arXiv subjects

Amina Helmi

Publications and source records attributed to Amina Helmi.

At least 19 recordsLinked to original sources

arXiv2026

Alignment of the Milky Way and M31 with their cosmic environment. New insights from constrained Local Group simulations

The large-scale environment is thought to play an important role in setting galaxy properties. The Milky Way (MW) and Andromeda (M31) reside in the Local Group, embedded in the Local Sheet (LS). To study the sheet's influence on the dark matter (DM) halo shapes, spins, and disks orientations of MW and M31 analogues, we use a new suite of constrained Local Group simulations that reproduce the observed configuration of the two main halos and the LS. We determine their shapes and alignments relative to the LS analogues, and the effect of infall and coalescence of massive mergers. We find that the DM halo shapes of our MW and M31 analogues are on average slightly rounder than literature reports for similar-mass galaxies in random environments. We find preferential alignment between the sheet normal and the halos' minor axes, but not with the halos' spins. The present-day disk angular momenta ($L_{\rm disk}$) closely align with the halos' minor axes (median $15^{+15}_{-8}$ degrees at $R_{\rm vir}$) and with the halos' spins. The direction of $L_{\rm disk}$ is often set by one of the two highest mass ratio mergers during the last 8-10 Gyr. While recent ($\leq 2$ Gyr) massive mergers can reorient the outer halo's minor axis leading to twisted shapes, $L_{\rm disk}$ retains the imprint of the earlier accretion event. These results can explain the peculiar alignment of the MW's disk and DM halo shape and their orientation relative to the LS. The prolate-like morphology and orientation of the MW's outer halo can be explained by the Magellanic Clouds (and perhaps Sagittarius) accreting from within the LS. As their orbital planes are nearly perpendicular to the Galactic disk, the disk orientation must be set earlier, possibly by the GES merger. This merger's estimated infall direction, highly inclined relative to the present-day LS, is broadly consistent with the LS's direction of maximum collapse.

astro-ph.GA↗

arXiv2026

Contrastive learning of extragalactic stellar streams. Sculpting a latent space of representations with DES DR2 photometry

We present a self-supervised approach for characterizing low surface brightness tidal features in wide-field imaging data by applying the nearest-neighbor contrastive learning of visual representations (NNCLR) algorithm to a curated subset of the Dark Energy Survey Data Release 2 (DES DR2). We construct 38,334 cutouts of well-resolved galaxies in the g, r, i bands, applying a novel "tiered sigmoid scaling function" to dynamically adjust image contrast according to the object's signal-to-noise and background level. A supplemental labeled sample of 366 galaxies enables qualitative assessment of the learned embeddings. We train a convolutional neural network with image augmentations including injection of simulated background stars, and project the resulting 512-dimensional representations into two dimensions using uniform manifold approximation and projection (UMAP) and its local density preserving variant (densMAP). We find that the NNCLR latent space recovers global trends corresponding to major merger features, yet does not reliably separate stellar streams without further supervision. To interpret the network's implicit attention, we compute gradient-based saliency maps averaged over the full dataset: these reveal that the tiered sigmoid scaling effectively attenuates information from the center of the image cutouts, thereby suppressing the learning of high surface brightness features of each image cutout's central galaxy. Our study provides a blueprint for leveraging contrastive methods to mine forthcoming survey data for faint tidal substructure, and highlights key preprocessing and interpretability considerations for robust stream detection.

astro-ph.GA↗

arXiv2026

Evidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger

The merger history of the Galaxy has been traced back firmly to redshift 2 (10 Billion years ago). While evidence for the existence of at least one more significant merger before this time has been presented, its interpretation is yet to be fully established. Here we show that the population of globular clusters around the Galaxy depicts three distinct age-metallicity sequences, one associated with the progenitor of the Milky Way, one with the merger with Gaia-Enceladus 10 billion years ago, and a third intermediate sequence associated to at least one merger which we estimate took place about 1.8 billion years before Gaia-Enceladus. This discovery has been possible thanks to exquisite Hubble Space Telescope data and sophisticated analysis that enables very precise relative age determination of globular clusters. The newly identified sequence reveals that this merger took place with an object of stellar mass similar to that of Gaia-Enceladus $(\simeq5\times10^8 M_{\odot})$, and which deposited most of its mass in the inner 6 kpc of the Milky Way. The identification of a third merger event in the inner Galaxy puts to rest earlier debates, and honoring previous works we name the progenitor system Low-energy-Kraken-Heracles, or LKH for short.

astro-ph.GA↗

arXiv2026

The mass distribution in and around the Local Group

Our Galaxy, Andromeda and their companion dwarf galaxies form the Local Group. Most of the mass in and around it is believed to be dark matter rather than gas or stars, so its distribution must be inferred from the effect of gravity on the motion of visible objects. Modelling efforts have long struggled to reproduce the quiet Hubble flow around the Local Group, as they require unrealistically little mass beyond the haloes of the two main galaxies. Here we revisit this using $\Lambda$CDM simulations of Local Group analogues with initial conditions constrained to match the observed dynamics of the two main haloes and the surrounding flow. The observations are reconcilable within $\Lambda$CDM, but only if mass is strongly concentrated in a plane out to 10 Mpc, with the surface density rising away from the Local Group and with deep voids above and below. This configuration, dynamically inferred, mirrors known structures in the nearby galaxy distribution. The resulting Hubble flow is quiet yet strongly anisotropic, a fact obscured by the paucity of tracers at high supergalactic latitude. This flattened geometry reconciles the dynamical mass estimates of the Local Group with the surrounding velocity field, thus demonstrating full consistency within the standard cosmological model.

astro-ph.GA↗

arXiv2025

Precise Asteroseismic Ages for the Helmi Streams

The Helmi streams are remnants of a dwarf galaxy that was accreted by the Milky Way and whose stars now form a distinct kinematic and chemical substructure in the Galactic halo. Precisely age-dating these typically faint stars of extragalactic origin has been notoriously difficult due to the limitations of using only spectroscopic data, interferometry, or coarse asteroseismic measurements. Using observations from NASA's Transiting Exoplanet Survey Satellite, we report the detailed asteroseismic modeling of two of the brightest red giants within the Helmi streams, HD 175305 and HD 128279. By modeling the individual oscillation mode frequencies and the spectroscopic properties of both stars, we determine their fundamental properties including mass, radius, and age ($\tau$). We report $\tau = 11.16 \pm 0.91$ Gyr for HD 175305 and $\tau = 12.52 \pm 1.05$ Gyr for HD 128279, consistent with previously inferred star-formation histories for the Helmi streams and the differential chemical abundances between the two stars. With precise ages for individual stream members, our results reinforce the hypothesis that the Helmi streams' progenitor must have existed at least 12 Gyr ago. Our results also highlight that the ages of metal-poor, $\alpha$-enhanced red giants can be severely underestimated when inferred using global asteroseismic parameters instead of individual mode frequencies.

astro-ph.SR↗

arXiv2025

On the chaos induced by the Galactic bar on the orbits of nearby halo stars

Many of the Milky Way's accreted substructures have been discovered and studied in the space of energy $E$, and angular momentum components $L_z$ and $L_{\bot}$. In a static axisymmetric system, these quantities are (reasonable approximations of) the integrals of motion of an orbit. However, in a galaxy like the Milky Way with a triaxial, rotating bar, none of these quantities are conserved, and the only known integral is the Jacobi energy $E_J$. This may result in chaotic orbits, especially for inner halo stars. Here, we investigate the bar's effect on the dynamics of nearby halo stars, and more specifically its impact on their distribution in $(E, L_z, L_{\bot})$ space. To this end, we have integrated and characterised the orbits of halo stars located within 1 kpc from the Sun. We computed their orbital frequencies and quantified the degree of chaoticity and associated timescales, using the Lyapunov exponent and the frequency diffusion rate. We find that the bar introduces a large degree of chaoticity on the stars in our sample: more than half are found to be on chaotic orbits, and this fraction is highest for stars on very bound and/or radial orbits. Such stars wander in $(E, L_z, L_{\bot})$ space on timescales shorter than a Hubble time. This introduces some overlap and hence contamination amongst previously identified accreted substructures with these orbital characteristics, although our assessment is that this is relatively limited. The bar also induces a number of resonances in the stellar halo, which are of larger importance for lower inclination, prograde orbits. Because the effect of the Galactic bar on the local halo is important for stars on very bound and/or radial orbits, clustering analyses in these regions should be conducted with care. Replacing the energy by $E_J$ in such analyses could be an improvement.

astro-ph.GA↗

arXiv2025

Group Accretion in Milky Way-like Stellar Haloes

As galaxies form hierarchically, larger satellites may accrete alongside smaller companions in group infall events. This coordinated accretion is likely to have left signatures in the Milky Way's stellar halo at the present day. Our goal is to characterise the possible groups of companions that accompanied larger known accretion events of our Galaxy, and infer where their stellar material could be in physical and dynamical space at present day. We use the AURIGA simulation suite of Milky Way-like haloes to identify analogues to these large accretion events and their group infall companions, and we follow their evolution in time. We find that most of the material from larger accretion events is deposited on much more bound orbits than their companions. This implies a weak dynamical association between companions and debris, but it is strongest with the material lost first. As a result, the companions of the Milky Way's earliest building blocks are likely to contribute stars to the solar neighbourhood, whilst the companions of our last major merger are likely found in both the solar neighbourhood and the outer halo. More recently infallen groups of satellites, or those of a smaller mass, are more likely to retain dynamical coherence, for example, through clustering in the orientation of angular momentum. We conclude that group infall has likely shaped the Milky Way's stellar halo. Disentangling this will be challenging for the earliest accretion events, although overlap with their less-bound debris may be particularly telling.

astro-ph.GA↗

arXiv2025

Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal) II. Unveiling the formation and evolution of the kinematically selected Thick and Thin Discs

Understanding the formation and evolution of the Milky Way's thin and thick discs is crucial to galaxy formation studies. We derive age and metallicity distributions of the kinematic thick and thin discs using the CMDft.Gaia pipeline and Gaia DR3 data within 250 pc of the Sun, covering 1 kpc in height. Our results show that the kinematic thick disc is mostly older than 10 Gyr, undergoing three main metallicity enrichment episodes: (1) over 12 Gyr ago, peaking at [M/H] $\sim$ -0.5 dex, (2) $\sim$11 Gyr ago, rapidly increasing to solar [M/H] and spanning [$\alpha$/Fe] from 0.3 to solar, and (3) just over 10 Gyr ago, reaching supersolar metallicities. Meanwhile, the kinematic thin disc began forming $\sim$10 Gyr ago, just as thick disc star formation ended, characterized by supersolar metallicities and low [$\alpha$/Fe]. This transition coincides with the Milky Way's last major merger: Gaia-Sausage Enceladus (GSE). We also identify a subset of kinematic thin disc stars older than 10 Gyr with high/intermediate [$\alpha$/Fe], indicating a transition phase. The age-metallicity relation of the thin disc suggests overlapping star formation episodes and radial mixing in the solar neighborhood, with the greatest spread $\sim$6 Gyr ago. Additionally, we detect an isolated thick disc star formation event at solar metallicity, coinciding with Sagittarius' first pericenter passage. These findings provide precise age-metallicity distributions and star formation rates, offering key insights for chemical evolution models and cosmological simulations.

astro-ph.GA↗

arXiv2025

Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal) -- III. Age and metallicity distribution of Gaia-Sausage-Enceladus stars near the Sun

Context. Gaia-Sausage-Enceladus is considered the last major merger that contributed to the formation of the Milky Way. Its remnants dominate the nearby accreted stellar halo of the Milky Way. Aim. We aim to characterise the star formation history of Gaia-Sausage-Enceladus through the age and metallicity of its stellar populations. Methods. From Gaia DR3 data, we dynamically define three Gaia-Sausage-Enceladus samples with different criteria and possible degrees of contamination from other substructures in the halo. Then, we derive the stellar age and metallicity distributions using the CMDfit.Gaia package. Results. We identify three main populations of stars and a fourth smaller one following an almost linear age-[M/H] relation. The three oldest populations correspond to the bulk of the star formation that lasted for, at least, $\sim$3-4 Gyr and ended about 10 Gyr ago, its metallicities ranging from $-$1.7 to $-$0.8. We categorise these populations into two main epochs: the evolution of GSE in isolation and the merger event. This separation finds independent support from the age-metallicty relation of GSE globular clusters (Aguado-Agelet et al., subm.). The fourth population is younger and more metal-rich, at $\sim$8.5 Gyr and [M/H]$\sim-0.4$; its link to GSE is unclear.

astro-ph.GA↗

arXiv2025

Unevolved Li-rich stars at low metallicity: a possible formation pathway through novae

A small fraction of low-mass stars have been found to have anomalously high Li abundances. Although it has been suggested that mixing during the red giant branch phase can lead to Li production, this method of intrinsic Li production cannot explain Li-rich stars that have not yet undergone the first dredge-up. To obtain clues about the origin of such stars, we present a detailed chemical abundance analysis of four unevolved Li-rich stars with $-2.1 < [\mathrm{Fe/H}] < -1.3$ and $2.9<A({\rm Li})<3.6$, $0.7-1.4$ dex higher Li abundance than typical unevolved metal-poor stars. One of the stars, Gaia DR3 6334970766103389824 (D25_6334), was serendipitously found in the stellar stream ED-3, and the other three stars have been reported to have massive ($M\gtrsim 1.3\,\mathrm{M_\odot}$) non-luminous companions. We show that three of the four stars exhibit abundance patterns similar to those of known unevolved Li-rich stars, namely normal abundances in most elements except for Li and Na. These abundance similarities suggest a common origin for the unevolved Li-rich stars and low-mass metal-poor stars with massive compact companions. We also made the first detection of N abundance to unevolved Li-rich stars in D25_6334, and found that it is significantly enhanced ($[\mathrm{N/Fe}]=1.3$). The observed abundance pattern of D25_6334, spanning from C to Si, indicates that its surface has been polluted by an intermediate-mass former companion star or a nova system that involves a massive ONe white dwarf. Using a population synthesis model, we show that the nova scenario can lead to the observed level of Li enhancement and also provide an explanation for Li-rich stars without companions and those with massive compact companions.

astro-ph.SR↗

arXiv2025

Chemical characterisation of small substructures in the local stellar halo

The local stellar halo of the Milky Way is known to contain the debris from accreted dwarf galaxies and globular clusters, in the form of stellar streams and over-densities in the space of orbital properties (e.g. integrals of motion). While several over-densities have been uncovered and characterised dynamically using Gaia data, their nature is not always clear. Especially for a complete understanding of the smaller halo substructures, the kinematic information from Gaia needs to be coupled with chemical information. In this work, we combine Gaia data with targeted high-resolution UVES spectroscopy of five small substructures that were recently discovered in the local halo, namely the ED-2, -3, -4, -5 and -6 (the ED streams). We present the chemical abundances measured from our newly obtained UVES spectra (20 stars) and from archival UVES spectra (nine stars). We compare these with homogeneously derived abundances from archive spectra of 12 Gaia Enceladus (GE) stars. The chemical abundances of all five substructures suggest that they are of accreted origin, except for two stars that present a high [{\alpha}/Fe] at high [Fe/H] more in line with an in situ origin. All but ED-2 present a significant spread in [Fe/H] suggestive of a dwarf galaxy origin. ED-3 and ED-4 tend to exhibit lower [{\alpha}/Fe] compared to GE stars. ED-5 and ED-6 are consistent with the GE chemical track and could be high-energy tails of GE that were lost earlier in the accretion process. We present new elemental abundances for 5 ED-2 stars, including more elements for the Gaia BH3 companion star. Our findings are in line with the picture that ED-2 is a disrupted ancient star cluster.

astro-ph.GA↗

arXiv2025

The effect of environment on the mass assembly history of the Milky Way and M31

We study the mass growth histories of the halos of Milky Way and M31 analogues formed in constrained cosmological simulations of the Local Group. These simulations constitute a fair and representative set of $\Lambda$CDM realisations conditioned on properties of the main Local Group galaxies, such as their masses, relative separation, dynamics and environment. Comparing with isolated analogues extracted from the TNG dark-matter-only simulations, we find that while our M31 halos have a comparable mass growth history to their isolated counterparts, our Milky Ways typically form earlier and their growth is suppressed at late times. Mass growth associated to major and minor mergers is also biased early for the Milky Way in comparison to M31, with most accretion occurring 1 - 4 Gyr after the Big Bang, and a relatively quiescent history at later times. 32% of our Milky Ways experienced a Gaia-Enceladus/Sausage (GES)-like merger, while 13% host an LMC-like object at the present day, with 5% having both. In one case, an SMC- and a Sagittarius-analogue are also present, showing that the most important mergers of the Milky Way in its Local Group environment can be reproduced in $\Lambda$CDM. We find that the material that makes up the Milky Way and M31 halos at the present day first collapsed onto a plane roughly aligned with the Local Sheet and Supergalactic plane; after $z \sim 2$, accretion occurred mostly within this plane, with the tidal effects of the heavier companion, M31, significantly impacting the late growth history of the Milky Way.

astro-ph.GA↗

arXiv2024

Characterisation of local halo building blocks: Thamnos and Sequoia

A crucial aspect of galaxy evolution is the pace at which galaxies build up their mass. We investigate this hierarchical assembly by uncovering and timing accretion events experienced by our Galaxy. In the Milky Way, accreted debris has been identified in the local halo, thanks to Gaia. We combine this dataset with advances in colour-magnitude diagram (CMD) fitting to characterise the Galaxy's building blocks based on their age and metallicity distributions. Here, we focus on the retrograde halo, specifically Thamnos and Sequoia. This study, part of the ChronoGal project, uses CMDft.Gaia to fit absolute CMDs of stars from these sub-structures, extracted from a local 5D Gaia DR3 dataset. By comparing their age and metallicity distributions with expected contamination from Gaia Enceladus (GE) and low-energy (LE) in situ populations, we identify distinct stellar population signatures for Sequoia and Thamnos. Both have metal-poor populations ([Fe/H] -2.5 to -1.5 dex) distinct from contamination. Their age distributions reveal the build-up pace of their progenitors: half of Sequoia's stars formed by 12 Gyr ago, while Thamnos appears slightly older and declines faster, forming half its stars by 12.3 Gyr. GE and LE populations formed half their stars by 12.1 Gyr and 12.9 Gyr, respectively. Caution is needed interpreting these distributions, especially for Sequoia, due to small sample sizes that can shift ages younger by up to 1 Gyr. Nonetheless, accounting for this and residual contamination, we conclude Thamnos, Gaia Enceladus, and Sequoia are predominantly old and were accreted within 1-2 Gyr of each other. We present, for the first time, age distributions for the retrograde halo sub-structures Sequoia and Thamnos, derived from photometric data using CMD fitting that also yields metallicity distributions consistent with spectroscopy.

astro-ph.GA↗

arXiv2024

First measurement of the triaxiality of the inner dark matter halo of the Milky Way

Stellar streams are particularly sensitive probes of the mass distribution of galaxies. In this work, we focus on the Helmi Streams, the remnants of an accreted dwarf galaxy orbiting the inner Milky Way. We examine in depth their peculiar dynamical properties, and use these to provide tight constraints on the Galactic potential, and specifically on its dark matter halo in the inner 20 kpc. We extract 6D phase-space information for the Helmi Streams from Gaia DR3, and confirm that the Streams split up into two clumps in angular momentum space, and that these depict different degrees of phase-mixing. To explain these characteristics we explore a range of Galactic potential models with a triaxial NFW halo, further constrained by rotation curve data. We find that a Galactic potential with a mildly triaxial dark matter halo, having $p=1.013^{+0.006}_{-0.006}$, $q=1.204^{+0.032}_{-0.036}$, $M_{\rm{discs}}=4.65^{+0.47}_{-0.57}\cdot10^{10} M_{\odot}$ and $M_{\rm{DM}}(< 15 \text{kpc})=1.14^{+0.11}_{-0.10}\cdot10^{11} M_{\odot}$, is required to form two clumps in angular momentum space over time. Their formation is driven by the fact that the clumps are on different orbital families and close to an orbital resonance. This resonance also explains the different degrees of mixing observed, as well as the presence of a dynamically cold subclump (also known as S2). This first and very precise measurement of the triaxiality of the inner dark matter halo of the Galaxy uniquely reveals the high sensitivity of phase-mixed streams to the exact form of the gravitational potential.

astro-ph.GA↗

arXiv2024

Testing MOND using the dynamics of nearby stellar streams

The stellar halo of the Milky Way is built up, at least in part, from debris from past mergers. Stars from such merger events define substructures in phase-space, for example in the form of streams, which are groups of stars moving on similar trajectories. The nearby Helmi streams discovered more than two decades ago are a well-known example. Using 6D phase-space information from the Gaia space mission, Dodd et al. (2022) have recently reported that the Helmi streams are split into two clumps in angular momentum space. Such substructure can be explained and sustained in time if the dark matter halo of the Milky Way takes a prolate shape in the region probed by the orbits of the stars in the streams. Here, we explore the behaviour of the two clumps identified in the Helmi streams in a Modified Newtonian Dynamics (MOND) framework to test this alternative model of gravity. We perform orbit integrations of Helmi streams member stars in a simplified MOND model of the Milky Way and using the more sophisticated Phantom of RAMSES simulation framework. We find with both approaches that the two Helmi streams clumps do not retain their identity and dissolve after merely 100 Myr. This extremely short timescale would render the detection of two separate clumps as very unlikely in MONDian gravity. The observational constraints provided by the streams, which MOND fails to reproduce in its current formulation, could potentially also be used to test other alternative gravity models.

astro-ph.GA↗

arXiv2024

Constrained cosmological simulations of the Local Group using Bayesian hierarchical field-level inference

We present a novel approach based on Bayesian field-level inference capable of resolving individual galaxies within the Local Group (LG), enabling detailed studies of its structure and formation via posterior simulations. We extend the Bayesian Origin Reconstruction from Galaxies (BORG) algorithm with a multi-resolution approach, allowing us to reach smaller mass scales and apply observational constraints based on LG galaxies. Our updated data model simultaneously accounts for observations of mass tracers within the dark haloes of the Milky Way (MW) and M31, their observed separation and relative velocity, and the quiet surrounding Hubble flow represented through the positions and velocities of galaxies at distances from one to four Mpc. Our approach delivers representative posterior samples of $\Lambda$CDM realisations that are statistically and simultaneously consistent with all these observations, leading to significantly tighter mass constraints than found if the individual datasets are considered separately. In particular, we estimate the virial masses of the MW and M31 to be $\log_{10}(M_{200c}/M_\odot) = 12.07\pm0.08$ and $12.33\pm0.10$, respectively, their sum to be $\log_{10}(\Sigma M_{200c}/M_\odot)= 12.52\pm0.07$, and the enclosed mass within spheres of radius $R$ to be $\log_{10}(M(R)/M_\odot)= 12.71\pm0.06$ and $12.96\pm0.08$ for $R=1$ Mpc and 3 Mpc, respectively. The M31-MW orbit is nearly radial for most of our $\Lambda$CDM LG's, and most lie in a dark matter sheet that aligns approximately with the Supergalactic Plane, even though the surrounding density field was not used explicitly as a constraint. The approximate simulations employed in our inference are accurately reproduced by high-fidelity structure formation simulations, demonstrating the potential for future high-resolution, full-physics $\Lambda$CDM posterior simulations of LG look-alikes.

astro-ph.GA↗

arXiv2024

On the Galactic rotation curve inferred from the Jeans equations Assessing its robustness using Gaia DR3 and cosmological simulations

Several works have recently applied Jeans modelling to Gaia-based datasets to infer the circular velocity curve for the Milky Way. Such works have consistently found evidence for a continuous decline in the rotation curve beyond $\sim$15kpc possibly indicative of a light dark matter halo. We used Gaia DR3 RVS data, supplemented with Bayesian distances to determine the radial variation of the second moments of the velocity distribution for stars close to the Galactic plane. We have used these profiles to determine the rotation curve using the Jeans equations under the assumption of axisymmetry and explored how they vary with azimuth and above and below the Galactic disk plane. We have applied the same methodology to an N-body simulation of a Milky Way-like galaxy impacted by a satellite akin the Sagittarius dwarf and to the Auriga suite of cosmological simulations. We reveal evidence of disequilibrium and deviations from axisymmetry closer in. We find that the second moment of $V_R$ flattens out at $R \gtrsim 12.5$kpc, and that the second moment of $V_{\phi}$ is different above and below the plane for $R \gtrsim 11$kpc. The simulations indicate that these features are typical of galaxies that have been perturbed by external satellites. They also suggest that the difference between the true circular velocity curve and that inferred from Jeans equations can be as high as 15$\%$, but is likely of order 10$\%$ for the Milky Way. This is of larger amplitude than the systematics inherent to Jeans equations. However, if the density of the tracer population were truncated at large radii, the erroneous conclusion of a steeply declining rotation curve can be reached. We find that steady-state axisymmetric Jeans modelling becomes less robust at large radii, indicating that particular caution is needed when interpreting the rotation curve inferred in those regions.

astro-ph.GA↗

arXiv2024

Filling in the Blanks: A Method to Infer the Substructure Membership and Dynamics of 5D Stars

We present and test a method to infer a probability density function (PDF) for the missing vlos of a star with 5D information within $2.5$ kpc. We use stars from the Gaia DR3 RVS catalogue to describe the local orbital structure in action space. This technique also allows us to infer the probability that a 5D star is associated with the Milky Way's stellar Disc or the stellar Halo, which can be further decomposed into known stellar substructures. The method is tested on a 6D Gaia DR3 RVS sample and a 6D Gaia sample crossmatched to groundbased spectroscopic surveys, stripped of their true vlos. The stars predicted vlos, membership probabilities, and inferred structure properties are then compared to the true 6D equivalents, allowing the method's accuracy and limitations to be studied in detail. Our predicted vlos PDFs are statistically consistent with the true vlos, with accurate uncertainties. We find that the vlos of Disc stars can be well constrained, with a median uncertainty of 26 kms. Halo stars are typically less well constrained with a median uncertainty of 72 kms, but those found likely to belong to Halo substructures can be better constrained. The dynamical properties of the total sample and subgroups, such as distributions of integrals of motion and velocities, are also accurately recovered. The group membership probabilities are statistically consistent with our initial labelling, allowing high quality sets to be selected from 5D samples by choosing a trade off between higher expected purity and decreasing expected completeness.

astro-ph.GA↗