Dark Halo Research Articles

Assessing the Dark Matter Content of Two Quasar Host Galaxies at z ∼ 6 through Gas Kinematics

Abstract We conduct a study of the gas kinematics of two quasar host galaxies at z ≳ 6 traced by the [C ii] emission line using the Atacama Large Millimeter/submillimeter Array. By combining deep observations at both low and high resolution, we recover the diffuse emission, resolve its structure, and measure the rotation curves from the inner region of the galaxy to its outskirts using DysmalPy and 3D Barolo. Assuming that both galaxies exhibit disk rotation driven by the gravitational potential of the galaxy, we find that the best-fit disk models have a V rot/σ ≈ 2 and inferred circular velocities out to ∼6–8 kpc scales, well beyond the likely stellar distribution. We then determine the mass profiles of each component (stars, gas, dark matter) with priors on the baryon and dark matter properties. We find relatively large dark matter fractions within their effective radii (f DM(R < R e) = 0.61 − 0.08 + 0.08 and 0.53 − 0.23 + 0.20 , respectively), which are significantly larger than those extrapolated from lower redshift studies and remain robust under different input parameters verified by Monte Carlo simulations. The large f DM(R < R e) corresponds to halo masses of ∼1012.5−1012.8 M ⊙, thus representative of the most massive halos at these redshifts. Notably, while the masses of these supermassive black holes (SMBHs) are approximately 1 dex higher than the low-redshift relationship with stellar mass, the closer alignment of SMBH and halo masses with a local relationship may indicate that the early formation of these SMBHs is linked to their dark matter halos, providing insights into the coevolution of galaxies and black holes in the early Universe.

Dark Galactic Subhalos and the Gaia Snail

Abstract Gaia has revealed a clear signal of disequilibrium in the solar neighborhood in the form of a spiral (or snail) feature in the vertical phase-space distribution. We investigate the possibility that this structure emerges from ongoing perturbations by dark 1 0 6 M ⊙ − 1 0 8 M ⊙ Galactic subhalos. We develop a probabilistic model for generating subhalo orbits based on a semianalytic model of structure formation, and combine this framework with an approximate prescription for calculating the response of the disk to external perturbations. We also develop a phenomenological treatment for the diffusion of phase-space spirals caused by gravitational scattering between stars and giant molecular clouds, a process that erases the kinematic signatures of old (t ≳ 0.6 Gyr) events. Perturbations caused by dark subhalos are, on average, orders of magnitude weaker than those caused by luminous satellite galaxies, but the ubiquity of dark halos predicted by cold dark matter makes them a more probable source of strong perturbation to the dynamics of the solar neighborhood. Dark subhalos alone do not cause enough disturbance to explain the Gaia snail, but they excite fluctuations of ∼0.1–0.5 km s−1 in the mean vertical velocity of stars near the Galactic midplane that should persist to the present day. Subhalos also produce correlations between vertical frequency and orbital angle that could be mistaken as originating from a single past disturbance. Our results motivate investigation of the Milky Way's dark satellites by characterizing their kinematic signatures in phase-space spirals across the Galaxy.

The stability of anisotropic compact stars influenced by dark matter under teleparallel gravity: an extended gravitational deformation approach

In our investigation, we pioneer the development of geometrically deformed strange stars within the framework of teleparallel gravity theory through gravitational decoupling via the complete geometric deformation (CGD) technique. The significant finding is the precise solution for deformed strange star (SS) models achieved through the vanishing complexity factor scenario. Further, we introduce the concept of space-time deformation caused by dark matter (DM) content in DM haloes, leading to perturbations in the metric potentials gtt and grr components. Mathematically, this DM-induced deformation is achieved through the CGD method, where the decoupling parameter α governs the extent of DM influence. To validate our findings, we compare our model predictions with observational constraints, including GW190814 (with a mass range of 2.5-2.67M⊙) and neutron stars (NSTRs) such as EXO 1785-248 [mass=1.3-0.2+0.2M⊙], 4U 1608-52 [mass=1.74-0.14+0.14M⊙], and PSR J0952-0607 [mass=2.35-0.17+0.17M⊙]. Our investigation delves into the stability of the model by considering causality conditions, Herrera’s cracking method, the adiabatic index, and the Harrison–Zeldovich–Novikov criterion. We demonstrate that the developed model mimics a wide range of recently observed pulsars. To emphasize its compatibility, we highlight the predicted mass and radius in tabular form by varying both the parameters α and ζ1. Notably, our findings are consistent with the observation of gravitational waves from the first binary merger event. Furthermore, we compare our results with those obtained for a slow-rotating configuration. In addition to this, we discuss the moment of inertia using the Bejger–Haensel approach in this formulation.

Gravitational Faraday-Cartan effect beyond gravitomagnetism due to dark matter intrinsic spin

We show that the spin of dark matter induces a gravitational analog of the electromagnetic Faraday effect, where the polarization of gravitational waves undergoes a rotation as they propagate through a dark matter halo with a non-vanishing axial hypermomentum. An expression for the gravitational rotation angle is provided, which is analogous to the Faraday rotation in optics, and evaluate its significance in astrophysical settings. Although the effect is expected to be small under current observational constraints, we discuss its potential importance in the early universe.Importantly, this effect is distinct from the known gravitational Faraday rotation in gravitomagnetism, where the geometry of general relativity is split into a background and a low-frequency gravitomagnetic perturbation. In that framework, the polarization of an electromagnetic wave (or a high-frequency GW perturbation) rotates relative to the background geometry. In contrast, this gravitational Faraday-Cartan effect arises from a non-vanishing dark matter axial hypermomentum that breaks the parallel transport of GW polarization, without invoking any gravitomagnetic approximation. Notably, it only rotates gravitational wave polarization without affecting the electromagnetic wave one.

Dynamics of the response of dark matter halo to galaxy evolution in IllustrisTNG

We present the dynamical evolution of the dark matter's relaxation response to galaxies and their connection to the astrophysical properties as simulated in the IllustrisTNG suite of cosmological hydrodynamical simulations. Our results show that the radially-dependent linear relaxation relation model from our previous work is applicable at least from redshift z=5. We focus on the offset parameter q 0, which characterizes the relaxation of dark matter shells without changing the enclosed mass. We perform multiple time-series analyses to determine the possible causal connections between the relaxation mechanism and astrophysical processes such as star formation and associated feedback processes, as well as feedback due to active galactic nuclei. We show that star formation activity significantly influences the halo relaxation response throughout its evolutionary history, with essentially immediate effects in the inner haloes and delayed effects of 2 to 3 Gyr in the outer regions. Metal content shows a weaker connection to relaxation than star formation rates, but the accumulated wind from feedback processes exhibits a stronger correlation. These findings enhance our understanding of halo relaxation mechanisms. Our estimates of the time-scales relevant for dark matter relaxation can potentially improve the description of halo profiles in existing baryonification schemes and semi-analytical galaxy formation models. Our results also show how the relaxation response of dark haloes can probe the evolutionary history of the galaxies they host.

Quasiperiodic oscillations around a Schwarzschild black hole surrounded by a Dehnen type dark matter halo

Quasiperiodic oscillations around a Schwarzschild black hole surrounded by a Dehnen type dark matter halo

Inner Radius and Energy Conditions of Dark Matter Halos Surrounding Schwarzschild Black Holes

Inner Radius and Energy Conditions of Dark Matter Halos Surrounding Schwarzschild Black Holes

A three-parameter model for dark matter halos based on general relativity and quantum field theory

In this paper, we present a three-parameter model for the distributions of “dark matter” around local spherically symmetric and static distributions of normal matter, usually referred to as “dark matter halos”. The model is based on the assumption that the spontaneous symmetry breaking mechanism involving the Higgs field in the standard model of elementary particle physics leads to the existence of a constant distribution of vacuum energy throughout space–time. This is to be interpreted as a constant distribution of proper energy, as measured by local observers in their proper reference frames. Due to the presence of this universal distribution of vacuum energy, the model requires the introduction of the cosmological term in the Einstein field equations, in order to regulate the behavior of the resulting geometry at cosmologically large distances. By implementing the model on galactic scales, we are able to establish the gravitational consequences of such a homogeneous distribution of proper energy density at these scales. This results in equivalent halo masses and orbital velocity curves that, at least qualitatively, match the observations for galactic “dark matter” halos. Although the detailed realization of the model presented and calculated here cannot be construed as a precise representation of galactic structure, given that most of the actual structure of the galaxy, such as the galactic disk itself, is ignored, and replaced by a single spherically symmetric bulge, in this paper we do introduce the conceptual framework that may be used to construct a more complete galactic model in the future.

Simulating realistic self-interacting dark matter models including small and large-angle scattering

Dark matter (DM) self-interactions alter matter distribution on galactic scales and alleviate tensions with observations. A feature of the self-interaction cross section is its angular dependence, which influences offsets between galaxies and DM halos in merging galaxy clusters. While algorithms for modelling mostly forward-dominated or mostly large-angle scatterings exist, incorporating realistic angular dependencies within N-body simulations remains challenging. To efficiently simulate models with a realistic angle dependence, such as light mediator models, we developed, validated, and applied a novel method. We combined existing approaches to describe small- and large-angle scattering regimes within a hybrid scheme. Below a critical angle, the scheme uses the effective description of small-angle scattering via a drag force combined with transverse momentum diffusion, while above the angle, it samples the dependence explicitly. We first verified the scheme using a test set-up with known analytical solutions, and we checked that our results are insensitive to the choice of the critical angle within an expected range. Next, we demonstrated that our scheme speeds up the computations by multiple orders of magnitude for realistic light mediator models. Finally, we applied the method to galaxy cluster mergers. We discuss the sensitivity of the offset between galaxies and DM to the angle dependence of the cross section. Our scheme ensures accurate offsets for mediator mass m_ϕ and DM mass m_χ within the range $0.1v/c m_ϕ/m_χ v/c$, while for larger (smaller) mass ratios, the offsets obtained for isotropic (forward-dominated) self-scattering are approached. Here, v is the typical velocity scale. Equivalently, the upper condition can be expressed as $1.1 tot /σ_ T 10$ for the ratio of the total and momentum transfer cross sections, with the ratio being $1$ (∞) in the isotropic (forward-dominated) limits.

COSMOS-Web: Stellar mass assembly in relation to dark matter halos across 0.2<z<12 of cosmic history

We study the stellar mass assembly of galaxies via the stellar mass function (SMF) and the coevolution with dark matter halos via abundance matching in the largest redshift range to date, $0.2<z<12$. We used the $0.53 deg ^2$ imaged by JWST from the COSMOS-Web survey, in combination with ancillary imaging in over 30 photometric bands, to select highly complete samples (down to log$ ⋆ /M_ ⊙ = 7.5-8.8$) in 15 redshift bins. Our results show that the normalization of the SMF monotonically decreases from z=0.2 to z=12 with strong mass-dependent evolution. At z>5, we find increased abundances of massive (log$ ⋆ /M_ ⊙ >10.5$) systems compared to predictions from semi-analytical models and hydrodynamical simulations. These findings challenge traditional galaxy formation models by implying integrated star formation efficiencies (SFEs) of ε_ ⋆ ≡ M_ ⋆ b M_ halo ≳ 0.5. We find a flattening of the SMF at the high-mass end that is better described by a double power law at z>5.5, after correcting for the Eddington bias. At z 5.5, it transitions to a Schechter law, which coincides with the emergence of the first massive quiescent galaxies in the Universe, indicating that physical mechanisms that suppress galaxy growth start to take place at z∼5.5 on a global scale. By integrating the SMF, we trace the cosmic stellar mass density and infer the star formation rate density, which at z>7.5 agrees remarkably with recent JWST UV luminosity function-derived estimates. This agreement solidifies the emerging picture of rapid galaxy formation leading to increased abundances of bright and massive galaxies in the first ∼ 0.7 Gyrs. However, at z 3.5, we find significant tension (∼ 0.3 dex) with the cosmic star formation (SF) history from instantaneous SF measures, the causes of which remain poorly understood. We infer the stellar-to-halo mass relation (SHMR) and the SFE from abundance matching out to z=12, finding a non-monotonic evolution. The SFE has the characteristic strong dependence with mass in the range of $0.02 - 0.2$, and mildly decreases at the low-mass end out to z∼3.5. At z∼3.5, there is an upturn and the SFE increases sharply from ∼ 0.1 to approach a high SFE of $0.8-1$ by z∼ 10 for h /M_ ⊙ )≈11.5$, albeit with large uncertainties. Finally, we use the SHMR to track the SFE and stellar mass growth throughout the halo history and find that they do not grow at the same rate -- from the earliest times up until z∼3.5 the halo growth rate outpaces galaxy assembly, but at z>3.5 halo growth stagnates and accumulated gas reservoirs keep the SF going and galaxies outpace halos.

Constraining the shape of dark matter haloes using only starlight. I. A new technique and its application to the galaxy Nube

We present a new technique to constrain the gravitational potential of a galaxy from the observed stellar mass surface density alone under a number of assumptions. It uses the classical Eddington inversion method to compute the phase-space distribution function (DF) needed for stars to reside in a given gravitational potential. In essence, each potential defines a set of density profiles, and it is the expansion of the observed profile in this database that provides the DF. If the required DF becomes negative, then the potential is inconsistent with the observed stars and can be discarded. It is particularly well suited for analyzing low-mass low surface brightness galaxies, where photometric but not spectroscopic data can be obtained. The recently discovered low surface brightness galaxy was used to showcase its application. For observed stellar core to be reproduced with a non-negative DF, cuspy NFW (Navarro, Frenk, and White) potentials are highly disfavored compared with potentials that have cores (Schuster-Plummer or ρ_230). The method assumes the stellar system to have spherical symmetry and isotropic velocity distribution; however, we discuss simple extensions that relax the need for isotropy and may help to drop the spherical symmetry assumption.

A candidate quadruple AGN system at z∼3

Multiple galaxies hosting active galactic nuclei (AGNs) at kiloparsec separations from each other are exceedingly rare, and in fact, only one quadruple AGN is known so far. These extreme densities of AGNs are expected to pinpoint protocluster environments and therefore should be surrounded by large galaxy overdensities. In this letter, we present another quadruple AGN candidate at z ∼ 3 including two SDSS quasars at a separation of roughly 480 kpc. The brighter quasar is accompanied by two AGN candidates (a type 1 AGN and a likely type 2 quasar) at a close (∼ 20 kpc) separation identified through emission line ratios, line widths, and high ionization lines, such as The extended Lyα emission associated with the close triple system is more modest in extent and brightness compared to similar multiple AGN systems and could be caused by ram-pressure stripping of the type 2 quasar host during infall into the central dark matter halo. The predicted evolution of the system into a z=0 galaxy cluster with the AGN host galaxies forming the brightest cluster galaxy needs to be further tested by galaxy overdensity studies on large scales around the quadruple AGN candidate. If confirmed as a quadruple AGN with X-ray observations or rest-frame optical line ratios, this system would represent a second AGN quartet and be the highest-redshift multiplet and the closest high-redshift triplet known.

Merger Response of Halo Anisotropy Properties

Abstract Anisotropy properties—halo spin, shape, position offset, velocity offset, and orientation—are an important family of dark matter halo properties that indicate the level of directional variation of the internal structures of halos. These properties reflect the dynamical state of halos, which in turn depends on the mass assembly history. In this work, we study the evolution of anisotropy properties in response to merger activity using the IllustrisTNG simulations. We find that the response trajectories of the anisotropy properties significantly deviate from secular evolution. These trajectories have the same qualitative features and timescales across a wide range of merger and host properties. We propose explanations for the behavior of these properties and connect their evolution to the relevant stages of merger dynamics. We measure the relevant dynamical timescales. We also explore the dependence of the strength of the response on time of merger, merger ratio, and mass of the main halo. These results provide insight into the physics of halo mergers and their effects on the statistical behavior of halo properties. This study paves the way toward a physical understanding of scaling relations, particularly to how systematics in their scatter are connected to the mass assembly histories of halos.

A Heavy Seed Black Hole Mass Function at High Redshift – Prospects for LISA

The advent of new and near-future observatories probing the earliest epochs of the Universe has opened the opportunity to investigate the formation and growth of the first massive black holes (MBHs). Additionally, the use of high resolution cosmological simulations to investigate these high-redshift environments is needed to predict the dark matter halos in which these MBH seeds will form. We use the 𝑅𝑒𝑛𝑎𝑖𝑠𝑠𝑎𝑛𝑐𝑒 simulations to analyse the formation and growth of so-called heavy seed black holes. Other past work has investigated the formation and growth of light (black hole) seeds with 𝑅𝑒𝑛𝑎𝑖𝑠𝑠𝑎𝑛𝑐𝑒 and found that these black holes do not grow in the environments in which they reside. In this work we seed MBHs, in post-processing, and track accretion onto the MBHs as well as mergers with other MBHs at high-redshift. We show that the heavy seeds struggle to achieve high accretion rates with only the most massive black holes ( ≳105M⊙) growing at close to the Eddington limit under optimistic conditions. Despite the lack of significant growth for these early MBHs, the signals from their merger events will be sufficiently strong (SNR ∼102) to be probed by the next generation of gravitational wave observatories, such as 𝐿𝐼𝑆𝐴. We predict that 𝐿𝐼𝑆𝐴 will observe of the order of 10 MBH merger events per year where the mergers occur at z ≳ 10 or at least begin their early inspiral phase at z ≳ 10.

Ellipticities of Galaxy Cluster Halos from Halo–Shear–Shear Correlations

Abstract We report the first detection of the halo ellipticities of galaxy clusters by applying the halo–shear–shear correlations (HSSCs), without the necessity of using major axis determination. We use the Fourier_Quad shear catalog based on the Hyper Suprime-Cam Survey and the group catalog from the Dark Energy Spectroscopic Instrument Legacy Surveys for the measurement of group/cluster lensing and HSSC. Our analysis includes off-centering effects. We obtain the average projected ellipticity of dark matter halos with mass 13.5 < log ( M G h / M ⊙ ) < 14.5 within a 1.3 virial radius to be 0.4 8 − 0.19 + 0.12 . We divide the sample into two groups based on mass and redshift, and we find that halos with higher mass tend to exhibit increased ellipticity. We also reveal that high-richness halos have larger ellipticities, confirming the physical picture from numerical simulation that high-richness halos have a dynamical youth and more active mass accretion phase.

Jetted Seyfert Galaxies at z = 0: Simulating Feedback Effects on Galactic Morphology and Beyond

Abstract We use high-resolution cosmological zoom-in simulations to model feedback from Seyfert-type supermassive black hole (SMBH) jets onto galaxies with identical dark matter (DM) halos of log M/M ⊙ ∼ 11.8. The low-mass, ∼106 M ⊙, seed SMBHs have been introduced when the parent DM halos have reached log M/M ⊙ ∼ 11. In a controlled experiment, we vary only the efficiency of the SMBH accretion and focus on galaxies and their immediate environment properties. Our results show that the active galactic nucleus jet feedback has a substantial effect on the basic properties of Seyfert-type galaxies, such as morphology, gas fraction and distribution, star formation rate and distribution, B/D ratio, DM halo baryon fraction, and properties of the circumgalactic medium and beyond. These have been compared to a galaxy with supernovae only feedback. We focus on the energy deposition by the jet in the interstellar medium (ISM) and intergalactic gas medium, and follow the expansion of the multiple jet cocoons to ∼2 Mpc. We find that the jet–ISM interaction gradually pushes the star formation to larger radii with increasing accretion efficiency, which results in increased mass of the outer stellar disk, which is best fit as a double-exponential disk. Furthermore, we compare our galaxies and their properties with the observed nearby Seyfert galaxies, including the scaling relations, and find a close agreement, although statistical analysis of observed Seyferts is currently missing. In a forthcoming paper, we focus on the evolution of these objects at z ≲ 10 and study the effect of the SMBH seeding redshift.

Signatures of dark and baryonic structures on weakly lensed gravitational waves

Gravitational lensing offers a powerful tool for exploring the matter distribution in the Universe. Thanks to their low frequencies and phase coherence, gravitational waves (GWs) allow for the observation of novel wave-optics features (WOFs) in lensing, inaccessible to electromagnetic signals. Combined with the existing accurate source models, lensed GWs can be used to infer the properties of gravitational lenses. The prospect is particularly compelling for space-borne detectors, where the high signal-to-noise ratio expected from massive black hole binary mergers allows WOFs to be distinguished deep into the weak lensing regime, drastically increasing the detection probability. Here, we investigate in detail the capacity of the LISA mission to detect WOFs caused by dark matter halos, galaxies, and the supermassive black holes (SMBHs) within them. We estimate the total optical depth to be λtot∼6×10−3 for the loudest binaries of total mass MBBH∼106M⊙, with the dominant contribution coming from SMBHs. We also find that WOFs in low-mass binaries MBBH∼104M⊙ are more likely due to the central galaxies. Within our model of gravitational lenses, we predict O(0.1)−O(1) weakly lensed events to be detectable during the five years of LISA mission, depending on the source population models. We show that WOFs signatures are very sensitive to the properties of dark matter halos with Mvir∈(106–108)M⊙: Increasing the compactness parameter by ∼3 in that range raises the detection rate by ∼26. Additionally, we show that collective effects from the complex inner halo structure can further enhance detectability. This suggests that lensed GWs in LISA will be an excellent probe of dark-matter theories, baryonic, and halo substructures. Published by the American Physical Society 2025

A halo model approach for mock catalogs of time-variable strong gravitational lenses

Time delays in both galaxy- and cluster-scale strong gravitational lenses have recently attracted a lot of attention in the context of the Hubble tension. Future wide-field cadenced surveys, such as the LSST, are anticipated to discover strong lenses across various scales. We generate mock catalogs of strongly lensed QSOs and SNe on galaxy-, group-, and cluster-scales based on a halo model that incorporates dark matter halos, galaxies, and subhalos. For the upcoming LSST survey, we predict that approximately 3500 lensed QSOs and 200 lensed SNe with resolved multiple images will be discovered. Among these, about 80 lensed QSOs and 10 lensed SNe will have maximum image separations larger than 10 arcsec, which roughly correspond to cluster-scale strong lensing. We find that adopting the Chabrier stellar IMF instead of the fiducial Salpeter IMF reduces the predicted number of strong lenses approximately by half, while the distributions of lens and source redshifts and image separations are not significantly changed. In addition to mock catalogs of multiple-image lens systems, we create mock catalogs of highly magnified systems, including both multiple-image and single-image systems. We find that such highly magnified systems are typically produced by massive galaxies, but non-negligible fraction of them are located in the outskirt of galaxy groups and clusters. Furthermore, we compare subsamples of our mock catalogs with lensed QSO samples constructed from the SDSS and Gaia to find that our mock catalogs with the fiducial Salpeter IMF reproduce the observation quite well. In contrast, our mock catalogs with the Chabrier IMF predict a significantly smaller number of lensed QSOs compared with observations, which adds evidence that the stellar IMF of massive galaxies is Salpeter-like. Our python code SL-Hammocks as well as the mock catalogs are made available online. (abridged)

Prediction of Individual Halo Concentrations Across Cosmic Time Using Neural Networks

The concentration of dark matter haloes is closely linked to their mass accretion history. We utilize the halo mass accretion histories from large cosmological N-body simulations as inputs for our neural networks, which we train to predict the concentration of individual haloes at a given redshift. The trained model performs effectively in other cosmological simulations, achieving the root mean square error between the actual and predicted concentrations that significantly lower than that of the model by Zhao et al. and Giocoli et al. at any redshift. This model serves as a valuable tool for rapidly predicting halo concentrations at specified redshifts in large cosmological simulations.

Segue 2 Recently Collided with the Cetus-Palca Stream: New Opportunities to Constrain Dark Matter in an Ultra-faint Dwarf

Abstract Stellar streams in the Milky Way are promising detectors of low-mass dark matter (DM) subhalos predicted by ΛCDM. Passing subhalos induce perturbations in streams that indicate the presence of the subhalos. Understanding how known DM-dominated satellites impact streams is a crucial step toward using stream perturbations to constrain the properties of dark perturbers. Here, we cross-match a Gaia Early Data Release 3 and SEGUE member catalog of the Cetus-Palca stream (CPS) with H3 for additional radial velocity measurements and fit the orbit of the CPS using this six-dimensional (6D) data. We demonstrate for the first time that the ultra-faint dwarf Segue 2 had a recent (77 ± 5 Myr ago) close flyby (within the stream's 2σ width) with the CPS. This interaction enables constraints on Segue 2’s mass and density profile at larger radii ( O ( 1 ) kpc) than are probed by its stars ( O ( 10 ) pc). While Segue 2 is not expected to strongly affect the portion of the stream covered by our 6D data, we predict that if Segue 2’s mass within ∼ 6 kpc is 5 × 109 M ⊙, the CPS's velocity dispersion will be ∼ 40 km s−1 larger at ϕ 1 > 20° than at ϕ 1 < 0°. If no such heating is detected, Segue 2’s mass cannot exceed 109 M ⊙ within ∼ 6 kpc. The proper motion distribution of the CPS near the impact site is mildly sensitive to the shape of Segue 2’s density profile. This study presents a critical test for frameworks designed to constrain properties of dark subhalos from stream perturbations.