arXiv subjects

Christopher Hopp

Publications and source records attributed to Christopher Hopp.

4 recordsLinked to original sources

arXiv2026

A Very Rich Bimodal Galaxy Cluster Merger: RXC J0032.1+1808

The galaxy cluster RXC J0032.1+1808 has been well-studied with optical imaging and gravitational lensing mass maps, both of which reveal an elongated morphology in the north-south direction. We find that its X-ray morphology is bimodal, suggesting that it is in the process of merging; combined with a previously reported detection of a radio relic, we suggest that the system is seen after first pericenter. We extract the global X-ray temperature and unabsorbed luminosity from archival XMM-Newton data, finding $T_X=8.5^{+1.1}_{-0.9}$ keV and $L_X=1.04 \pm 0.03 \times 10^{45}$ erg s$^{-1}$ at 90% confidence in the 0.5-10 keV energy range. We conduct a redshift survey of member galaxies and find that the line-of-sight relative velocity between the two subclusters is 1002-1785 km/s at 68% confidence, although the BCGs have an even larger difference of 2466 km/s. We use publicly available hydrodynamic simulations to show that it cannot be a head-on merger; it is consistent with an impact parameter of 0.5 or 1 Mpc. We also use the simulation set to find initial constraints on the time since pericenter (${\approx}$315-550 Myr) and the locus of possible lines of sight.

astro-ph.HE↗

arXiv2026

Toward Detecting the Moving Lens Effect with Optical Spectroscopy

Relativity predicts that sky-plane motion of a gravitational lens changes the wavelengths of lensed photons. This moving lens effect is orders of magnitude smaller than cosmological and Doppler effects on the same photons, but can be isolated by comparing different strongly lensed images of a given source. With a massive, fast-moving lens such as a galaxy cluster falling into another cluster, lensed spectra of a given source could differ in wavelength by up to 1 part in $10^6$. We examine the feasibility of detecting this effect with optical spectroscopy, using Keck/Deimos data on the merging cluster RM J001938.7+033557.3 as a case study. We also quantify the predicted sky-plane motions of systems likely to be identified as merging clusters in a $Λ$CDM cosmology, and investigate how information on sky-plane motions could help constrain merger models. We find that measurement of the moving lens effect would disambiguate between models in which the subclusters are outbound after first pericenter and those in which the subclusters have returned to the same separation after first apocenter.

astro-ph.CO↗

arXiv2026

X-SORTER (X-ray Survey Of meRging clusTErs in Redmapper): X-ray and Spectroscopic Characterization of 12 Optically Selected Galaxy Cluster Merger Candidates

Merging galaxy clusters offer a unique probe of dark matter (DM) interactions through the spatial offsets between galaxies, the intracluster medium, and the DM halo. Systems that are binary, near the plane of the sky, and observed shortly after first pericenter provide the cleanest constraints on the DM self-interaction cross-section. The X-SORTER (X-ray Survey Of meRging clusTErs in redMaPPer) program aims to systematically identify such mergers using optical indicators of binarity in the redMaPPer cluster catalog and to follow up promising candidates with X-ray and spectroscopic observations. We select massive clusters where the top redMaPPer brightest cluster galaxy (BCG) probability is below 0.98, the top two BCGs are separated by at least 0.95 arcmin, and the optical richness exceeds $λ= 120$. We present XMM and Keck/DEIMOS observations of twelve clusters with no previous XMM-Newton or Chandra archival data meeting these criteria. The X-ray data reveal that most targets are morphologically disturbed, with several clear post-pericenter, dissociative systems exhibiting X-ray peaks between the BCGs. Spectroscopy confirms cluster membership and rules out foreground or background contamination. Together, these results demonstrate that optical BCG properties provide an efficient means of identifying dynamically active clusters, including clean dissociative mergers suitable for detailed, multi-wavelength studies of dark matter and cluster evolution.

astro-ph.CO↗

arXiv2026

Improving Precision in Kinematic Weak Lensing with MIRoRS: Model-Independent Restoration of Reflection Symmetries

We present a novel, model-independent technique for fitting the cross-component of weak lensing shear, $γ_\times$, along a line of sight by combining kinematic and photometric measurements of a single lensed galaxy. Rather than relying on parametric models, we fit for the shear parameter that best transforms the velocity field to restore its underlying symmetries, while also incorporating photometric data for the change in position angle due to shear. We first validate our technique with idealized mock data, exploring the method's response to variations in shear, position angle, inclination, and noise. On this idealized mock data, our combined kinematic and photometric model demonstrates superior performance compared to traditional parametric or kinematic-only approaches. We also explore the effects of asymmetric warps and show that rotation direction can impart a small bias on the fit of $γ_\times$. Subsequently, we apply our method to a dataset of 358 halos from the Illustris TNG simulations, achieving a notable reduction in the uncertainty of $γ_\times$ to 0.039, marking a substantial improvement over previous analysis of the dataset with a parametric model. Finally, we introduce an outlier rejection method based on Moran's $I$ test for spatial autocorrelation. Identifying and filtering out halos with spatially correlated residuals reduces the overall uncertainty to 0.028. Our results underscore the efficacy of combining kinematic and photometric data for weak lensing studies, providing a more precise and targeted measurement of shear along an individual line of sight.

astro-ph.CO↗