arXiv subjects

Richard A. N. Brooks

Publications and source records attributed to Richard A. N. Brooks.

9 recordsLinked to original sources

arXiv2026

GSE vs. LMC: reshaping of radially biased stellar haloes by satellites

Perturbations from the Large Magellanic Cloud (LMC) of the Milky Way's stellar and dark matter haloes are well-established. However, studies have generally not considered the high radial anisotropy of the Milky Way's inner halo caused by the accreted debris of Gaia Sausage-Enceladus (GSE). We run a series of test particle simulations of stellar haloes being perturbed by the LMC, with different halo velocity anisotropies $β\in[0.5,0.9]$. The LMC causes these initially axisymmetric haloes to become approximately triaxial. Their major axes are aligned with its orbital plane and tilted by up to $\sim14^\circ$ with respect to a fixed Galactic disc. These effects become much more dramatic as $β$ increases, causing the halo to fractionate spatially according to anisotropy. This confirms the expectations of an analytical model, which predicts that orbits with eccentricities $e\gtrsim0.95$ should azimuthally align with the tidal field of the LMC. The reshaping of the $β=0.9$ halo creates strong overdensities of $\sim40\%$ at heliocentric distances as close as 15 kpc. These coincide with the well-known Virgo Overdensity (VOD) and Hercules-Aquila Cloud (HAC), which have previously been associated with the GSE. We propose that the HAC and VOD were created by the dynamical alignment of highly eccentric orbits by the LMC, and are not necessarily relics of the GSE merger geometry. We conclude that previous works have significantly underestimated perturbations from the LMC in the inner stellar halo by not considering sufficiently high velocity anisotropy. This effect should be corrected for when constructing equilibrium models of the GSE debris.

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arXiv2026

The Milky Way - Large Magellanic Cloud Interaction with Simulation Based Inference

The infall of the Large Magellanic Cloud (LMC) into the Milky Way (MW) has displaced the MW's centre of mass, manifesting as an observed reflex motion in the velocities of outer halo stars. We use a Simulation Based Inference framework to constrain properties of the MW, LMC and the induced reflex motion using the dynamics of outer MW halo stars. Specifically, we use the mean radial and tangential velocities of outer halo stars calculated in a set of distance and on-sky bins. We train neural networks to estimate parameter posterior distributions using a set of $128,000$ rigid MW--LMC simulations conditioned upon velocity data from the Dark Energy Spectroscopic Instrument (DESI) and the combined H3+SEGUE+MagE outer halo surveys. We constrain the reflex motion velocity and the enclosed LMC mass within $50 \, \rm kpc$ using the DESI or H3+SEGUE+MagE dataset while varying the survey sky coverage and depth. Using the radial and tangential velocity data from the H3+SEGUE+MagE survey and on-sky quadrants, we report a distance-averaged reflex motion velocity for the outer halo samples, the speed at which the MW lurches towards the LMC, of $v_{\rm{travel}} = 26.4^{+5.5}_{-4.4} \, \rm km \, \rm s^{-1}$, while simultaneously finding an enclosed LMC mass of $M_{\rm LMC}(< 50 \, \rm kpc) = 9.2^{+1.9}_{-2.3} \times 10^{10}\, \rm M_{\odot}$. Quoted uncertainties are statistical. Our results suggest that the LMC's total mass is at least $\approx 10-15 \%$ of that of the MW. This inference framework is flexible such that it can provide rapid constraints when applied to any future survey measuring the velocities of outer halo stars.

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arXiv2026

Quantifying the Milky Way, LMC and their interaction using all-sky kinematics of outer halo stars

The recent pericentric passage of the Large Magellanic Cloud (LMC) has dislodged the Milky Way's (MW) centre of mass, inducing dynamical disequilibrium, the reflex motion, in the kinematics of outer stellar halo stars. Using data out to $160 \, \rm kpc$ from the combined H3+SEGUE+MagE outer halo survey, we constrain the mass of the MW and LMC, as well as the resulting reflex motion and the stellar halo velocity anisotropy. Using a suite of 32,000 rigid MW--LMC simulations, each with a MW stellar halo evolved to the present day in the combined MW--LMC potential, we perform Simulation Based Inference by training a neural posterior estimator on the means and dispersions of the radial and tangential velocities of stars from the combined H3+SEGUE+MagE outer halo sample. Relative to halo stars at $100 \, \rm kpc$, we find the magnitude of the reflex velocity to be $v_{\rm travel} = 38.6^{+8.3}_{-7.8}\,\rm km \, s^{-1}$. Simultaneously, we determine the enclosed MW mass, $M_{\rm MW}(< 50 \, \rm kpc) = 3.36 \pm 0.15 \times 10^{11}\, \rm M_{\odot}$ and the enclosed LMC mass, $M_{\rm LMC}(< 50 \, \rm kpc) = 8.76^{+1.94}_{-1.77} \times 10^{10}\, \rm M_{\odot}$. Our results suggest that the total LMC mass must be at least $\sim20\%$ that of the MW. The velocity anisotropy prior to the LMC's infall is constrained to be $β_0 = 0.68 \pm 0.02$. Finally, we demonstrate that neglecting the LMC in models biases the estimated MW mass to prefer more massive values.

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arXiv2026

The velocity field of our Milky Way outer stellar halo based on DESI DR2

Using 64,000 halo K giants from Dark Energy Spectroscopic Instrument (DESI) second Data Release (DR2), we decompose the Milky Way (MW) stellar halo between 3 and 160 kpc into metal-rich (MR) and metal-poor (MP) components via a Gaussian mixture model (GMM). The two populations are nearly equal in number but chemically and kinematically distinct: MR stars occupy highly radial orbits with velocity anisotropy of beta ~0.94 and metallicity dispersion sigma([Fe/H]) ~0.17 dex, without obvious dependence on distance, and are mainly contributed by Gaia-Sausage/Enceladus (GSE) debris. MR component dominates the inner 30 kpc and re-emerges beyond 50 kpc, implying GSE debris can extend to ~70-80 kpc. MP stars exhibit a weaker radial bias of beta ~0.46, decreasing to -0.5 beyond 80 kpc, and with a larger metallicity dispersion of sigma([Fe/H]) ~0.46 dex, showing signatures of multiple minor mergers. Both components exhibit net prograde rotation at ~10-30 kpc with a stronger azimuthal signal in the MP population. The non-equilibrium motions of the outer halo (>50 kpc) are quantified with a dipole-plus-contraction velocity field. We find that the outer halo is simultaneously contracting (~-19 km/s, distance-independent) and subject to reflex motions (increases from -19 to -44 km/s with radius), reflecting the perturbation from the Large Magellanic Cloud (LMC). We also confirm a linear dependence of mean polar velocity for the outer stellar halo on the dipole velocity field, a direct consequence of the LMC and MW interaction. Our results provide a quantitative distance-resolved description of the MW's last major accretion event and its ongoing response to the first infall of the LMC.

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arXiv2025

A Simulation Based Inference Approach to the Dynamics of the MW-LMC System -- Validation

The infall of the LMC into the Milky Way (MW) has generated dynamical disequilibrium throughout the MW. The interaction has displaced the MW's centre of mass, manifesting as an apparent 'reflex motion' in velocities of outer halo stars. Often, expensive high fidelity MW--LMC simulations are required to model these effects, though the range of model parameter spaces can be large and complex. We investigate the ability of lower fidelity, rigid MW-LMC simulations to reliably infer the model parameters of higher fidelity N-body and hydrodynamical cosmological zoom-in MW--LMC simulations using a Simulation-Based Inference (SBI) approach. We produce and release a set of 128,000 MW--LMC rigid potentials, with stellar haloes evolved to present-day, each adopting a unique combination of model parameters including the MW mass, the LMC mass and the dynamical friction strength. For these simulation parameters, we use SBI to find their posterior distributions. We find that our SBI framework trained on rigid MW--LMC simulations is able to correctly infer the true simulation LMC mass within a $1σ$ confidence interval from both N-body and cosmological simulations when knowledge of the induced MW reflex motion is provided as data. This motivates future applications of the presented SBI framework to observational data, which will help constrain both MW and LMC properties, as well as the dynamics of the MW's reflex motion.

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arXiv2024

LMC Calls, Milky Way Halo Answers: Disentangling the Effects of the MW--LMC Interaction on Stellar Stream Populations

The infall of the LMC into the Milky Way (MW) has dynamical implications throughout the MW's dark matter halo. We study the impact of this merger on the statistical properties of populations of simulated stellar streams. Specifically, we investigate the radial and on-sky angular dependence of stream perturbations caused by the direct effect of stream--LMC interactions and/or the response of the MW dark matter halo. We use a time-evolving MW--LMC simulation described by basis function expansions to simulate streams. We quantify the degree of perturbation using a set of stream property statistics including the misalignment of proper motions with the stream track. In the outer halo, direct stream--LMC interactions produce a statistically significant effect, boosting the fraction of misaligned proper motions by ~25% relative to the model with no LMC. Moreover, there is on-sky angular dependence of stream perturbations: the highest fractions of perturbed streams coincide with the same on-sky quadrant as the present-day LMC location. In the inner halo, the MW halo dipole response primarily drives stream perturbations, but it remains uncertain whether this is a detectable signature distinct from the LMC's influence. For the fiducial MW--LMC model, we find agreement between the predicted fraction of streams with significantly misaligned proper motions, $\bar{\vartheta}>10^{\circ}$, and Dark Energy Survey data. Finally, we predict this fraction for the Rubin Observatory Legacy Survey of Space and Time (LSST) footprint. Using LSST data will improve our constraints on dark matter models and LMC properties as it is sensitive to both.

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arXiv2024

Action and energy clustering of stellar streams in deforming Milky Way dark matter haloes

We investigate the non-adiabatic effect of time-dependent deformations in the Milky Way (MW) halo potential on stellar streams. Specifically, we consider the MW's response to the infall of the Large Magellanic Cloud (LMC) and how this impacts our ability to recover the spherically averaged MW mass profile from observation using stream actions. Previously, action clustering methods have only been applied to static or adiabatic MW systems to constrain the properties of the host system. We use a time-evolving MW--LMC simulation described by basis function expansions. We find that for streams with realistic observational uncertainties on shorter orbital periods and without close encounters with the LMC, e.g. GD-1, the radial action distribution is sufficiently clustered to locally recover the spherical MW mass profile across the stream radial range within a 2 sigma confidence interval determined using a Fisher information approach. For streams with longer orbital periods and close encounters with the LMC, e.g. Orphan-Chenab (OC), the radial action distribution disperses as the MW halo has deformed non-adiabatically. Hence, for OC streams generated in potentials that include a MW halo with any deformations, action clustering methods will fail to recover the spherical mass profile within a 2 sigma uncertainty. Finally, we investigate whether the clustering of stream energies can provide similar constraints. Surprisingly, we find for OC-like streams, the recovered spherically averaged mass profiles demonstrate less sensitivity to the time-dependent deformations in the potential.

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arXiv2023

Origins of the north-south asymmetry in the ALFALFA HI velocity width function

The number density of extragalactic 21-cm radio sources as a function of their spectral line-widths -- the HI width function (HIWF) -- is a tracer of the dark matter halo mass function. The ALFALFA 21-cm survey measured the HIWF in northern and southern Galactic fields finding a systematically higher number density in the north; an asymmetry which is in tension with $Λ$ cold dark matter models which predicts the HIWF should be identical everywhere if sampled in sufficiently large volumes. We use the Sibelius-DARK N-body simulation and semi-analytical galaxy formation model GALFORM to create mock ALFALFA surveys to investigate survey systematics. We find the asymmetry has two origins: the sensitivity of the survey is different in the two fields, and the algorithm used for completeness corrections does not fully account for biases arising from spatial galaxy clustering. Once survey systematics are corrected, cosmological models can be tested against the HIWF.

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arXiv2023

The north-south asymmetry of the ALFALFA HI velocity width function

The number density of extragalactic 21-cm radio sources as a function of their spectral line-widths -- the HI width function (HIWF) -- is a sensitive tracer of the dark matter halo mass function (HMF). The $Λ$ cold dark matter model predicts that the HMF should be identical everywhere provided it is sampled in sufficiently large volumes, implying that the same should be true of the HIWF. The ALFALFA 21-cm survey measured the HIWF in northern and southern Galactic fields and found a systematically higher number density in the north. At face value, this is in tension with theoretical predictions. We use the Sibelius-DARK N-body simulation and the semi-analytical galaxy formation model GALFORM to create a mock ALFALFA survey. We find that the offset in number density has two origins: the sensitivity of the survey is different in the two fields, which has not been correctly accounted for in previous measurements; and the $1/V_{\mathrm{eff}}$ algorithm used for completeness corrections does not fully account for biases arising from spatial clustering in the galaxy distribution. The latter is primarily driven by a foreground overdensity in the northern field within $30\,\mathrm{Mpc}$, but more distant structure also plays a role. We provide updated measurements of the ALFALFA HIWF (and HIMF) correcting for the variations in survey sensitivity. Only when systematic effects such as these are understood and corrected for can cosmological models be tested against the HIWF.

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