the physical properties of quasars and their host galaxies. A high fraction of the most luminous high-redshift quasars was predicted to be lensed due to magnification bias. However, no multiple imaged quasar was found at z>5 in previous surveys. We report the discovery of J043947.08+163415.7, a strongly lensed quasar at z=6.51, the first such object detected at the epoch of reionization, and the brightest quasar yet known at z>5. High-resolution HST imaging reveals a multiple imaged system with a maximum image separation theta ~ 0.2", best explained by a model of three quasar images lensed by a low luminosity galaxy at z~0.7, with a magnification factor of ~50. The existence of this source suggests that a significant population of strongly lensed, high redshift quasars could have been missed by previous surveys, as standard color selection techniques would fail when the quasar color is contaminated by the lensing galaxy.

astro-ph.GA↗
arXiv2018

Probing the nature of dark matter by forward modeling flux ratios in strong gravitational lenses

The free streaming length of dark matter particles determines the abundance of structure on sub-galactic scales. We present a statistical technique, amendable to any parameterization of subhalo density profile and mass function, to probe dark matter on these scales with quadrupole image lenses. We consider a warm dark matter particle with a mass function characterized by a normalization and free streaming scale $m_{\rm{hm}}$. We forecast bounds on dark matter warmth for 120-180 lenses, attainable with future surveys, at typical lens (source) redshifts of 0.5 (1.5) in early-type galaxies with velocity dispersions of 220-270 km/sec. We demonstrate that limits on $m_{\rm{hm}}$ deteriorate rapidly with increasing uncertainty in image fluxes, underscoring the importance of precise measurements and accurate lens models. For our forecasts, we assume the deflectors in the lens sample do not exhibit complex morphologies, so we neglect systematic errors in their modeling. Omitting the additional signal from line of sight halos, our constraints underestimate the true power of the method. Assuming cold dark matter, for a low normalization, corresponding the destruction of all subhalos within the host scale radius, we forecast $2σ$ bounds on $m_{\rm{hm}}$ (thermal relic mass) of $10^{7.5} \ (5.0)$, $10^{8} \ (3.6)$, and $10^{8.5} \ (2.7) \ M_{\odot} \left(\rm{keV}\right)$ for flux errors of $2\%$, $4\%$, and $8\%$. With a higher normalization, these constraints improve to $10^{7.2} \ (6.6)$, $10^{7.5} \ (5.3) $, and $10^{7.8} \ (4.3) \ M_{\odot} \left(\rm{keV}\right)$ with 120 systems. We are also able to measure the normalization of the mass function, which has implications for baryonic feedback models and tidal stripping.

astro-ph.CO↗