arXiv subjects

Gustavo E. Medina

Publications and source records attributed to Gustavo E. Medina.

At least 19 recordsLinked to original sources

arXiv2026

Discovery, Characterization, and Potential Origins of a Stream in the Stellar Halo of Nearby LMC-Mass Galaxy NGC 55

We present a previously undetected stellar stream in the halo of the LMC-mass dwarf galaxy NGC 55 (2 Mpc), as part of an ongoing effort to characterize the stellar halos of LMC/SMC-mass dwarfs in the DEEP component of the DECam Local Volume Exploration (DELVE) survey. This structure is aligned with an outflow traced by H$α$ emission, a spur and cloud of neutral hydrogen, and the ultra-diffuse and possibly disrupting satellite galaxy NGC 55-dw1. We investigate possible ex-situ progenitor scenarios for this stream through the local luminosity-metallicity relation and a set of toy dynamical models that include gaseous, stellar, and dark matter components. If the stream is a product of a previous merger, the progenitor galaxy had an absolute magnitude $M_V \leq -7.2$ and a stellar mass $M_* \geq 10^5 M_{\odot}$ based on extrapolations of the detected stellar populations. If it is disrupted material from the ultra-diffuse satellite, their shared progenitor had an absolute magnitude $M_V \leq -8.3$ and a stellar mass $M_* \geq 10^{5.2} M_{\odot}$. We compare our stream and progenitor scenarios to those for NGC 300, a galaxy with a stellar mass similar to that of NGC~55 and known to host multiple structures in its halo. Our results demonstrate that even relatively isolated LMC-mass galaxies can have complex accretion histories, providing new tests of hierarchical galaxy formation at low masses.

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arXiv2026

The Cocytos Stream: A Disrupted Globular Cluster from our Last Major Merger?

The census of stellar streams and dwarf galaxies in the Milky Way provides direct constraints on galaxy formation models and the nature of dark matter. The DESI Milky Way survey -- with a footprint of $14{,}000$ deg$^2$ and a depth of $r<19$ mag -- delivers the largest sample of distant metal-poor stars compared to previous optical fiber-fed spectroscopic surveys. This makes DESI an ideal survey to search for previously undetected streams and dwarf galaxies. We present a detailed characterization of the Cocytos stream, which was re-discovered using a clustering analysis with a catalog of giants in the DESI year 3 data, supplemented with Magellan/MagE spectroscopy. Our analysis reveals a relatively metal-rich ([Fe/H]$=-1.3$) and thick stream (width$=1.6^\circ$) at a heliocentric distance of $\approx 25$ kpc, with an internal velocity dispersion of $6$--$9$ km s$^{-1}$. The stream's metallicity, radial orbit, and proximity to the Virgo stellar overdensities suggest that it is most likely a disrupted globular cluster that came in with the Gaia-Enceladus merger. We also confirm its association with the Pyxis globular cluster. Our result showcases the ability of wide-field spectroscopic surveys to kinematically discover faint disrupted dwarfs and clusters, enabling constraints on the dark matter distribution in the Milky Way.

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arXiv2026

Discovery of the Distant, Ultra-Faint Milky Way Satellite Aquarius IV with the Vera C. Rubin Observatory Early Data Preview 2

We present the discovery of Aquarius IV (Rubin J2201$-$0234) -- the first ultra-faint Milky Way satellite to be identified using data from the Vera C. Rubin Observatory. This system was detected at $\sim$8$σ$ significance using Rubin Early Data Preview 2 (EDP2) photometry and independently confirmed at $\sim$6$σ$ significance in archival Dark Energy Camera imaging. Jointly fitting its morphology and distance, we find that Aquarius IV is a low-luminosity ($M_V=-1.9^{+0.6}_{-1.0}$), compact ($r_{1/2} = 19^{+4}_{-6}$ pc; $r_h = 0.60^{+0.14}_{-0.17}$ arcmin) stellar system in the outer Galactic halo ($D_{\odot} = 109^{+6}_{-8}\ \mathrm{kpc}$). Its stellar population is consistent with an ancient, metal-poor stellar isochrone ($τ= 13$ Gyr, $Z=0.0001$). These properties closely resemble those of the smallest and faintest confirmed ultra-faint dwarf galaxies, though a globular cluster classification is not ruled out. Given the small number of detected member stars in Rubin EDP2, deeper imaging and spectroscopy will be critical for determining the properties and classification of Aquarius IV at higher confidence.

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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

Boötes III is a Tidally Disrupting Ultra-Faint Dwarf Galaxy on an Eccentric Polar Orbit

We present updated systemic properties of the ultra-faint dwarf galaxy Boötes III from the Southern Stellar Stream Spectroscopic Survey (S$^5$). We identify 21 high-probability members and measure a velocity dispersion of $σ_{v} = 1.69^{+1.03}_{-0.85}$ km s$^{-1}$, about six times smaller than the previously reported $10.7 \pm 3.5$ km s$^{-1}$, and a mean metallicity of [Fe/H] $= -2.34 \pm 0.11$. The revised dispersion brings Boötes III in line with other tidally disrupting dwarfs such as Antlia II and Crater II. Orbit integrations in a Milky Way (MW) + Large Magellanic Cloud (LMC) potential confirm a highly eccentric ($e \approx 0.8$), polar ($i \approx 89.5^\circ$) orbit with a recent pericentric passage $\sim 0.14$ Gyr ago at $r_{\rm peri} \approx 9.5$ kpc. Boötes III is thus likely actively tidally disrupting, as its tidal radius at pericenter, $r_t \approx 164$ pc, is only $\sim 0.35$ of its half-light radius. The unusually low dispersion also implies that Boötes III has either lost most of its dark matter to tides or hosts a cored inner density profile, making it a probe of the nature of dark matter. Simulated tidal streams are broadly consistent with the Styx stellar stream, though the predicted track and kinematics are sensitive to the MW halo mass, LMC mass, and solar velocity. Boötes III overlaps the Typhon stream in integrals-of-motion space but has a much lower mean metallicity, suggesting the two are not the same system but may have had a common group infall origin. Sagittarius-stream contamination prevents a direct tidal-tail detection, so deep spectroscopic follow-up remains essential, both to confirm Styx as a genuine stream and to establish it as Boötes III's tidal tail.

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arXiv2026

Signatures of a Tidally Induced Spiral Arm at the Anticenter of the Milky Way and a Kinematically Extended Anticenter Stream Using DESI DR2

Using the Dark Energy Spectroscopic Instrument Milky Way Survey (DESI MWS), we examine the 6D space of the anticenter region of the stellar disk (150$^\circ$ $<$ Galactic longitude $<$ 220$^\circ$) using 61,883 main-sequence turnoff stars. We focus on two well-known stellar overdensities in the anticenter, the Monoceros Ring (MRi) and Anticenter Stream (ACS). We find that the MRi overdensity has kinematics consistent with a tidally induced spiral arm, a type of dynamic spiral arm created by an interaction with a satellite galaxy, most likely the Sagittarius dwarf spheroidal galaxy (Sgr). We use the kinematics of the MRi to calculate the two most recent passage times of Sgr are 0.25 $\pm$ 0.09 Gyrs and 1.10 $\pm$ 0.23 Gyrs from the present day. We validate that the ACS is kinematically decoupled from the MRi because they are moving in opposite radial and vertical directions. We find that the kinematics associated with the ACS are not confined to our defined overdensity. The features we see in the ACS region are likely part of a broader distribution of stars with the same kinematic signature as detected in other places, like the vertical wave in the outer disk and phase spiral.

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arXiv2026

The DECam MAGIC Survey: Uncovering the Tidal Tails of the Crater II Dwarf Galaxy

Crater II (CraII), a large and low-density dwarf spheroidal galaxy, has unusual observed properties that are difficult to reproduce in cold dark matter simulations. Ongoing tidal disruption may help explain the discrepancies, as evidenced by the recent discovery of tidal tails. Here we present metallicity-sensitive narrowband photometry of the Ca II H and K lines from the Dark Energy Camera, covering $128$ deg$^2$ across the center and identified tidal tails of CraII as part of the Mapping the Ancient Galaxy in CaHK (MAGIC) survey. Our combined photometric metallicity, color-magnitude, proper motion, and parallax selections identify 162 CraII candidates. Of these, 37 candidates are located in the tidal tails which extend at least $7^\circ$ ($\sim 95$ kpc) from the center of CraII, suggesting it has lost $\gtrsim 25$% of its initial stellar mass. We confirm low contamination rates with dedicated control fields and highlight the extremely low surface brightness stellar features that can be uncovered with CaHK data, as faint as $\sim 36$ mag arcsec$^{-2}$. We also make the first detection of a metallicity gradient ($-0.34\pm0.17~{\rm dex}~{\rm deg}^{-1}$) in the center of the galaxy and infer a stream width of $w\sim 0.8^\circ$, roughly 50% larger than the CraII half-light radius. The detection of candidates in the most distant CraII pointings from its center implies that the tidal tails extend beyond our footprint. We compare the CraII stream to $N$-body models with "cored" and "cuspy" dark matter halo progenitors, determining that CraII's density profile is still ambiguous and warrants further modeling.

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arXiv2026

The DESI Y1 RR Lyrae catalog I: Empirical modeling of the cyclic variation of spectroscopic properties and a chemodynamical analysis of the outer halo

We present the catalog of RR Lyrae stars (RRLs) observed in the first year of operations of the Dark Energy Spectroscopic Instrument (DESI) survey. This catalog contains 6,240 RRLs out to $\sim120$\,kpc from the Galactic center and over 12,000 individual epochs with homogeneously-derived stellar atmospheric parameters. We introduce a novel methodology to model the cyclical variation of the spectroscopic properties of RRLs from single-epoch measurements. We employ this method to infer the systemic velocities and mean temperatures of fundamental and first-overtone mode RRLs in our sample (without distinguishing between individual spectral lines). For fundamental mode pulsators, we obtain radial velocity curves with amplitudes of $\sim$30--80\,km\,s$^{-1}$ and effective temperature curves with 300--1,000\,K variations, whereas for first-overtone pulsators these amplitudes are $\sim20$\,km\,s$^{-1}$ and $\sim 600$\,K, respectively. We use our sample to study the metallicity distribution of the halo and its dependence on Galactocentric distance ($R_{\rm GC}$). Using a radius-dependent mixture model, we split the data into chemodynamically distinct components and find that our inner halo sample ($R_{\rm GC}\lesssim50$\,kpc) is predominantly composed of stars with [Fe/H] $\sim-1.5$ and largely radial orbits (with an anisotropy parameter $β\sim0.94$), that we associate with the Gaia-Sausage-Enceladus merger. Stars in the halo field exhibit a broader and more metal-poor [Fe/H] distribution with more circular orbits ($β\sim0.39$). The metallicity gradient of the metal-rich and the metal-poor components is found to be $0.005$ and $0.010$\,dex\,kpc$^{-1}$, respectively. Our catalog highlights DESI's tantalizing potential for studying the Milky Way and the pulsation properties of RRLs in the era of large spectroscopic surveys.

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arXiv2026

The DESI Y1 RR Lyrae catalog II: The metallicity dependency of pulsational properties, the shape of the RR Lyrae instability strip, and metal rich RR Lyrae

RR Lyrae stars (RRLs) are valuable probes of both Milky Way assembly and stellar-evolution physics. Using a sample 6,240 RRLs obtained in the first year of the Dark Energy Spectroscopic Instrument (DESI) survey, we investigate the metallicity of RRLs and its correlation with their pulsation properties. We find that (1) a clear correlation between period and [Fe/H] reinforces the view that the long-standing Oosterhoff dichotomy arises from the scarcity of intermediate-metallicity Galactic globular clusters hosting sizeable RRL samples; (2) high-amplitude short-period and small-amplitude short-period variables are comparatively metal-rich, with mean [Fe/H] = $-1.39 \pm 0.27$ and $-1.30 \pm 0.28$, respectively; (3) in double-mode pulsators (RRd) the metallicity declines smoothly with increasing fundamental-mode period, and anomalous RRd stars occupy a remarkably narrow [Fe/H] range relative to classical RRd stars; (4) this spectroscopic sample let us, for the first time, place empirical constraints on the metallicity-dependent topology of the instability strip using phase-corrected effective temperatures and a large number of RRLs, where we observe an instability strip that moves towards cooler $T_{\rm eff}$ with declining [Fe/H] with a width roughly consistent with stellar-evolution models; and (5) a subset of metal-rich RRLs exhibits orbits consistent with disk membership and halo kinematics. Our results confirm the tantalizing potential of DESI for Galactic and stellar astrophysics and highlight the importance of the even larger samples of RRLs and data-processing improvements forthcoming in future DESI data releases.

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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

Characterizing the GD-1 Stream with DESI DR2 Data: Thin Stream and Hot Cocoon

GD-1 is among the longest, coldest stellar streams in the Milky Way, making it an ideal target for probing dark matter substructure through dynamical heating. We present a catalog of 608 spectroscopically confirmed GD-1 members from the first three years of Dark Energy Spectroscopic Instrument (DESI) observations. This constitutes the largest homogeneous spectroscopic sample of GD-1, doubling the number of members previously available only through heterogeneous compilations combining multiple surveys with different systematics. Using these data, we derive updated stream tracks in sky position, proper motion, and radial velocity that extend over $100^\circ$ of the stream. We apply a Gaussian mixture model to decompose the stream into a dynamically cold thin component ($σ_V = 2.49\pm 0.28$ km s$^{-1}$, width $= 0.23\pm0.01^\circ$) and a kinematically hot cocoon ($σ_V = 6.13\pm0.75$ km s$^{-1}$, width $= 2.18\pm0.17^\circ$). The cocoon contains $\sim30\%$ of members and its velocity dispersion is consistent with $\sim11$ Gyr of heating by cold dark matter subhalos. We also detect a large proper motion dispersion ($41.36\pm4.98$ km s$^{-1}$) along the stream direction in the cocoon component. This feature indicates a significant line-of-sight distance spread in the cocoon, and its origin will be further explored in a forthcoming paper. These measurements demonstrate the power of DESI spectroscopy for characterizing the multi-component phase-space structure of stellar streams and constraining small-scale dark matter substructure.

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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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arXiv2026

A second-generation star in a relic dwarf galaxy

Stars that contain only trace amounts of elements heavier than helium, referred to as having low "metallicity", preserve the chemical fingerprints of the first generation of stars and supernovae. In the Milky Way, the lowest metallicity stars show an extreme over-abundance of carbon relative to other elements, which has been hypothesized to be a unique result of the first low-energy supernovae. However, the origin of this signature has remained a mystery, since no such stars have been discovered in the ancient dwarf galaxies where they are thought to have formed. Here, we present observations of a star in the >10 billion year old ultra-faint dwarf galaxy Pictor II, that shows the lowest iron and calcium abundances outside the Milky Way (<1/43,000th solar and ~1/160,000th solar), with a factor of >3000x relative carbon enhancement. As the first unambiguous second-generation star in a relic dwarf galaxy, this object demonstrates that carbon-enhanced second-generation stars can originate in primordial small-scale systems. This star supports the hypothesis that carbon-enhancement is produced by low-energy-supernovae, since the yields of energetic supernovae are harder to retain in small-scale environments. This key local signature of chemical enrichment by the first stars traces a regime inaccessible to current high-redshift observations, which cannot detect the early enrichment of the smallest galaxies.

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arXiv2026

The Kinematically Hot, Extremely Metal-Poor C-19 Stellar Stream in DESI DR2

Stellar streams are the result of a host galaxy's gravitational potential tidally disrupting satellite dwarf galaxies and globular clusters (GCs), causing them to grow leading and trailing tidal tails. The C-19 stellar stream is an extremely metal-poor stellar population, showing chemical abundance patterns characteristic of a globular cluster. However, its large velocity dispersion is difficult to reconcile with a conventional, purely baryonic, disrupting-GC progenitor. Current techniques for stream characterization are primarily applied to Gaia DR3, relying heavily on proper motion measurements. Using the Dark Energy Spectroscopic Instrument (DESI), which provides radial velocities and metallicites for over 10 million stars reaching significantly fainter magnitudes than comparable surveys, we employ a mixture model approach to jointly characterize stream populations in proper motions, radial velocities, and metallicities against a Milky Way halo background. By applying this framework to the C-19 stellar stream, we identify a total of 47 spectroscopically confirmed member stars, of which 41 are newly identified and only 6 were previously reported in the literature. In this work, we measure a velocity dispersion of $7.8^{+1.5}_{-1.3}$ km s$^{-1}$ and a mean metallicity of [Fe/H] = $-3.36^{+0.12}_{-0.10}$. We further identify a novel 'spur' feature within the stream. We conclude that our measurements are in line with previous works identifying C-19 as a 'hot', metal-poor stream. In forthcoming work, we will apply this approach to many more streams in the DESI footprint, enabling population-level comparisons with predictions from simulations.

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arXiv2026

Epicyclic Density Variations in the Indus Stellar Stream

Longitudinal density fluctuations observed in stellar streams can result from gravitational interactions with massive perturbers in the Milky Way, such as dark matter subhalos. Analysing these density variations provides a powerful probe of properties (motion, mass, size, etc.) of the perturbing objects. However, caution is needed because density variations may arise naturally from internal dynamics of streams, namely epicycles. In this work, we focus on the Indus stellar stream, a remnant of an ancient dwarf satellite of the Galaxy. An Indus stream spanning $\sim 90^\circ$ is revealed in the southern Galactic sky using a comprehensive matched-filter analysis utilizing data from the Gaia mission. A spatial density model is fitted to the filtered map to quantitatively characterize the morphology, which demonstrates episodic density peaks and gaps in the stream. Through N-body simulations, we show that there are strong epicyclic motions of stars happening during tidal disruptions. The present-day longitudinal densities from simulations are comparable to the measurement from data, with similar numbers and locations of peaks and gaps, suggesting that the observed density should mainly be caused by epicycles. We also find that a cuspy dark matter halo for the Indus dwarf is likely to produce milder stellar epicyclic peaks compared to a cored halo which results in steeper peaks. This arises from different instantaneous mass loss due to distinct central mass distributions of halos, where a cored halo usually leads to severer tidal stripping. The observed density exhibits moderate peak sharpness, implying that Indus may have originally possessed a cuspy halo.

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arXiv2026

The Milky Way stellar halo is twisted and doubly broken: insights from DESI DR2 Milky Way Survey observation

Using K giants from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) Milky Way (MW) Survey, we measure the shape, orientation, radial profile, and density anisotropies of the MW stellar halo over 8 kpc$<r_\mathrm{GC}<200$ kpc. We identify a triaxial stellar halo (axes ratio $10:8:7$), 43 degrees tilted from the disk, showing two break radii at $\sim16$ kpc and $\sim76$ kpc, likely associated with Gaia-Sausage/Enceladus (GSE) and Large Magellanic Cloud (LMC), respectively. The inner stellar halo ($<30$ kpc) is oblate and aligned with the disk, whereas the outer stellar halo becomes prolate and perpendicular to the disk, consistent with the Vast Polar Structure of MW satellites. The twisted halo may arise from the disk-halo angular momentum shift triggered by the infall of a massive satellite. The anisotropic density distribution of the stellar halo is also measured, with successful re-identification of the Hercules-Aquila Cloud South/North (HAC-N/-S) and Virgo overdensities (VOD). Break radii are found at 15/30 kpc for VOD/HAC-N(-S). We identify the LMC transient density wake with a break radius at 60 kpc in the Pisces overdensity region. We also find new observational evidence of the LMC collective density wake, by showing a break radius at $\sim$100 kpc in the northern Galactic cap with a clear density peak at 90 kpc. In the end, we found that more metal-poor halo stars are more radially extended. Our results provide important clues to the assembly and evolution of the MW stellar halo under the standard cosmic structure formation framework.

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arXiv2025

Detailed Chemical Abundance Analysis of the Brightest Stars in the Turranburra and Willka Yaku Stellar Streams

We present a detailed chemical abundance analysis of the three brightest known stars from each of the Turranburra and Willka Yaku stellar streams using high-resolution Magellan/MIKE spectra. Abundances for 27 elements, ranging from carbon to dysprosium, were derived. Our results support the original classification that Turranburra, with a low average metallicity of $\mathrm{[Fe/H]=-2.45} \pm 0.07$, likely originates from a dwarf-galaxy progenitor. Willka Yaku has a low average metallicity of $\mathrm{[Fe/H]=-2.35 \pm 0.03}$ with a small scatter in the abundances, consistent with a globular cluster progenitor as suggested by previous studies. Both streams exhibit mild enhancements in neutron-capture elements, with averages of $\mathrm{[Eu II/Fe]}=$ $0.47 \pm{0.09}$ for Turranburra and $0.44 \pm{0.05}$ for Willka Yaku, consistent with enrichment from an $r$-process event. A similar enrichment is observed in other stellar streams, and we further discuss this signature as it relates to the potential enrichment histories of these two streams.

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arXiv2025

The dark matter content of Milky Way dwarf spheroidal galaxies: Draco, Sextans and Ursa Minor

The Milky Way Survey of the Dark Energy Spectroscopic Instrument (DESI) has so far observed three classical dwarf spheroidal galaxies (dSphs): Draco, Sextans and Ursa Minor. Based on the observed line-of-sight velocities and metallicities of their member stars, we apply the axisymmetric Jeans Anisotropic Multi-Gaussian Expansion modeling (JAM) approach to recover their inner dark matter distributions. In particular, both the traditional single-population Jeans model and the multiple population chemodynamical model are adopted. With the chemodynamical model, we divide member stars of each dSph into metal-rich and metal-poor populations. The metal-rich populations are more centrally concentrated and dynamically colder, featuring lower velocity dispersion profiles than the metal-poor populations. We find a diversity of the inner density slopes $γ$ of dark matter halos, with the best constraints by single-population or chemodynamical models consistent with each other. The inner density slopes are $0.71^{+0.34}_{-0.35}$, $0.26^{+0.22}_{-0.12}$ and $0.33^{+0.20}_{-0.16}$ for Draco, Sextans and Ursa Minor, respectively. We also present the measured astrophysical J and D factors of the three dSphs. Our results indicate that the study of the dark matter content of dSphs through stellar kinematics is still subject to uncertainties behind both the methodology and the observed data, through comparisons with previous measurements and data sets.

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