arXiv subjects

Guillaume F. Thomas

Publications and source records attributed to Guillaume F. Thomas.

At least 19 recordsLinked to original sources

arXiv2026

A direct measurement of the tilting rate of the Milky Way disc

The Milky Way disc is typically described, at lowest order, as a flattened rotating stellar component with a fixed spin axis. However, interactions and mergers with smaller galaxies are expected to have an impact on the disc properties. Such interactions can leave long-lasting signatures within galactic discs, including warps and corrugation waves that have already been detected in the Milky Way, but also a slow reorientation of the disc known as tilting, which has never been directly measured up to now. Here, using the vertical proper motions of 110 million stars from the Gaia DR3 catalogue, we present the first-ever direct measurement of the tilting rate of our Galaxy's disc. The detected signal of 14 +/- 2 deg/Gyr around the Sun-Galactic Centre axis is statistically significant and is consistent with typical values found in cosmological simulations. By comparing our measurements with high-resolution tailored simulations, we infer that the observed tilting rate is likely the long-term consequence of a major accretion event that occurred around 8-10 billion years ago. The signal is consistent with the merger of a massive satellite galaxy on a retrograde orbit with a ratio of about 1:4, probably associated with the Gaia-Enceladus/Sausage event. These results provide independent evidence for a key episode in the Milky Way's formation history and introduce a new way to reconstruct the assembly histories of disc galaxies from their present-day motions.

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arXiv2026

Reconstructing the orbits of Milky Way dwarf galaxies: A Large Magellanic Cloud perspective

The orbital histories of the dwarf satellites of the Milky Way (MW) are key to understanding their evolution and placing their present-day properties in a dynamical context. We present the results of the orbit integration of 72 dwarfs in the vicinity of the MW, based on accurate 6D phase-space coordinates from the literature and a suite of six realistic, time-evolving gravitational potentials that account for the mutual interaction of the MW and the Large Magellanic Cloud (LMC). We provide the largest catalogue of orbital parameters for MW dwarfs to date, in terms of both galaxy sample size and range of potentials explored. We also assess the binding status of the dwarfs and estimate their infall times, finding that the majority of them have spent the last 5 Gyr within the MW virial radius. From the reconstructed orbits, we identify ten likely LMC satellites, several of which have experienced very close passages within the LMC stellar disc. For the Small Magellanic Cloud (SMC), we find that its most recent pericentre about the LMC ($\sim$8 kpc, $\sim$170 Myr ago) is consistent with predictions from the direct collision scenario proposed to explain the LMC's offset and tilted bar. We also note a broad temporal coincidence between previous SMC pericentres and star formation rate peaks reported in both Magellanic Clouds, suggesting a causal connection. Finally, we identify Grus II and Tucana IV as possible MW satellites recently captured by the LMC, based on their pronounced orbital deflections and velocities relative to the LMC.

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arXiv2026

Characterizing the disruption of Böotes III: a missing link in the Galactic halo?

The Böotes III (Boo3) dwarf galaxy has long been suspected of being the progenitor of Styx, a ~50°-long stellar stream that was simultaneously discovered in the same region of sky. Boo3's diffuse morphology, large velocity dispersion, small pericenter, and excess of candidate stars at large radii suggest it is undergoing active tidal disruption. A link to Styx is therefore logical; however, a clear connection between these structures has not yet been clearly demonstrated. Here, we re-examine the Boo3-Styx association by searching for Boo3's tidal debris using a combination of Gaia-selected members, new CaHK narrow-band imaging with CFHT/MegaCam, and stellar tracer catalogues of blue horizontal branch and red giant branch stars. We also conduct a broad search for a putative stream using matched filter techniques applied to SDSS DR17 and DELVE DR2. Despite our extensive search, we find no observational evidence directly linking Boo3 to Styx. Furthermore, our results suggest that either Boo3's extended substructure is too diffuse to be detected with current data, or that its particular orbit may have erased a coherent tidal signature. Boo3 thus remains an enigmatic system, and exemplifies the need for spectroscopic follow-up to properly disentangle the nature between this faint Milky Way satellite and nearby stream.

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arXiv2026

Tracing the very early disruption of the Sagittarius dwarf galaxy in the distant Milky Way halo

Current models predict that at distances beyond 80 kpc in the Milky Way halo, we can find the earliest escaped stars from the merging Sagittarius dwarf galaxy. However, observational data on the Sagittarius stream at these distances is limited. This study examines an overdensity of red giant branch (RGB) stars potentially linked to Sagittarius merger debris. Using the Magellan Inamori Kyocera Echelle spectrograph of Las Campanas Observatory's Clay Telescope, we measured the radial velocities and metallicities of these stars. We compared their properties with model predictions of Sagittarius' disruption and other stellar tracers from the Dark Energy Spectroscopic Instrument Data Release 1 and RR Lyrae catalogs. Our spectral analysis confirms the significant tight clustering of four of these RGBs in full 6D phase space. This tight clump is embedded within a larger spur-like feature of the Sagittarius stream in the southern sky. A comparison with Sagittarius stream models further strengthens this hypothesis and shows that this far spur could be composed of stars originally in the halo of the Sagittarius dwarf galaxy, stripped in the earliest phases of the interaction. The metallicity dispersion of the four stars of $0.15 ^ {+0.17} _ {-0.08}$ around the average of [Fe/H] = $-1.46 ^ {+0.11} _ {-0.09}$ is very low. This study provides the first spectroscopic view of the distant southern spur of Sagittarius, composed of stars likely stripped from Sagittarius's halo.

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arXiv2026

Constraining the Galactic bar using the M92 stellar stream

Stellar streams are excellent probes of the gravitational potential in which they evolve. In the Milky Way (MW), globular cluster (GC) streams are routinely used to infer properties about time-dependent perturbations of the underlying potential. This implies that streams with Galactocentric radii small enough to be perturbed by the MW bar should offer constraints on it, such as its pattern speed, which currently has a wide range of values reported in the literature and is important when studying stellar kinematics. The GC M92 has a small pericentre and should be affected by the bar. It has a diffuse stellar stream, but confirming stream members has previously been hindered by a lack of spectroscopic data. In this paper, we use Dark Energy Spectroscopic Instrument (DESI) observations together with photometric and astrometric data to obtain spectroscopic members of the M92 stream for the first time. We identify a clear spatial distribution and gradients in distance moduli, proper motions, and radial velocities that confirm the stream's existence. We compare the observed stream to mock streams generated in different barred potentials and estimate the MW bar's pattern speed $Ω= 29.1^{+0.7}_{-0.4}$ km s$^{-1}$ kpc$^{-1}$ and $\dot Ω= 0.7^{+3.5}_{-2.3}$ km s$^{-1}$ kpc$^{-1}$ Gyr$^{-1}$. This is the first time a stellar stream is used to probabilistically infer these bar properties, and it opens up an exciting realm of inner Galactic potential characterisation using stellar streams.

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arXiv2026

Systematic census of RR Lyrae stars in Milky Way stellar streams

Context. Nearly 150 tidal streams are known in the Milky Way, but full phase-space information exists for only a few. RR Lyrae stars (RRL), as standard candles, provide a powerful way to probe these structures, yet they have been identified in less than a dozen streams. Aims. We study the RRL population in all known stellar streams with reported proper motions in the galstreams library, performing the first systematic census of these stars. Our goals are to identify likely RRL members, map distances along streams, and compare RRL populations in streams and their progenitors. Method. We use a union of the largest RRL catalogs (Gaia DR3 SOS, PS1, and ASAS-SN-II) to construct a Bayesian probabilistic membership model and find 361 RRL in the 56 streams studied. Results. i) We find that 32 of the 56 streams have RRL in their tidal tails -- 13 with progenitors and 19 without; 13 of these contain more than 3 RRL in their tails. ii) We report new RRL detections in 31 of these streams, anchoring distances and, in particular, inferring new distance gradients for 5 of them. iii) Our method provides intrinsic dispersion estimates in distance and proper motion for each track and statistically quantifies the expected contamination. iv) The census revealed some complex origin histories, such as the new plausible origin scenario we propose for M92 with multiple progenitors. v) We find that the presence of RRL in the tidal tails is linked to the late stages of progenitor dissolution. Conclusions. This census represents a first step toward identifying which of the studied stellar streams contain a significant number of RRL based on currently reported tracks while also providing a homogeneous and robust catalog of RRL members with precise empirical distances, crucial for a full phase-space analysis of these structures and their use as probes of the Galaxy's history and gravitational potential.

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arXiv2026

Estimating the dynamical masses of dwarf galaxies in the presence of binary-star contamination

Ultra-faint dwarf galaxies (UFDs) show extreme dynamical mass-to-light ratios of approximately 100-5000 in solar units within the half-light radius, making them critical tests for cosmological models. However, it is a concern whether the line-of-sight (l.o.s.) velocity component of the orbital motion of undetected binary stars is significantly inflating the observed l.o.s. velocity dispersions and, consequently, UFDs dynamical mass estimates. We correct the current estimates of these quantities for UFDs to account for the presence of undetected binaries with single-epoch data. We use the latest binary population models in the solar neighborhood to compute the expected velocity distribution of binary stars. We then convolve this distribution with a Gaussian to model the l.o.s. velocity distribution of UFDs in a mixture model, in which the binary fraction is a free parameter. We apply this methodology to observed UFDs whose dynamical masses are potentially inflated by binaries. In order to generalize to the multi-epoch data case, we compute the velocity distribution of undetected binaries by applying the same cuts to the models as one would apply to the observed data to remove binaries. We find that estimated dynamical masses of UFDs decrease by a factor of 1.5 to 3 once undetected binaries are accounted for. These corrections significantly affect considerations about DM models based on these systems, even challenging the classification of Leo IV, Unions I and Sagittarius II as galaxies. We find that a dedicated multi-epoch campaign spanning one year could substantially mitigate the impact of binaries. Finally, we find that the expected level of binary-star contamination in DM halo density profile inferences from dynamical models of classical dwarf spheroidal galaxies is negligible.

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arXiv2026

HR-GO II: chemical abundances of low-$E$ retrograde dynamically-tagged-groups: Revealing Thamnos as a very metal-poor substructure

Milky Way halo substructures identified in dynamical space are known to suffer from contamination from the Milky Way in-situ stars, which makes their accreted origins uncertain. We present detailed chemical abundances of 35 stars belonging to two sets of dynamically tagged groups, Rg8 and Rg9, to investigate their accreted nature. Both groups are composed of stars with low orbital energy and very retrograde orbits. We find that Rg8 and Rg9 are chemically indistinguishable across all elements, from C to Eu, strongly indicating that they belong to the same structure. The iron-abundance distribution of this low-$E$ retrograde group has a prominent peak at [Fe/H] $\approx-2.1$, revealing that its main population is very metal-poor, and a secondary peak at [Fe/H] $\approx-1.5$, very likely due to contamination from Milky Way in-situ stars. These groups also heavily overlap with the Thamnos substructure in dynamical space, and we thus use them to investigate the chemical properties of Thamnos. The dominant, low-metallicity population provides strong evidence for the ex-situ origin of Thamnos, as well as its very metal-poor nature. We do not see any evidence of an $α$ knee in our sample, which is consistent with previous studies. Comparison with the Cetus-Palca stream in the chemical space shows similar abundance distributions, and thus it suggests that the Thamnos progenitor dwarf galaxy had a truncated star formation history due to its early merger with the Milky Way.

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arXiv2025

GALATEA: The 15-m Galactic Archaeology Spectroscopic Surveyor

\textsc{GALATEA} (the \emph{Galactic Archaeology and Local-group Astrophysics Telescope for Extended Areas}) is a concept for a dedicated 15-m, wide-field, 10,000-fibre spectroscopic survey facility in the northern hemisphere, optimized for degree-scale, multi-object spectroscopy. With a $\sim 1~\mathrm{deg}^2$ corrected field-of-view and both medium- ($R \simeq 5{,}000$--$10{,}000$) and high-resolution ($R \simeq 20{,}000$--$25{,}000$) modes, \textsc{GALATEA} would open a new regime in Galactic and Local Group astronomy: deep, chemically detailed spectroscopy of vast samples of individual stars in the outer disc, warp, flare, halo substructures, M31, M33 and their dwarf satellites, far beyond the reach of current surveys. By delivering precise radial velocities and detailed chemical abundances for stars with exquisite astrometry and photometry from \emph{Gaia} and its proposed near-infrared successor \emph{GaiaNIR}, \textsc{GALATEA} will complete and fully exploit the 6D phase-space and chemodynamical information for these populations. Compared to existing northern multi-object spectroscopic facilities (BOSS, APOGEE, DESI, LAMOST, WEAVE, PFS), \textsc{GALATEA} delivers an order-of-magnitude jump in survey power ($\propto D^{2} \times N_{\mathrm{fibres}}$) by combining a 15-m aperture, $\sim 1~\mathrm{deg}^2$ field, and 10{,}000 fibres in a single dedicated facility. It is also strongly complementary to 30--40\,m ELTs: GALATEA provides the wide-field, high-multiplex discovery and chemodynamical mapping, while ELTs deliver deep, high-resolution follow-up of the faintest or most complex targets.

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arXiv2025

Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: Galactic Archaeology from the Northern Sky

By the 2040s--50s, facilities such as \emph{Gaia}, WEAVE, 4MOST, Rubin, \emph{Euclid}, \emph{Roman}, and the ESO ELT will have transformed our global view of the Milky Way. Yet key questions will remain incompletely resolved: a detailed reconstruction of the Galaxy's assembly from its earliest building blocks, and robust tests of dark matter granularity using the fine structure of the stellar halo and outer disk -- particularly in the Galactic anticenter. Addressing these questions requires high-resolution spectroscopy of faint main-sequence stars (typically 1--2 mag below the turnoff) and turnoff stars ($r \sim 21$--23) in low-surface-brightness structures: halo streams and shells, ultra-faint dwarf galaxies, the warped and flared outer disk, and anticenter substructures. We argue that addressing this science case requires a 30\,m-class telescope in the northern hemisphere, equipped with wide-field, highly multiplexed, high-resolution spectroscopic capabilities. Such a facility would enable (i) a Northern Halo Deep Survey of $\sim 10^{5}$--$10^{6}$ faint main-sequence and turnoff stars out to $\sim 150$--200\,kpc, (ii) chemodynamical mapping of dozens of streams to measure perturbations from dark matter subhalos, and (iii) tomographic studies of the anticenter and outer disk to disentangle perturbed disk material from accreted debris. A northern 30\,m telescope would provide the essential complement to ESO's southern ELT, enabling genuinely all-sky Milky Way archaeology and delivering stringent constraints on the small-scale structure of dark matter.

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arXiv2025

Asymmetries in stellar streams induced by a galactic merger

Stellar streams are sensitive to perturbations from, e.g., giant molecular clouds, bars and spiral arms, infalling dwarf galaxies, or globular clusters which can imprint gaps, clumps, spurs, and asymmetries in tails. In addition to these effects, the impact of a galactic major merger on a population of stellar streams remains to be explored. Here, we focus on the emergence and longevity of asymmetries between the leading and trailing tails of streams caused by such interactions. We run collisionless N-body simulations of a Milky Way-like galaxy hosting 36 globular cluster streams and merging with a perturber galaxy. We propose a new asymmetry metric to quantify the structural differences between both tails from their respective cumulative density profiles. We find that the over- and under-densities along streams induced by the merger depend on the orbital characteristics of their progenitors. The non-simultaneity of this effect from stream to stream implies that global asymmetry signatures are less prominent than in individual cases. These population-averaged imprints remain detectable over only 2.5 Gyr but asymmetric signatures can persist over much longer periods for individual streams with wide orbits that have been perturbed prior to coalescence. We thus caution that the interpretation of streams' morphology in the context of dark matter mapping is strongly subject to degeneracies and should be performed considering the merger history of the host.

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arXiv2025

Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal): IV. On the inner Milky Way stellar age distribution

The Milky Way's inner region is dominated by a stellar bar and a boxy-peanut shaped bulge. However, which stellar populations inhabit the inner Galaxy or how star formation proceeded there is still unknown. The difficulty in studying these stars stems from their location in dense regions that are strongly impacted by extinction and crowding effects. In this work, we use star formation histories computed in the solar neighbourhood using Gaia Colour-Magnitude Diagram fitting to shed light onto the evolution of the central regions of our Galaxy. For that, we have obtained precise age distributions for the non-negligible amount of super metal-rich stars ([M/H] $\sim$ 0.5) in the solar neighbourhood (more than 5$\%$ of the total stars within 400 pc of the plane). Assuming that these stars were born in the inner Galaxy and migrated outwards, those distributions should be indicative of the true stellar age distribution in the inner Galaxy. Surprisingly, we find that these age distributions are not continuous but show clear signs of episodic star formation ($\sim$~13.5, 10.0, 7.0, 4.0, 2.0 and less than 1~Gyr ago). Interestingly, with the exception of the 4~Gyr event, the timings of the detected events coincide with the formation of the primitive Milky Way and with known merging events or satellite encounters (Gaia-Enceladus-Sausage, Sagittarius dwarf galaxy, and the Magellanic Clouds), suggesting that these could have induced enhanced and global star-forming episodes. These results are compatible with a scenario in which Gaia-Enceladus-Sausage is responsible for the formation of the bar 10 Gyr ago. However, we cannot associate any accretion counterpart with the 4-Gyr-ago event, leaving room for a late formation of the bar, as previously proposed. A qualitative comparison with the Auriga Superstars simulations suggesting a possible link to bar dynamics and satellite accretion. [Abridged]

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arXiv2025

Chemo-dynamics of the stellar component of the Sculptor dwarf galaxy II: dynamical properties and dark matter halo density

Dwarf galaxies satellite of the Milky Way are excellent laboratories for testing dark matter (DM) models and baryonic feedback implementation in simulations. The Sculptor 'classical' dwarf spheroidal galaxy, a system with two distinct stellar populations and high-quality data, offers a remarkable opportunity to study DM distributions in these galaxies. In this work, we infer the DM halo density distribution of Sculptor, applying a method based on spherically symmetric distribution functions depending on actions to fit the stellar structural and kinematic properties of Sculptor. The galaxy is represented via four components: two distinct stellar populations based on distribution functions, tracers within a fixed and dominant DM potential, plus the contribution of a third stellar component that accounts for possible sources of contamination. The model-data comparison accounts for the kinematics and metallicities of individual stars, allowing us to assign probabilities of membership to each star. The modeling is applied on the largest available set of spectroscopic data, which have not been previously analyzed with this objective. We find the DM distribution of Sculptor to have a logarithmic inner slope of 0.39+0.23-0.26 and a scale radius of 0.79+0.38-0.17 kpc at 1 sigma confidence level. Our results show that Sculptor DM density profile deviates from predictions of DM-only simulations at a 3 sigma level over a large range of radii. Our analysis suggests that the velocity distribution of Sculptor's two main stellar components is isotropic in the center and becomes radially anisotropic in the outskirts. Additionally, we provide predictions for the projected radial and tangential velocity dispersion profiles. We also present updated DM annihilation and decay J- and D-factors, finding J = 18.15+0.11-0.12 and D = 18.07+0.10-0.10 for an angular aperture of 0.5 degrees.

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arXiv2025

The Backup Program of the Dark Energy Spectroscopic Instrument's Milky Way Survey

The Milky Way Backup Program (MWBP), a survey currently underway with the Dark Energy Spectroscopic Instrument (DESI) on the Nicholas U. Mayall 4-m Telescope, works at the margins of the DESI Main surveys to obtain spectra of millions of additional stars from the Gaia catalog. Efficiently utilizing twilight times (<18 deg) and poor weather conditions, the MWBP extends the range of stellar sources studied to both brighter magnitudes and lower Galactic latitude and declination than the stars studied in DESI's Main Milky Way Survey. While the MWBP prioritizes candidate giant stars selected from the Gaia catalog (using color and parallax criteria), it also includes an unbiased sample of bright stars (i.e., 11.2 < G < 16 mag) as well as fainter sources (to G < 19 mag). As of March 1, 2025, the survey had obtained spectra of ~7 million stars, approximately 1.2 million of which are included in the DESI Data Release 1. The full survey, when completed, will cover an area of more than 21,000 square degrees and include approximately 10 million Gaia sources, roughly equal to the number of stellar spectra obtained through the DESI Main Survey, while only utilizing <9% of all DESI observing time.

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arXiv2025

The Pristine survey: XXV. The very metal-poor Galaxy: Chemodynamics through the follow-up of the Pristine-Gaia synthetic catalogue

The Pristine-\textit{Gaia} synthetic catalogue provides reliable photometric metallicities for $\sim$30 million FGK stars using the Pristine survey model and Gaia XP spectra. We perform the first low-to-medium-resolution spectroscopic follow-up of bright (G<15) and distant (up to 35 kpc) very and extremely metal-poor (V/EMP, [Fe/H]<-2.5) red giant branch stars from this. We use Isaac Newton Telescope/Intermediate Dispersion Spectrograph (INT/IDS) observations centred around the calcium triplet region ideal for V/EMP stars. We find that 76\% of our stars indeed have [Fe/H]<-2.5 with these inferred spectroscopic metallicities and only 3\% are outliers with [Fe/H] > -2.0. We report a success rate of 77\% and 38\% in finding stars with [Fe/H]<-2.5 and -3.0 respectively. This will allow for 10,000-20,000 homogeneously analysed EMP stars using the WEAVE survey follow-up of Pristine EMP candidates. We associate 20\%, 46\%, and 34\% of the stars to be confined to the disc plane, or to have inner and outer halo orbits, respectively. We also associate these V/EMP stars to known accretion events such as Gaia-Enceladus-Sausage (GES), LMS-1/Wukong, Thamnos, Helmi streams, Sagittarius, Sequoia, etc. For the stars that orbit close to the disc plane, we find that the prograde region with low vertical action is overdense with a significance of 4$σ$ as compared to its retrograde counterpart. We also find three new (brightest) members of the most metal-poor stellar stream, C-19, one of which is 50$^\circ$ away from the main body of the stream. Our measured mean metallicity, velocity dispersion, and stream width are consistent with the literature, but our results favour a higher distance ($\sim$21.5 kpc) for the stream. We publish a catalogue (and 1D spectra) of 215 V/EMP stars from this spectroscopic follow-up and showcase the power of chemokinematic analysis of V/EMP end.

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arXiv2025

How well can we unravel the accreted constituents of the Milky Way stellar halo? A test on cosmological hydrodynamical simulations

Context. One of the primary goals of Galactic Archaeology is to reconstruct the Milky Way's accretion history. To achieve this, significant efforts have been dedicated to identifying signatures of past accretion events. In particular, the study of integrals-of-motion (IoM) space has proven to be highly insightful for uncovering these ancient mergers and understanding their impact on the Galaxy's evolution. Aims. This paper evaluates the effectiveness of a state-of-the-art method for detecting debris from accreted galaxies, by testing it on four Milky Way-like galaxies from the Auriga suite of cosmological magneto-hydrodynamical simulations. Methods. We employ the innovative method from Lövdal et al. (2022) to identify substructures in the integrals-of-motion space within the local stellar halos of the four simulated galaxies. This approach enables us to evaluate the method's performance by comparing the properties of the identified clusters with the known populations of accreted galaxies in the simulations. Additionally, we investigate whether incorporating chemical abundances and stellar age information can help to link distinct structures originating from the same accretion event. Results. This method is very effective in detecting debris from accretion events that occur less than 6-7 Gyr ago but struggles to detect most of the debris from older accretion. Furthermore, most of the detected structures suffer from significant contamination by in-situ stars. Our results also show that the method may also generate artificial detections. Conclusions. Our work show that the Milky Way's accretion history remains uncertain, and question the reality of some detected structures in the Solar vicinity.

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arXiv2025

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

Stellar streams in the Milky Way are promising detectors of low-mass dark matter (DM) subhalos predicted by $Λ$CDM. 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 towards using stream perturbations to constrain the properties of dark perturbers. Here, we cross-match a \textit{Gaia} EDR3 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 6-D data. We demonstrate for the first time that the ultra-faint dwarf Segue 2 had a recent (77$\pm$5 Myr ago) close flyby (within the stream's 2$σ$ width) with the CPS. This interaction enables constraints on Segue 2's mass and density profile at larger radii ($\mathcal{O}(1)$ kpc) than are probed by its stars ($\mathcal{O}(10)$ pc). While Segue 2 is not expected to strongly affect the portion of the stream covered by our 6-D data, we predict that if Segue 2's mass within $\sim 6$ kpc is $5\times 10^9\,M_\odot$, the CPS's velocity dispersion will be $\sim 40$ km s$^{-1}$ larger at $ϕ_1>20^\circ$ than at $ϕ_1<0^\circ$. If no such heating is detected, Segue 2's mass cannot exceed $10^9\,M_\odot$ within $\sim 6$ 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.

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arXiv2024

Chemo-dynamics of the stellar component of the Sculptor dwarf galaxy I: observed properties

Aims. Recently, both the presence of multiple stellar chemo-kinematic components and rotation in the Sculptor dwarf spheroidal galaxy have been put into question. Therefore, we re-examine the chemo-kinematic properties of this galaxy making use of the best spectroscopic data-set available containing both line-of-sight velocities and metallicities of individual stars. Methods. We carry out a detailed, quantitative analysis on the recent spectroscopic data-set from Tolstoy et al. (2023) that contains high precision velocities and metallicities for 1339 members of Sculptor. In particular, we assess whether Sculptor is best represented by a single stellar population with a negative metallicity gradient or by the super-position of two or more components with different mean metallicity, spatial distribution and kinematic properties. For this analysis, we also include the incompleteness of the spectroscopic data-set. Results. We find that Sculptor is better described by a two-populations model than by a single-population model with a metallicity gradient. Moreover, given the assumptions of the current modeling, we find evidence of a third population, composed of few stars, that is more extended and metal-poor than the two other populations. This very metal-poor group of stars shows a shift of around 15 km/s in its average l.o.s. velocity (vlos) with respect to the rest of the galaxy. We discuss several possible origins for this new population, finding a minor merger as the most likely one. We also find a vlos gradient of 4.0 +1.5 -1.5 km s-1 deg-1 but its statistical evidence is inconclusive and, moreover, its detection is partially driven by the group of stars with off-set velocities.

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