The inner Galaxy is the most complex region of the Milky Way, comprising the early bulge, inner thin and thick discs, and inner halo stars; moreover, the formation of the bar caused transfer of gas and stars from the disc to the inner Galaxy. Moreover, accretion of dwarf galaxies took place along the Galaxy's lifetime, merging with the original bulge. In this work, we sought to constrain the metal-rich stars of the earliest spheroidal bulge. With the aim of studying the oldest bulge stars, which show a distribution in a spheroid, we applied a selection based on kinematical and dynamical criteria, in the metal-rich range Fe/H $> -$0.8. This analysis complements our previous work on a symmetric sample with Fe/H $< -$0.8. We derived the individual abundances through spectral synthesis for the elements C, N, O, Al, P, S, K, Mn, and Ce using the stellar physical parameters available for our sample from Data Release 17 of the Apache Point Observatory Galactic Evolution Experiment (APOGEE DR17) project in the H band. We also compared the present results, together with literature data, with chemical-evolution models. The abundances of the alpha elements Mg Si, and Ca, and iron-peak elements V, Cr, Co, and Ni from APOGEE DR17 follow the expected behaviour as compared with the chemical-evolution models. Mn shows the expected secondary behaviour. S and K show a large star-to-star spread, but remain broadly compatible with the model predictions. Phosphorus and cerium display a clear abundance excess around Fe/H ∼ -0.7 that is more pronounced than in our metal-poor sample, suggesting a distinctive chemical signature for the earliest bulge population. Diagnostic diagrams involving Mg/Mn versus Al/Fe and Ni/Fe versus (C+N)/O indicate an in situ origin of the bulk of the sample. At super-solar metallicities, a subset of stars shows enhanced K and Mn (and possibly S) together with low Ce/Fe ratios, hinting at enrichment processes linked to the nuclear disc and bar. These stars may therefore trace a chemically distinct population shaped by the unique dynamical and star formation conditions of the innermost Galaxy.

Abstract To unravel the formation history of the Milky Way, we estimate the accretion times of six phase-space substructures in the stellar halo, using the orbital frequencies toward two spatial directions ( r , ϕ ) in spherical coordinates. These substructures, identified in our previous studies, are located in the solar neighbourhood and therefore have high-precision astrometry from Gaia. The uncertainties of the results are determined using the Monte Carlo method, and the significance is established through comparison with random halo samples. The results for the substructure GL-1 in both directions show good consistency and high significance (4.3 σ and 3.9 σ ), yielding a combined accretion time of 5.6 ± 0.1 Gyr ago, where the uncertainties quoted are statistical only. The substructures GL-4 and GR-1, with smaller pericenters, exhibit higher significance in the less massive potential of the Milky Way, implying that the more massive potential may overestimate the central mass, especially the bulge. The accretion times of GL-4 and GR-1 are 6.9 ± 0.3 Gyr with a confidence of 3.7 σ , and 2.0 ± 0.1 Gyr with a confidence of 4.4 σ , respectively. Further constraints on the accretion times of phase-space substructures require more precise astrometric data, e.g., by Gaia DR4, China Space Station Survey Telescope and Roman space telescope.

Abstract Resolved metallicity studies of local disk galaxies have revealed that their interstellar media (ISMs) are far from chemically homogeneous, displaying significant (∼0.05 dex) variations in the metallicity on characteristic scales of a few hundred parsecs. Such data is at odds with most analytical models, where the ISM is predicted to be more well-mixed. Here, we suggest that the observed small-scale features seen in galaxies may be superbubbles of metal-enriched gas created by a collection of core collapse supernovae with tight spatial (and temporal) correlation. In this scenario, the size of the metallicity fluctuations (superbubble radius, φ) is set by the disk scale height of the galaxy in question (after which point shock breakout favours preferential expansion along directions perpendicular to the dense disc), and the amount of additional metals contained within a fluctuation is proportional to the star formation efficiency in superbubble regions (ε). To test this theory, we analysed metallicity maps from the PHANGS-MUSE sample of galaxies using a geostatistical forward-modelling approach. We find φ ≃ 300 pc and ε = 0.1 − 0.2, in good agreement with our theoretical model. Further, these small-scale parameters are found to be related to the global galaxy properties, suggesting that the local structure of the interstellar medium of galaxies is not universal. Such a model of star formation paints a new picture of galaxy evolution in the modern universe: in large local galaxies, star formation appears steady and regular when averaged over large scales. However, on small scales, these large galaxies remain intrinsically bursty like their smaller, high-redshift counterparts.

Abstract The spherical Jeans equation is commonly used to infer dark matter distributions in dwarf spheroidal satellites of the Milky Way to constrain the nature of dark matter. One of its assumptions is that of dynamical equilibrium while the dwarfs are under the influence of Galactic tides. We carry out tailored simulations of Carina, Draco, Fornax, Sculptor and Ursa Minor and test the accuracy of dark matter density profiles and annihilation rates (J-factors) recovered with the Jeans analysis code pyGravSphere. We find that tides do not significantly affect the quality of density profile inference; however, pyGravSphere tends to underestimate the inner densities of dwarf galaxies, which, together with tidal mass loss, leads to an inference of flatter density slopes, although all of our dwarfs have cuspy Navarro-Frenk-White haloes. This is because the default broken power-law model is unable to describe the outer halo density profile. The recovered J-factors are generally underestimated. While the difference with the true J-factor is small, the error bars are also often underestimated. We also test the accuracy of the Wolf et al. 2010 mass estimator and find that it can be sensitive to orbital stage and eccentricity. Still, for our sample of dwarf galaxies, the estimates agree with the truth within 10 per cent. Consistency of our simulated dwarfs with the mass-concentration relation in ΛCDM requires a light Milky Way, or limited action of tides, which may be in tension with a “tidal stirring” origin of dwarf spheroidals.

Abstract We construct a model by integrating observational constraints from the Milky Way and nearby galaxies to predict cloud-scale star formation rates (SFRs). In the model, we first estimate the initial total mass of clumps in a cloud based on the cloud mass, and then generate the initial clump population of the cloud using the initial clump mass function. Next, we model the star formation histories (SFHs) of the cloud to assign an age to each clump. We then sort out the intermediate-age clumps and calculate the total embedded cluster mass. Finally, we predict the SFR based on the duration of the embedded phase. The model-predicted SFR is broadly consistent with the observed SFR, supporting the plausibility of the model. The model primarily provides a theoretical framework that integrates a wide range of observational results, thereby clarifying the tasks for future observations.

Neutrinos as a new tool to characterize the Milky Way Center

The nuclear stellar disc (NSD) of the Milky Way is a dense, rotating stellar system in the central ∼200,pc. The NSD is thought to be primarily fuelled by bar-driven gas inflows from the inner Galactic disc. As part of the LEGARE project, we aim to construct the first chemical-evolution models for the NSD using a Bayesian approach tailored to reproducing the observed metallicity-distribution functions and compared with the available abundance ratios for Mg, Si, and Ca relative to Fe. In particular, we intend to test whether the flowing gas from the inner Galactic disc, which feeds the NSD, can reproduce the observed abundances. We adopted a state-of-the-art chemical-evolution model in which the gas responsible for the formation of the NSD is assumed to be driven by the Galactic-bar-induced inflows. The chemical composition of the accreted material is assumed to reflect that of the Galactic disc at a radius of ∼4 kpc. A Bayesian framework based on Markov Chain Monte Carlo (MCMC) techniques was then employed to fit the metallicity-distribution functions of different samples of NSD stars. If we take the NSD data at face value, without considering possible contamination from bulge stars, we find that a formation scenario based on the inner disc's flowing gas is inconsistent with the low-metallicity tail of the observed metallicity-distribution function. This is because the inner disc's metallicity, at the epoch of bar formation, was already near solar. On the other hand, models invoking dilution from additional metal-poor inflows successfully reproduce the observations. Models with different levels of gas dilution share similar gas infall timescales (ranging from 3.7 to 5.2 Gyr) and negligible galactic winds (mass-loading factors, ω, between 0.001 and 0.030). The best-fit model corresponds to an inflow with a metallicity five times lower than that of the inner disc and a moderate star-formation efficiency. The same model successfully reproduces the observed α/Fe - Fe/H abundance trends and predicts a star formation history consistent with the most recent estimates. However, if we assume that the metallicity distribution function is contaminated by metal-poor bulge stars and is restricted to stars with Fe/H > -0.3 dex, there is no longer any need for gas dilution. In this case, the best-fit model is characterised by a very low star formation efficiency, coupled with a mild galactic wind. Our analysis indicates that dilution of the inflowing gas forming the NSD is necessary to reproduce its observed chemical properties, if bulge contamination in the data is not considered. This implies that, in addition to bar-driven inflows from the inner thin disc, lower metallicity gas -- possibly originating from the thick disc or from more recent accretion events -- contributed to the formation of the NSD. On the other hand, when contamination by bulge stars is assumed, dilution is no longer required.

Abstract Complete catalogs of molecular clouds in the Milky Way allow analysis of the molecular medium and the star formation properties of the Milky Way that closely follows the method used for nearby galaxies. We explore whether the big dip in the radial distribution of molecular gas in the Milky Way is peculiar and find several other galaxies with similar patterns, all with similar morphological classifications of YClxxGnR, indicating a clearly defined, long bar leading to a grand-design spiral. This category is fairly rare among galaxies in the PHANGS sample, but all galaxies with this classification have some evidence for dips in the radial distribution of CO emission. The lengths of the bars correlate with the extents of the dips. The Milky Way and the other galaxies with dips have similar stellar masses and star formation rates, both lying near the high ends of the distributions for all PHANGS galaxies.

Abstract Kinematic and spectroscopic studies in the past few years have revealed coherent azimuthal metallicity variations across the disk of the Milky Way (MW) that may be the result of dynamical process associated with nonaxisymmetric features of the Galaxy. At the same time, stellar kinematics from Gaia have uncovered ridgelike features in the velocity space, raising the question of whether these chemical and dynamical substructures share a common origin. Using a sample of disk stars from Gaia Data Release 3, we find that azimuthal metallicity variations are correlated with kinematic ridges in the V ϕ – R plane, suggesting a shared origin. We utilize a suite of MW test-particle simulations to assess the role of transient spiral arms, the bar, and interactions with a Sagittarius (Sgr)-like dwarf galaxy in simultaneously shaping both chemical and kinematic substructures. Among the physical mechanisms explored, bar and spiral arm interactions are the ones that consistently reproduce both the chemokinematic features and alignment observed in the Gaia data. While our model of an interaction with a Sgr-like dwarf galaxy can also induce kinematic and metallicity substructure, the amplitude of the azimuthal metallicity variations is too weak, suggesting this is likely not the dominant influence. Although additional contributing processes cannot be ruled out, the azimuthal metallicity variations observed in Gaia are best explained by a dynamical origin. Our results support the view that azimuthal metallicity variations in the Galaxy are driven by similar dynamical mechanisms responsible for generating the kinematic ridges and comoving groups.

Abstract We present spectroscopic evidence for tidal debris associated with the bulge globular cluster NGC 6569, based on medium-resolution ( R ∼ 11,000) spectra of 303 stars obtained with the Anglo-Australian Telescope. Targets were selected using Blanco DECam Bulge Survey photometry with Gaia DR3 astrometry, and span ∼ 7 ′ – 3 0 ′ (i.e., 1–5 r t , where r t is the King-model tidal radius) from the cluster center. Theoretical modeling shows that the Jacobi radii can vary between 8 ′ – 1 1 ′ and 1 8 ′ – 2 2 ′ over the orbit, likely leaving stars around the cluster that are transitioning into the predicted leading and lagging tidal tails. We identify 40 stars in this sample that exhibit chemical and kinematic properties consistent with previous, or borderline, cluster membership. The seven best candidates have S/N > 30, with [Fe/H] = –0.83 ± 0.14 dex and [ α /Fe] = +0.38 ± 0.06 dex, consistent with NGC 6569’s bound population. Our findings provide evidence that NGC 6569 is actively losing stars through tidal stripping, contributing to the bulge field population at a present rate of 1.0–1.6 M ⊙ Myr −1 , which corresponds to ≈5.6% ± 1.3% of its present-day mass per Gyr. This work is part of the Milky Way Bulge Extra-Tidal Star Survey and represents our first detailed study of a massive bulge globular cluster in this context.

Abstract We present a kinematical study of the outer halo ( r GC ∼ 60–160 kpc) of the Milky Way (MW) based on spectroscopy of 55 RR Lyrae stars using the Echelle Spectrograph and Imager instrument on the Keck II telescope. Our spectroscopic targets were selected from three photometric surveys: NGVS, DES, and Pan-STARRS 1. We derive center-of-mass radial velocities with uncertainties of 6–35 km s −1 . The halo velocity dispersion measured with our sample is 70 ± 7 km s −1 . The velocity field shows a possible dipole-like structure, with redshifted northern and blueshifted southern hemispheres. Fitting a MW–Large Magellanic Cloud (LMC) dipole perturbation model yields a weak/marginal dipole signal, with an amplitude of − 3 0 − 20 + 16 km s −1 and an apex direction ( l , b ) = ( − 38 . 2 − 31.5 + 42.4 , − 41 . 3 − 23.8 + 27.9 ) deg, along with a bulk compression velocity of −16 ± 11 km s −1 . While limited by sky coverage and sample size, our results are consistent with the presence of LMC-induced disequilibrium in the distant halo beyond 100 kpc. Aside from the 55 RR Lyrae stars, our spectroscopic analysis reveals that 10 additional phometrically selected RR Lyrae candidates are, in fact, quasar/blazar contaminants; this provides a cautionary tale about the presence of such contaminants in sparsely sampled photometric surveys. Our study demonstrates that single-epoch spectroscopy of RR Lyrae stars is a viable method for probing the kinematics of the outer halo, and future surveys like Rubin/LSST and the Dark Energy Spectroscopic Instrument (DESI-II) have the potential to significantly advance this effort.

Abstract We present Hubble Space Telescope (HST) imaging of Pegasus V and Pisces VII, along with a re-analysis of the archival imaging of Pegasus W, and Karl Jansky Very Large Array (VLA) neutral gas (H i ) observations of all three. These three ultra-faint dwarfs (UFDs) are all within the Local Group in the approximate direction of M31. The VLA observations place stringent upper limits on their H i content, with all having M HI < 10 4 M ⊙ . As the red giant branches of these UFDs are sparsely populated, we determined distances from the HST photometry of horizontal branch (HB) stars in comparison to a fiducial HB population (from M92), with all three falling in the range 0.7–1 Mpc. Using a new Python -based star formation history (SFH) fitting code (based on StarFISH ), we derive SFHs of all three UFDs. As found previously, the best-fit SFH for Pegasus W includes significant star formation well beyond the end of reionization, while the SFHs calculated for Pegasus V and Pisces VII are consistent with them having quenched over 10 Gyr ago. These findings for the latter two objects indicate that, like those in the vicinity of the Milky Way, lower-mass UFDs in the vicinity of M31 likely quenched at early times.

Abstract In addition to comprehensive surveys of the Milky Way (MW) bulge, disk, and halo, the Apache Point Galactic Evolution Experiment (APOGEE) project observed seven dwarf spheroidal satellites (dSphs) of the MW: Carina, Sextans, Sculptor, Draco, Ursa Minor, Bootes 1, and Fornax. APOGEE radial velocities, stellar parameters, and Gaia EDR3 proper motions are used to identify member stars from the targets in the vicinity of each dwarf; for seven dwarfs, new member stars are identified. To properly analyze the abundance patterns of these galaxies, a novel procedure was developed to determine the measurable upper limits of the APOGEE chemical abundances as a function of the effective temperature and the spectral signal-to-noise ratio. In general, the APOGEE abundance patterns of these galaxies (limited to [Fe/H] > −2.5) agree with those found in high-resolution optical studies in the literature after abundance offsets are applied. Most of the galaxies studied here have abundance patterns that are distinctly different from the majority of stars found in the MW halo, suggesting that these galaxies contributed little to the MW halo above [Fe/H] > −2.0. From these abundance patterns, we also find that these dSphs tend to follow two types of chemical evolution paths: episodic and continuous star formation, a result that is broadly consistent with previous photometric studies of the star formation histories (SFHs) of these galaxies. We explore whether mass and/or environment have an impact on whether a galaxy has an episodic or continuous SFH, finding tentative evidence that, in addition to the galaxy’s mass, proximity to a larger galaxy and the cessation of star formation may drive the overall shape of the chemical evolution.

The Milky Way Project: Bridging Intermediate- and High-mass Star Formation with the MIRION Catalog of Yellowballs

Abstract We present a non–local thermodynamic equilibrium (NLTE) abundance analysis of the light neutron-capture elements Sr, Y, and Zr in a sample of 103 very metal-poor stars spanning −4.3 ≲ [Fe/H] ≲ −1.7, based on high-resolution Subaru/HDS spectra and atmospheric parameters from Paper I. This is the first NLTE study of Sr−Y−Zr abundances using a large, homogeneous sample optimized for tracing Galactic chemical evolution (GCE). We find a mild positive trend in [Sr, Y, Zr/Fe] with [Fe/H], consistent with GCE models that incorporate s -process, electron-capture supernovae, compact object mergers, and magnetorotational supernovae, without invoking additional light-element primary processes. We report a statistically significant correlation between [Sr/Fe] and [Na/Fe] in our stellar sample. This finding supports the contribution of fast-rotating massive stars (spinstars) to the simultaneous production of sodium through hydrogen burning and light neutron-capture elements via the weak s -process in low-metallicity environments. We identify five Sr-poor stars exhibiting significantly lower Sr, Y, and Zr abundances than other stars at similar [Fe/H], pointing to chemical inhomogeneities in early star-forming environments. These light neutron-capture elements therefore provide valuable tracers of early nucleosynthetic processes and the Milky Way’s assembly history.

Abstract We introduce a new photometric catalog of RR Lyrae (RRL) variables (∼300,000) mainly based on data available in public datasets. We also present the largest and most homogeneous spectroscopic dataset of RRLs and blue horizontal branch (BHB) stars ever collected. This includes radial velocity measurements (∼16,000) and iron abundances (Δ S method for 8140 RRLs, plus 547 from literature). Elemental abundances based on high-resolution spectra are provided for 487 RRLs and 64 BHB stars. We identified candidate RRLs associated with the main Galactic components and their iron distribution function (IDF) becomes more metal rich when moving from the halo ([Fe/H] = −1.56) to the thick disk (TCD; [Fe/H] = −1.47) and thin disk (TND; [Fe/H] = −0.73). Furthermore, halo RRLs and RRLs in retrograde orbits are α enhanced ([ α /Fe]=0.27, σ = 0.18), while TCD RRLs are either α enhanced ([Fe/H] ≤ −1.0) or α poor ([Fe/H] > −1.0), and TND RRLs are mainly α poor ([ α /Fe] = −0.01, σ = 0.20). We also identified RRLs associated with the main stellar streams—Gaia–Sausage–Enceladus (GSE); Sequoia, Helmi, and Sagittarius—and we found that their IDFs are quite similar to halo RRLs. However, GSE RRLs lack the metal-poor/metal-rich tails and their α -element distribution is quite compact. The iron radial gradient in Galactocentric distance for TND, TCD, and halo RRLs is negative and it decreases from −0.026, to −0.010, and to −0.002 dex kpc −1 . The iron radial gradient based on dry halo (halo without substructures) RRLs is, within the errors, equal to the global halo. We also found a strong similarity between iron and [ α /Fe] radial gradients of Milky Way RRLs and M31 globular clusters throughout the full range of galactocentric distances covered by the two samples.

Abstract We combine new spectroscopic observations of the ultrafaint dwarf (UFD) galaxy Boötes I (Boo I) from the Southern Stellar Stream Spectroscopic Survey ( S 5 ) with ∼15 yr of archival spectroscopic data to create the largest sample of stellar kinematics and metallicities to date for any Milky Way UFD. Our combined sample includes 148 members extending out to ∼7 half-light radii ( r h ), including 24 newly confirmed members, 18 binary candidates, 15 RR Lyrae stars, and 92 [Fe/H] measurements. Using this larger and more spatially extended sample, we provide updated constraints on Boo I’s systemic properties, including its radial population gradients. Properly accounting for perspective rotation effects in a UFD for the first time, we detect a 4 σ line-of-sight velocity gradient of 1.2 ± 0.3 km s −1 r h − 1 aligned along Boo I’s orbit and discuss its potential tidal origins. We also infer a metallicity gradient of −0.10 ± 0.02 dex r h − 1 in agreement with previous studies. Using an axisymmetric Jeans model, we provide updated constraints on Boo I’s dark matter density profile, which weakly favors a cusped ( γ = 1 . 0 − 0.6 + 0.5 ) dark matter profile. Lastly, we reanalyze Boo I’s metallicity distribution function with a one-zone galactic chemical evolution model and place new constraints on its rapid, inefficient star formation and strong galactic outflows.

The Magellanic Clouds, the closest star-forming galaxies to the Milky Way, offer an excellent environment to study high-mass X-ray binaries (HMXBs). While the Small Magellanic Cloud (SMC) has been thoroughly investigated with over 120 systems identified, the Large Magellanic Cloud (LMC) has lacked a complete survey due to its large angular size. Most prior studies targeted central or high-star-formation regions. The all-sky surveys now enable a comprehensive coverage of the LMC, particularly due to its close vicinity to the south ecliptic pole. This work aims to improve our understanding of the HMXB population in the LMC by building a flux-limited catalogue. This allows us to compare sample properties with those of HMXB populations in other nearby galaxies. Using detections during the first all-sky survey (eRASS1), we cross-matched X-ray positions with optical and infrared catalogues to identify candidate HMXBs. We assigned flags based on multi-wavelength follow-up observations and archival data, using properties of known LMC HMXBs. These flags defined confidence classes for our candidates. We detect sources down to X-ray luminosities of a few 10^34 erg s -1 , resulting in a catalogue of 53 objects, including 28 confirmed HMXBs and 21 new detections. Compared to the SMC, the LMC hosts fewer HMXBs and more systems with supergiant companions. We identify several likely supergiant systems, including a candidate supergiant fast X-ray transient with phase-dependent flares. We also find three Be stars with likely white dwarf companions. Two of the candidate Be/WD binaries show steady luminosities across four scans, unlike the post-nova states seen in the majority of previous Be/WD reports. Our catalogue is the first to cover the entire LMC since the _ era, providing a basis for statistical population studies. Using the HMXB population, we estimate the LMC star-formation rate to be $(0.22^ +0.06 -0.07 -1 , which is in agreement with results using other tracers.

Abstract Based on the spectral data from the LAMOST Medium-Resolution Spectroscopic Survey (LAMOST-MRS) DR11-v1.1, combined with a publicly available open cluster catalog, we built a large-sample database of open clusters with medium-resolution spectroscopic parameters. This sample encompasses radial velocity measurements from medium-resolution spectroscopy for 1,033 open clusters, among which 446 clusters further offer metallicity profiles and abundance distributions for dozens of chemical elements. Based on this comprehensive dataset, we performed statistical analyses on key parameters of open clusters, including radial velocities, metallicities, and chemical element abundances. Notably, when the star cluster radial velocities are compared with results from the high-resolution spectroscopic survey (APOGEE), the mean difference is constrained within 1 km s$^{-1}$, with a standard deviation less than 10 km s$^{-1}$. For metallicity [Fe/H] comparisons with published high-resolution literature values, the average discrepancy falls in the range of 0.02 - 0.04 dex, accompanied by a standard deviation of 0.06 - 0.08 dex. These findings demonstrate that open cluster properties derived by LAMOST-MRS exhibit reliable precision, making them suitable as probes for in-depth investigations into the chemical evolution of the Milky Way. Finally, we have compiled a catalog of 1,033 open clusters, complemented by parameter lists for approximately 7,000 member stars - all including their LAMOST-MRS spectroscopic parameters. This dataset provides a valuable resource for advancing galactic astrophysics research.

The Milky Way's Disk Keeps Getting Weirder: The disk of our galaxy is not flat but warped and waving.