arXiv subjects

Hanneke C. Woudenberg

Publications and source records attributed to Hanneke C. Woudenberg.

10 recordsLinked to original sources

arXiv2026

The Pristine survey XXXI. A stream offset from the Stream: very metal-poor stellar debris trailing 100$^{\circ}$ from the LMC-SMC system

Context: The stellar counterpart to the $200^\circ$-long HI Stream from the Clouds probes their tidally stripped stellar populations and constrains their interaction history and structure before infall into the Milky Way. Aims: We investigate the chemodynamics and possible origin of very metal-poor (VMP; [Fe/H] $<-2$) stars in the stellar Stream, targeting distant red giants selected from the Pristine--Gaia synthetic metallicity catalogue. Methods: We obtained high-resolution Gemini/GHOST spectroscopy of 33 candidate members, deriving radial velocities and metallicities. We combine these data with literature red giant members and assess their association with the Clouds using orbital integrations in a time-dependent Milky Way--LMC potential. We also compare their distribution with collisionless test particles initially placed in a diffuse LMC stellar halo. Results: We identify 12 high-confidence members based on energy and angular momentum: one intermediate metal-poor star ([Fe/H] $\sim-1.6$), ten VMP stars, and one extremely metal-poor (EMP) star ([Fe/H] $<-3$). The new VMP stars trace over $100^\circ$ of the trailing Stream, follow the Clouds' kinematics, and lie $\sim20^\circ$ from both the HI Stream and previously known, more metal-rich debris, suggesting a metallicity-dependent spatial distribution. Combined with the literature, they approximately double the known red giant sample and extend its metallicity range from [Fe/H] $\sim-0.5$ to $-4.3$. Orbital integrations support an association with the Clouds, while test particles reproduce the debris' broad spatial distribution. Conclusions: VMP and EMP stars contribute to the Clouds' tidal debris. They may originate in the extended stellar halos of the LMC or SMC, or in disrupted satellites. Their spatial and kinematic properties can constrain future simulations of the Milky Way--LMC--SMC interaction.

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arXiv2026

The Pristine Survey. XXX. Mapping substructures in the inner and outer halo with blue horizontal branch stars

Blue horizontal branch (BHB) stars serve as good standard candles and kinematic tracers of the Milky Way halo. The halo displays various overdensities and kinematical substructures of different shapes and sizes. Our objective is to investigate the substructure in three dimensions and the bulk motion of the halo up to distances well exceeding the limit of the Gaia parallaxes. We selected BHB stars in the $\left(u-CaHK\right) _ 0$-$(g-r) _ 0$ colour-colour diagram with a hierarchical Bayesian mixture model of three populations: BHB stars, blue straggler stars, and outliers. Using $CaHK$ photometry derived from Gaia XP spectra and the Pristine survey, we provide both a shallow and a deep catalogue of BHB stars of 7,057 and 8,824 stars, respectively, where the deep catalogue is restricted to the deeper Pristine footprint compared to the shallower with Gaia XP. Using the shallow catalogue, we trace to larger distances 6D substructures identified earlier within a 2.5 kpc from the Sun, including accepted populations; namely, Gaia-Enceladus, Thamnos, Helmi streams, Sequoia, and other smaller halo substructures such as ED-3, ED-4, and ED-6. The deep catalogue provides reliable distance measurements for a more detailed 3D mapping of the Sagittarius stream and of the Pisces Overdensity. We add new candidate BHB stars with line-of-sight velocities to existing catalogues and confirm previously reported differences between the northern and southern Galactic hemispheres, likely due to the infall of the Large Magellanic Cloud. This research demonstrates that probabilistic photometric classification can effectively identify distant candidate BHB stars, and can provide catalogues of these stars that enhance our understanding of the 3D and 6D structures in the halo of the Milky Way.

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arXiv2026

Understanding the formation of alpha-Fe bimodality in the Milky Way using constrained cosmological simulations of the Local Group

We use a novel suite of constrained cosmological simulations of the Local Group to investigate the formation of alpha--Fe bimodalities in stellar discs of Milky Way analogues. In agreement with past studies using the Auriga model, we show that in our simulations the high-alpha sequence forms in a merger-induced starburst, which further quenches star formation, allowing the new stellar populations to form at low [alpha/Fe]. Producing a bimodality that is prominent at large radii, with a significant [alpha/Fe] gap and overlapping metallicity as in the Milky Way, requires the quenching of star formation across all radii where the bimodality is observed. The quenching can be achieved either with a galaxy-wide starburst, or with a retrograde merger reducing angular momentum in the outer disc and allowing more efficient dilution by the metal-poor gas in the circumgalactic medium (CGM). We show that mergers can also tilt the disc, changing the direction of galactic outflows and enriching a larger volume of the CGM with galactic metals, leading to low [alpha/Fe] scatter in the subsequently formed low-alpha sequence. We argue that the slopes in [alpha/Fe] of the two alpha sequences are crucial in narrowing down the mechanism that leads to their formation. We additionally uncover an indirect role that mergers play in enhancing the accretion of metal-poor gas onto galaxies. The smoothly accreted gas can dominate the gas content in the CGM and aid in the formation of distinct chemical sequences, especially at the low-metallicity end.

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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.

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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.

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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 [α/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 [α/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

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.

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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↗

arXiv2023

Swarming in stellar streams: Unveiling the structure of the Jhelum stream with ant colony-inspired computation

The halo of the Milky Way galaxy hosts multiple dynamically coherent substructures known as stellar streams that are remnants of tidally disrupted systems such as globular clusters (GCs) and dwarf galaxies (DGs). A particular case is that of the Jhelum stream, which is known for its complex morphology. Using the available data from Gaia DR3, we extracted a region on the sky that contains Jhelum. We then applied the novel Locally Aligned Ant Technique (LAAT) on the position and proper motion space of stars belonging to the selected region to highlight the stars that are closely aligned with a local manifold in the data and the stars belonging to regions of high local density. We find that the overdensity representing the stream in proper motion space is composed of two components, and show the correspondence of these two signals to the previously reported narrow and broad spatial components of Jhelum. We made use of the radial velocity measurements provided by the $S^5$ survey to confirm, for the first time, a separation between the two components in radial velocity. We show that the narrow and broad components have velocity dispersions of $4.84^{+1.23}_{-0.79}$~km/s and $19.49^{+2.19}_{-1.84}$~km/s, and metallicity dispersions of $0.15^{+0.18}_{-0.10}$ and $0.34^{+0.13}_{-0.09}$, respectively. These measurements, and the difference in component widths, could be explained with a scenario where Jhelum is the remnant of a GC embedded within a DG that were accreted onto the Milky Way during their infall. Although the properties of Jhelum can be explained with this merger scenario, other progenitors of the narrow component remain possible such as a nuclear star cluster or a DG. To rule these possibilities out, we would need more observational data of member stars of the stream. Our analysis highlights the importance of the internal structure of streams with regards to their formation history.

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arXiv2022

Characterization and dynamics of the peculiar stream Jhelum, A tentative role for the Sagittarius dwarf galaxy

Stellar streams are a promising tool to study the Milky Way's dark matter subhalo population, as interactions with subhalos could produce substructure in streams. However, other possible causes for substructure first need to be well understood. Here we studied the kinematics and the unusual morphology of the stellar stream Jhelum. Using a combination of ground-based photometry and Gaia EDR3 astrometry, we characterized the morphology of Jhelum. We combined this new data with radial velocities from the literature to perform orbit integrations of the stream in static Galactic potentials. We also carried out N-body simulations in the presence of the Sagittarius dwarf galaxy. The new data reveal a previously unreported tertiary component in the stream, as well as several gaps and a kink-like feature in its narrow component. We find that for a range of realistic Galactic potentials, no single orbit is able to reproduce Jhelum's radial velocity data entirely. A generic property of the orbital solutions is that they share a similar orbital plane to Sagittarius and this leads to repeated encounters with the stream. Using N-body simulations that include a massive Sagittarius, we explored its effect on Jhelum, and we show that these encounters can qualitatively reproduce the narrow and broad components in Jhelum, as well as create a tertiary component in some cases. We also find evidence that such encounters can result in an apparent increase in the velocity dispersion of the stream by a factor up to four due to overlapping narrow and broad components. Our findings suggest that the Jhelum stream is even more complex than once thought; however, its morphology and kinematics can tentatively be explained via the interactions with Sagittarius. In this scenario, the formation of Jhelum's narrow and broad components occurs naturally, yet some of the smaller gap-like features remain to be explained.

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