Recent Gaia-based measurements of the Milky Way rotation curve and stellar-stream modeling give significantly different estimates of the Galactic dynamical mass beyond Galactocentric radii of 15 kpc. The stream-based model predicts an outer halo five times more massive than that predicted by the Gaia rotation curve. We aim to test the impact of analytic assumptions used in stream modeling and to assess whether the currently available stream constraints can distinguish between low- and high-mass Galactic potentials. We first compared globular-cluster disruption in analytic and N-body Milky Way potentials. We then modeled Palomar 5 and ATLAS–Aliqa Uma, which are unique in probing the outer region beyond R_GC=15 kpc and are the most relevant to understanding the mass discrepancy. Both streams have usable constraints on sky position, proper motion, line-of-sight velocity, and RR Lyrae distance. They were modeled for both a rotation-curve-based and a stream-based Galactic potential. In the N-body simulations, tidal shocks have a stronger effect on the closer orbit than on the more distant orbit, and therefore do not naturally explain the outer-Galaxy mass discrepancy. For the streams Palomar 5 and ATLAS–Aliqa Uma, simulations performed for both low- and high-mass Galactic potentials provide comparably good fits of their morphologies and kinematics. Neither potential provides a uniformly better match to all observables, and their differences are comparable to the present observational and modeling uncertainties. Current stream data do not discriminate between the low- and high-mass Milky Way models over the radial range probed by Palomar 5 and ATLAS–Aliqa Uma. This resolves the apparent tension between the rotation-curve- and stream-based constraints over this radial range.

We investigate galaxies in the Galaxies at All Redshifts Deciphered and Explained with the NIRSpec microshutter array (MSA; GARDEN) survey that exhibit auroral emission lines, enabling spatially resolved measurements of electron temperature and direct oxygen abundances. Two galaxies in this survey have spectra suitable for this analysis: CANDELS 8005 at z = 3.794 and CANDELS 7986 at z = 4.702. For both galaxies, we measure auroral and key nebular emission-line fluxes across their full extent, allowing direct-method oxygen abundance determinations in individual spatial pixels (spaxels). These observations demonstrate the viability of deep JWST/NIRSpec MSA spectroscopy for spatially resolved chemical analyses at high redshift, aided by weak nebular continua and low interstellar extinction. We derive global direct abundances of log(O/H)+12 = 8.008 (+0.025)(-0.027) for CANDELS 8005 and 7.89 (+0.027)(-0.028) for CANDELS 7986. Emission-line diagnostics indicate neither galaxy hosts an active galactic nucleus. A first-order kinematic analysis suggests a potential merger in CANDELS 8005. The direct abundances are consistent with strong-line estimates based on our data and recent high-redshift calibrations. We build emission line, radial velocity, strong-line abundance indices, electron temperature, and direct abundance maps for both galaxies, thanks to the excellent spatial resolution. From the direct abundance maps, we measure linear radial metallicity gradients of -0.111(-0.025)(+0.026 )dex kpc(-1) for CANDELS 8005 (statistically significant) and -0.0928 +/- 0.0880 dex kpc(-1) for CANDELS 7986, where the large uncertainties limit the significance of the result. These results provide a rare direct measurement of a radial metallicity gradient at z > 0 from direct-method abundances, offering key observational support for inside-out galaxy growth with feedback-regulated chemical enrichment.

Several studies have associated globular clusters (GCs) with former Galactic accretion events by comparing their positions in the energy-angular momentum ( E - L z ) plane, an approach further supported by similarities in their age-metallicity relations. However, recent merger simulations suggest that GCs initially associated with the Gaia -Sausage-Enceladus (GSE) disc may have lost their orbital energy and thus may not reliably trace this accretion event. We extend this framework by considering three N-body simulations of the Milky Way-GSE merger with different initial masses, mass ratios, and gas content. In addition to GCs belonging to the GSE disc progenitor, we accounted for GCs in its halo and, in gas-rich models, a population of GCs formed during the Milky Way-GSE merger. We confirm that most GCs originating in the disc have lost a significant part of their orbital energy during repeated passages through the dense disc medium, and we conjecture that associated tidal shocks may have destroyed many of them. In contrast, GCs from the halo and GCs formed during the merger have largely retained their orbital energy, which remains comparable to that of GSE stars even up to 9 Gyr after the completion of the merger. By using a more realistic GC population and GSE modelling, we find that most GCs linked to GSE can be associated with Milky Way accretion events in the E - L z plane, which supports previous observational associations based on a combination of energy-angular momentum and age-metallicity relations.

The discovery of dark matter-free (DM-free) dwarf galaxies in the NGC 1052 neighborhood has had a considerable impact on modern cosmology. The galaxies have been explained through a dwarf–dwarf head-on collision, a rare event. We find that they could alternatively be associated with a head-on 1:1 merger after it was tuned to generate the E4 morphology of NGC 1052. Our simulations show that such mergers produce long-lived tidal features, are associated with the remnant galaxy, and are in the form of large tidal tails, including tidal dwarf galaxies (TDGs). We emphasise that such tidal features are predicted by the hierarchical scenario in which massive galaxies are formed by galaxy mergers. The latter can reproduce both the tidal features in the NGC 1052 outskirts and the observed dwarf galaxies. The simulated TDGs have sizes similar to those observed, while they are ten times smaller in the bullet dwarf scenario. However, we cannot reproduce the luminous globular cluster systems due to resolution limitations. Resolving the radial distance between the DM-free dwarfs is necessary to identify the scenario of their formation. We suggest that there should be many other examples of DM-free dwarf galaxies in the neighbourhood of local massive galaxies and galaxy groups.

Context. The impact of extremely low-density environments such as the diffuse intergalactic medium (IGM) on the neutral gas distribution of dwarf galaxies remains poorly explored observationally. Aims. We present deep MeerKAT H I 21 cm observations of the Local Group dwarf irregular galaxy Sextans B that achieve a spectral resolution of 1.4 km s −1 and reach column-density sensitivities down to N HI ∼ 3.3 × 10 18 cm −2 , allowing us to trace the extended H I disc and faint outer structures with high sensitivity. Methods. We analysed the H I distribution and compared it with the stellar component. Three-dimensional kinematic modelling of the H I cube was performed using TiRiFiC. We performed hydrodynamical simulations tailored to Sextans B, which show that the IGM ram-pressure acting on the outer gas disc can produce asymmetric gas distributions, filamentary structures, and kinematic perturbations. Results. The low-column-density H I distribution is asymmetric and reveals a remarkable filamentary structure arranged in a rosette superposed on the H I disc. A comparison with the stellar distribution shows spatial offsets between the gaseous and stellar components, with the stellar disc remaining relatively symmetric and the H I envelope becoming increasingly disturbed. Three-dimensional kinematic modelling of the H I cube using TiRiFiC reproduces the global velocity gradient but reveals systematic differences between the approaching and receding sides of the rotation curve at large radii, indicating that the outer velocity field departs from simple axisymmetric rotation. While stellar feedback can produce small-scale cavities and turbulence in dwarf galaxies, it cannot generate the H I filamentary structure, the large-scale asymmetric outer H I envelope, or the systematic divergence between the approaching and receding rotation curves observed here. This is consistent with the effects expected from interaction with a diffuse IGM. Conclusions. The combination of morphological and kinematic signatures therefore suggests that the outer H I disc of Sextans B is affected by a ram-pressure interaction with the diffuse IGM in the outskirts of the Local Group. This is the second strong example in the Local Group, after WLM, showing that even a very low-density IGM can significantly influence the gas distribution and kinematics in the outer parts of dwarf galaxies.

The Wolf-Lundmark-Melotte (WLM) galaxy is an archetypal dwarf irregular galaxy that has not experienced interactions with major Local Group galaxies within the past 8 Gyr. It has recently been shown that WLM is losing its gas due to ram pressure forces exerted by the surrounding intergalactic medium. In this work, we explored how ram pressure can also affect the WLM gas kinematics, and we show that its dynamics is especially perturbed at its outskirts, which explains the asymmetric rotation between the approaching and receding sides. Moreover, we have been able to decompose WLM into two main components, a compact one with a solid-body rotation that resembles a bar-like structure, and a more extended one with a characteristic double-horn profile suggesting an edge-on disc. The former is relatively unaffected by ram pressure while the dynamics of the latter is considerably affected. This study shows that mass estimates of a dwarf galaxy like WLM should involve a full modelling of its dynamical components, especially its asymmetric rotation curve.

Several studies have associated globular clusters (GCs) with former Galactic accretion events by comparing their positions in the energy-angular momentum (E-L_z) plane, an approach further supported by similarities in their age-metallicity relations. However, recent merger simulations suggest that GCs initially associated with the Gaia-Sausage-Enceladus (GSE) disc may have lost their orbital energy and thus may not reliably trace this accretion event. We extend this framework by considering three N-body simulations of the Milky Way-GSE merger with different initial masses, mass ratios, and gas content. In addition to GCs belonging to the GSE disc progenitor, we accounted for GCs in its halo and, in gas-rich models, a population of GCs formed during the Milky Way-GSE merger. We confirm that most GCs originating in the disc have lost a significant part of their orbital energy during repeated passages through the dense disc medium, and we conjecture that associated tidal shocks may have destroyed many of them. In contrast, GCs from the halo and GCs formed during the merger have largely retained their orbital energy, which remains comparable to that of GSE stars even up to 9 Gyr after the completion of the merger. By using a more realistic GC population and GSE modelling, we find that most GCs linked to GSE can be associated with Milky Way accretion events in the E-L_z plane, which supports previous observational associations based on a combination of energy-angular momentum and age-metallicity relations.

The ΛCDM paradigm provides a successful framework for the formation of cosmic structures on large scales, yet persistent tensions might remain on galactic and sub-galactic scales. The Milky Way and the Local Universe offer a uniquely accessible laboratory to probe these discrepancies through detailed chemo-dynamical studies of resolved stellar populations. In the era of large photometric and spectroscopic surveys, understanding disk assembly, dynamical heating, and galactoseismology requires not only wide-field coverage but also high-angular-resolution, near-infrared spectroscopy capable of penetrating dust-obscured regions and resolving crowded stellar environments. We propose a set of science cases that exploits the advanced capabilities of SHARP, a multi-mode near-infrared spectrograph designed for Extremely Large Telescopes and future space facilities such as the Habitable World Observatory. Operating over 0.95–2.45μm in a single exposure, SHARP enables efficient access to key stellar and gas tracers across the Galactic bulge, disk, halo, and nearby galaxies. The NEXUS mode provides adaptive-optics-assisted multi-object spectroscopy over a 1.2′×1.2′ field with spectral resolutions up to R∼17,000 for point sources, enabling gas and stellar kinematics. Complementarily, the VESPER multi-IFU mode delivers spatially resolved spectroscopy in crowded and complex regions, allowing the reconstruction of internal kinematics and stellar population gradients on sub-arcsecond scales. By combining SHARP observations with ongoing and upcoming surveys such as Gaia, APOGEE, LAMOST, LSST, 4MOST, and ELT–MOSAIC, this project will map six-dimensional phase-space structures of disk and bulge stars, stellar streams, and nearby disk galaxies (e.g. LMC, SMC). The resulting data will enable detailed investigations of bars, spiral arms, warps, flares, and satellite interactions, providing critical constraints on disk evolution, merger history, and dark matter substructure. These efforts will significantly advance our understanding of the dynamical assembly of the Milky Way and the Local Universe, and provide stringent tests of ΛCDM predictions on the smallest accessible scales.

We use Hubble Space Telescope optical imaging from the Panchromatic Hubble Andromeda Southern Treasury (PHAST) to measure the spatially resolved recent star formation history (SFH) across the southern disk of M31. We fit color-magnitude diagrams (CMDs) of over 6500 individual 0.01 kpc^2 regions to measure SFHs over the last ∼500 Myr. The resulting maps show coherent structure that traces the ringed morphology of the disk. We find a clear global decline in the recent SFR, with a pronounced drop in the last ∼40 Myr that is most evident in the region closest to M32. Combining PHAST and PHAT measurements, we now cover two thirds of M31's star-forming disk with homogeneous SFHs, yielding the highest-resolution spatially resolved SFHs of M31. Inside the joint footprint, we measure mean SFRs of 0.445 ±0.006 M_⊙ yr^-1 over the last 100 Myr and 0.285 ± 0.014 M_⊙ yr^-1 over the last 20 Myr, implying total disk SFRs of ∼0.67 and ∼0.43 M_⊙ yr^-1, respectively. The observed decline is interpreted as the late stage of a multi-Gyr wind-down from a previously more active state. Because recent star formation in M31 is concentrated primarily in the rings, the global decline is driven mainly by decreasing activity within those features. We also compare the CMD-based SFR surface densities to those inferred from FUV+24 μm prescriptions and find that the FUV-based calibration underestimates the CMD-based 100 Myr average by a factor of ∼2.1. However, the PHAST SFHs produce a synthetic GALEX FUV image that agrees well with observations, indicating that the CMD-derived SFHs provide an accurate description of recent star formation. The mismatch with the FUV+24 μm estimates underscores that tracers implicitly averaged over ∼100 Myr are not reliable when the recent SFR is evolving.

A significant part of the Milky Way (MW) dwarf galaxies orbit within a Vast POlar Structure (VPOS), which is perpendicular to the Galactic disc and whose origin has not yet been identified. It includes the Large Magellanic Cloud (LMC) and its six dynamically associated dwarf galaxies. Andromeda Galaxy (M31) experienced a major merger two to three billion years ago, and its accurate modelling predicts that an associated tidal tail is pointing towards the Galaxy. Here, we tested a possible association between M31 tidal tail particles and MW dwarf galaxies, focusing first on the LMC and its associated dwarfs since they are less affected by ram pressure. We traced back these dwarf galaxy orbits by one billion years and calculated their association with the tidal tail particles in the 6D phase space, based on their proper motion from Gaia DR3. We find that for low-mass MW models (total mass less than 5 × 10 11 M ⊙ ), the separation in the 6D space can be less than 1 σ for most of the M31 modelling, albeit with a significant degree of freedom due to the still unknown proper motion of M31. We further discover that many other dwarfs could also be associated with the M31 tidal tails if their motions had been radially slowed, as expected from the ram pressure exerted by the MW corona. This intriguing coincidence could explain the origin of the VPOS, which resulted from a matter exchange between M31 and MW.

The Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) is a "collaboration of collaborations" that is using the Canada-France-Hawai'i Telescope, the Pan-STARRS telescopes, and the Subaru Observatory to obtain $ugriz$ images of a core survey region of 6250 deg$^2$ of the northern sky. The $10\sigma$ point source depth of the data, as measured within a 2-arcsecond diameter aperture, are $[u,g,r,i,z] = [23.7, 24.5, 24.2, 23.8, 23.3]$\ in AB magnitudes. UNIONS is addressing some of the most fundamental questions in astronomy, including the properties of dark matter, the growth of structure in the Universe from the very smallest galaxies to large-scale structure, and the assembly of the Milky Way. It is set to become the major ground-based legacy survey for the northern hemisphere for the next decade and provides an essential northern complement to the static-sky science of the Vera C. Rubin Observatory's Legacy Survey of Space and Time. UNIONS supports the core science mission of the {\it Euclid} space mission by providing the data necessary in the northern hemisphere for the calibration of the wavelength dependence of the {\it Euclid} point-spread function and derivation of photometric redshifts in the North Galactic Cap. This region contains the highest quality sky for {\it Euclid}, with low backgrounds from the zodiacal light, stellar density, extinction, and emission from Galactic cirrus. Here, we describe the UNIONS survey components, science goals, data products, and the current status of the overall program.

Recent observations from the ESA Gaia satellite and with the ESO VLT, have identified the presence of a population of young, 0.5 to 2 Gyr old, stars in the halo and in dwarf spheroidal galaxies surrounding the Milky Way. It suggests that MW dwarf galaxies, currently devoid of gas, had, until recent times, enough gas to sustain a burst of star formation. The recent loss of gas coincides with their arrival in the vicinity of the Milky Way, in agreement with orbital predictions from Gaia that indicate that most dwarf galaxies reached the Milky Way halo less than 3 Gyr years ago. This completely changes the interpretation of their dynamics, mass, and dark matter content.

The timing argument (TA) aims to find the total mass of the Local Group (LG) from the relative motions of the Milky Way (MW) and Andromeda galaxy (M31). However, the classical TA always overestimates the LG mass, presumably because it does not account for the hierarchical scenario and other interactions, such as that with the Large Magellanic Cloud (LMC). We focused on the impact of M31's recent major merger by using three merger models to find the peculiar motion of M31 within the simple two-body, point-mass scenario of the TA. We find that the merger correction affects the TA mass by plus or minus 10-15% depending on the M31 tangential motion, which has very large uncertainties. If we consider an M31 merger configuration that reduces the TA mass by 10-15%, to which we add the impact due to the LMC infall into the MW as reported in the literature, the TA mass is consistent with the LG mass from Hubble-Lema & icirc;tre flow. Galaxies are believed to have experienced about 16 major mergers each since z = 11.5. Assuming all these mergers had a similar impact on the TA mass as the most recent M31 merger, the ratio of LG mass to TA mass would be 0.85-0.37+0.65 0 . 85 - 0.37 + 0.65 $ 0.85<^>{+0.65}_{-0.37} $ , and such a TA mass is consistent with all the LG mass estimates. Our result also agrees with findings that used LG analogues in cosmological simulations. We find that the TA mass estimate is limited by the hierarchical scenario, since it is not possible to track the progenitors of both the MW and M31 through so many mergers. We conclude that the MW-M31 dynamical system is far too complex to be modelled as a simple two-body point-mass system.

The Panchromatic Hubble Andromeda Southern Treasury (PHAST) is a large 195-orbit Hubble Space Telescope program imaging ∼0.45 deg 2 of the southern half of M31's star-forming disk at optical and near-ultraviolet (NUV) wavelengths. The PHAST survey area extends the northern coverage of the Panchromatic Hubble Andromeda Treasury (PHAT) down to the southern half of M31, covering out to a radius of ∼13 kpc along the southern major axis and in total ∼two-thirds of M31's star-forming disk. This new legacy imaging yields stellar photometry of over 90 million resolved stars using the Advanced Camera for Surveys in the optical (F475W and F814W), and the Wide Field Camera 3 (WFC3) in the NUV (F275W and F336W). The photometry is derived using all overlapping exposures across all bands, and achieves a 50% completeness-limited depth of F475W ∼ 27.7 in the lowest surface density regions of the outer disk and F475W ∼ 26.0 in the most crowded, high surface brightness regions near M31's bulge. We provide extensive analysis of the data quality, including artificial star tests to quantify completeness, photometric uncertainties, and flux biases, all of which vary due to the background source density and the number of overlapping exposures. We also present seamless population maps of the entire M31 disk, which show relatively well-mixed distributions for stellar populations older than 1–2 Gyr, and highly structured distributions for younger populations. The combined PHAST + PHAT photometry catalog of ∼0.2 billion stars is the largest ever produced for equidistant sources and is available for public download by the community.

Mass estimates of a spiral galaxy derived from its rotation curve must account for the galaxy’s past accretion history. There are several lines of evidence indicating that M31 experienced a major merger 2 to 3 Gyr ago. In this work, we generated a dynamical model of M31 as a merger remnant that reproduces most of its properties, including from the central bar to the outskirts. The model accounts for M31’s past major merger and reproduces the details of its rotation curve, including its 14 kpc bump and the observed increase of velocity beyond 25 kpc. We find non-equilibrium and oscillatory motions in the gas of the merger-remnant outskirts caused by material in a tidal tail returning to the merger remnant. A total dynamical M31 mass of 4.5 × 10 11 M ⊙ within 137 kpc was obtained after scaling it to the observed HI rotation curve. Within this radial distance, we find that 68% of the total dynamical mass is dark.

The detection of low-surface-brightness galaxies beyond the Local Group poses significant observational challenges, yet these faint systems are fundamental to our understanding of dark matter, hierarchical galaxy formation, and cosmic structure. Their abundance and distribution provide crucial tests for cosmological models, particularly regarding the small-scale predictions of ΛCDM. We present a systematic detection and classification framework for unresolved dwarf galaxy candidates in the large-scale Ultraviolet Near Infrared Optical Northern Survey (UNIONS) imaging data. The main survey region covers 4861 deg 2 . Our pipeline preprocesses UNIONS data in three ( gri ) of the five bands ( ugriz ), including binning, artifact removal, and stellar masking before employing the software MTOBJECTS (MTO) to detect low-surface-brightness objects. Following a set of parameter cuts using known dwarf galaxies from the literature and cross-matching between the three bands, we were left with an average of ∼360 candidates per deg 2 . With ∼4000 deg 2 in g , r and i , this amounts to ∼1.5 million candidates that form our GOBLIN (Galaxies OBserved as Low-luminosity Identified Nebulae) catalog. For the final classification of these candidates, we finetuned the deep learning model Z OOBOT , which was pretrained based on labels from the Galaxy Zoo project. We created our training dataset by visually inspecting dwarf galaxy candidates from existing literature catalogs within our survey area and assigning probability labels based on averaged expert assessments. This approach captures both consensus and uncertainty among experts. When applied to all detected MTO objects, our method identified 42 965 dwarf galaxy candidates with probability scores of >0.8, of which 23 072 have probabilities exceeding 0.9. The spatial distribution of high-probability candidates reveals a correlation with the locations of massive galaxies (log ( M ∗ / M ⊙ )≥ 10) within 120 Mpc. While some of these objects may have been previously identified in other surveys, we present this extensive catalog of candidates, including their positions, structural parameter estimates, and classification probabilities, as a resource for the community to enable studies of galaxy formation, evolution, and the distribution of dwarf galaxies in different environments.

Primordial dark matter halos are well understood from cold dark matter-only simulations. Since they can contract significantly as baryons settle into their centers, direct comparisons with observed galaxies are complicated. We present an approach to reversing the halo contraction by numerically calculating the halo response to baryonic infall and iterating the initial condition. This allowed us to derive spherically averaged primordial dark matter halos for observed galaxies. We applied this approach to the Milky Way and found that the latest Gaia measurements for the rotation velocities imply an odd primordial Galactic halo: Its concentration and total mass differ by more than 3σ from the predictions, and the density profile presents an inner core that is too shallow and an outer decline that is too steep to be compatible with the cold dark matter paradigm.

We present the discovery of Ursa Major III/UNIONS 1, the least luminous known satellite of the Milky Way, which is estimated to have an absolute V-band magnitude of $+2.2^{+0.4}_{-0.3}$ mag, equivalent to a total stellar mass of 16$^{+6}_{-5}$ M$_{\odot}$. Ursa Major III/UNIONS 1 was uncovered in the deep, wide-field Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and is consistent with an old ($\tau > 11$ Gyr), metal-poor ([Fe/H] $\sim -2.2$) stellar population at a heliocentric distance of $\sim$ 10 kpc. Despite being compact ($r_{\text{h}} = 3\pm1$ pc) and composed of so few stars, we confirm the reality of Ursa Major III/UNIONS 1 with Keck II/DEIMOS follow-up spectroscopy and identify 11 radial velocity members, 8 of which have full astrometric data from $Gaia$ and are co-moving based on their proper motions. Based on these 11 radial velocity members, we derive an intrinsic velocity dispersion of $3.7^{+1.4}_{-1.0}$ km s$^{-1}$ but some caveats preclude this value from being interpreted as a direct indicator of the underlying gravitational potential at this time. Primarily, the exclusion of the largest velocity outlier from the member list drops the velocity dispersion to $1.9^{+1.4}_{-1.1}$ km s$^{-1}$, and the subsequent removal of an additional outlier star produces an unresolved velocity dispersion. While the presence of binary stars may be inflating the measurement, the possibility of a significant velocity dispersion makes Ursa Major III/UNIONS 1 a high priority candidate for multi-epoch spectroscopic follow-ups to deduce to true nature of this incredibly faint satellite.

Dwarf spheroidal galaxies are known to be dominated by old stellar populations. This has led to the assumption that their gas-rich progenitors lost their gas during their infall in the Milky Way (MW) halo at distant look-back times. Here, we report a discovery of a tiny but robustly detected population of possibly young (∼1 Gyr old) and intermediate-mass (1.8 M⊙ ≤ M < 3 M⊙) stars in MW dwarf spheroidal galaxies. This was established on the basis of their positions in color–magnitude diagrams, after filtering out the bulk of the foreground MW using Gaia DR3 proper motions. We have considered the possibility that this population is made of evolved blue stragglers. For Sculptor, it seems unlikely, because 95.5% of its stars are older than 8 Gyr, leading to masses smaller than 0.9 M⊙. This would only allow blue straggler masses of less than 1.8 M⊙, which is much lower than what we observed. Alternatively, it would require the merger of three turnoff stars, which appears even more unlikely. On the other hand, the recent Gaia proper motion measurements of MW dwarf galaxies infer their low binding energies and large angular momenta, pointing to a more recent, ≤3 Gyr, infall. Although the nature of the newly discovered stars still needs further confirmation, we find that they are consistent with the recent infall of the dwarf galaxies into the MW halo, when star formation occurred from the ram pressurization of their gas content before its removal by the hot Galactic corona. The abundance of this plausibly young population of stars is similar to the expectations drawn from hydrodynamical simulations. These results point to a novel origin for MW dwarf spheroidal galaxies.

Galactic halos are known to grow hierarchically, inside out. This implies a correlation between the infall lookback time of satellites and their binding energy. Cosmological simulations predict a linear relation between the infall lookback time and the logarithm of the binding energy, with a small scatter. Gaia measurements of the bulk proper motions of globular clusters and dwarf satellites of the Milky Way are sufficiently accurate to establish the kinetic energies of these systems. Assuming the gravitational potential of the Milky Way, we can deduce the binding energies of the dwarf satellites and those of the galaxies that were previously accreted by the Milky Way. This can be compared to cosmological simulations for the first time. The relation of the infall lookback time versus binding energy we found in a cosmological simulation matches that for the early accretion events when the simulated total Milky Way mass within 21 kpc was rescaled to 2 x 10(11) M-circle dot. This agrees well with previous estimates from globular cluster kinematics and from the rotation curve. However, the vast majority of the dwarf galaxies are clear outliers to this rescaled relation, unless they are very recent infallers. In other words, the very low binding energies of most dwarf galaxies compared to Sgr and previous accreted galaxies suggests that most of them were accreted much later than 8 or even 5 Gyr ago. We also found that the subhalo systems in some cosmological simulations are too dynamically hot when they are compared to identified Milky Way substructures. This leads to an overestimated impact of satellites on the Galaxy rotation curve.