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.

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.

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.

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.

We report a new high-sensitivity H i mapping observation of the NGC 5055 galaxy group over an area of 1.degrees 5 x 0.degrees 75 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Our observation reveals that the warped H i disk of NGC 5055 is more extended than what was previously observed by WSRT, out to 23.' 9 (61.7 kpc). The total H i mass of NGC 5055 is determined to be similar to 1.1 x 10(10) M-circle dot. We identified three H i clouds with H i masses of the order of similar to 10(7) M-circle dot at the southeastern edge of the H i disk, as well as a candidate high-velocity cloud with an H i mass of (1.2 +/- 0.5) x 10(6) M-circle dot to the north of NGC 5055. The H i content of UGCA 337 is robustly detected for the first time by the FAST observations. It has a narrow H i linewidth of W 50 = 17.4 +/- 3.8 km s-1 with a total H i mass of (3.5 +/- 0.3) x 10(6) M-circle dot. Comparing the gas content and g-r color of UGCA 337 with typical low-mass dwarf galaxies, UGCA 337 appears relatively gas-poor despite its blue color. This suggests that UGCA 337 may have undergone gas stripping in the past. We also analyzed the possible origin of the diffuse H i clouds located at the outskirts of NGC 5055, and speculate that they might be the remnant features of a merger event in the past.

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.

MOSAIC is the Multi-Object Spectrograph (MOS) for the 39m Extremely Large Telescope (ELT) of the European Southern Observatory (ESO), with unique capabilities in terms of multiplex, wavelength coverage and spectral resolution. It is a versatile multi-object spectrograph working in both the Visible and NIR domains, designed to cover the largest possible area (similar to 40 arcmin(2)) on the focal plane, and optimized to achieve the best possible signal-to-noise ratio on the faintest sources, from stars in our Galaxy to galaxies at the epoch of the reionization. In this paper we describe the main characteristics of the instrument, including its expected performance in the different observing modes. The status of the project will be briefly presented, together with the positioning of the instrument in the landscape of the ELT instrumentation. We also review the main expected scientific contributions of MOSAIC, focusing on the synergies between this instrument and other major ground-based and space facilities.

Previous studies have revealed that the Galactic warp is a long-lived, nonsteady, and asymmetric structure. There is a need for a model that accounts for the warp's long-term evolution. Given that this structure has persisted for over 5 Gyrs, its timeline may coincide with the completion of Gaia-Sausage-Enceladus (GSE) merger. Recent studies indicate that the GSE, the significant merger of our Galaxy, was likely a gas-rich merger and the large amount of gas introduced could have created a profound impact on the Galactic morphology. This study utilizes GIZMO simulation code to construct a gas-rich GSE merger. By reconstructing the observed characteristics of the GSE, we successfully reproduce the disk warp and capture nearly all of its documented features that aligns closely with observational data from both stellar and gas disks. This simulation demonstrates the possibility that the single major merger could generate the Galactic warp amplitude and precession. Furthermore, the analysis of the warp's long-term evolution may offer more clues into the formation history of the Milky Way.

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 (similar to 1 Gyr old) and intermediate-mass (1.8 M-circle dot <= M < 3 M-circle dot) 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-circle dot. This would only allow blue straggler masses of less than 1.8 M-circle dot, 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.

We use the rotation curve from Gaia data release (DR) 3 to estimate the mass of the Milky Way. We consider an Einasto density profile to model the dark matter component. We extrapolate and obtain a dynamical mass $M=2.75^{+3.11}_{-0.48}\times 10^{11} M_\odot$ at $112$ kpc. This lower-mass Milky Way is consistent with the significant declining rotation curve, and can provide new insights into our Galaxy and halo inhabitants.

Most Milky Way dwarf galaxies are much less bound to their host than are relics of Gaia-Sausage-Enceladus and Sgr. These dwarfs are expected to have fallen into the Galactic halo less than 3 Gyr ago, and will therefore have undergone no more than one full orbit. Here, we have performed hydrodynamical simulations of this process, assuming that their progenitors are gas-rich, rotation-supported dwarfs. We follow their transformation through interactions with the hot corona and gravitational field of the Galaxy. Our dedicated simulations reproduce the structural properties of three dwarf galaxies: Sculptor, Antlia II and, with somewhat a lower accuracy, Crater II. This includes reproducing their large velocity dispersions, which are caused by ram-pressure stripping and Galactic tidal shocks. Differences between dwarfs can be interpreted as due to different orbital paths, as well as to different initial conditions for their progenitor gas and stellar contents. However, we failed to suppress in a single orbit the rotational support of our Sculptor analog if it is fully dark-matter dominated. In addition, we have found that classical dwarf galaxies like Sculptor may have stellar cores sufficiently dense to survive the pericenter passage through adiabatic contraction. On the contrary, our Antlia II and Crater II analogs are tidally stripped, explaining their large sizes, extremely low surface brightnesses, and velocity dispersion. This modeling explains differences between dwarf galaxies by reproducing them as being at different stages of out-of-equilibrium stellar systems.

We study how structural properties of globular clusters and dwarf galaxies are linked to their orbits in the Milky Way halo. From the inner to the outer halo, orbital energy increases and stellar-systems gradually move out of internal equilibrium: in the inner halo, high-surface brightness globular clusters are at pseudo-equilibrium, while further away, low-surface brightness clusters and dwarfs appear more tidally disturbed. Dwarf galaxies are the latest to arrive into the halo as indicated by their large orbital energies and pericentres, and have no time for more than one orbit. Their (gas-rich) progenitors likely lost their gas during their recent arrival in the Galactic halo. If dwarfs are at equilibrium with their dark matter (DM) content, the DM density should anticorrelate with pericentre. However, the transformation of DM dominated dwarfs from gas-rich rotation-supported into gas-poor dispersion-supported systems is unlikely accomplished during a single orbit. We suggest instead that the above anticorrelation is brought by the combination of ram-pressure stripping and of Galactic tidal shocks. Recent gas removal leads to an expansion of their stellar content caused by the associated gravity loss, making them sufficiently fragile to be transformed near pericentre passage. Out of equilibrium dwarfs would explain the observed anticorrelation of kinematics-based DM density with pericentre without invoking DM density itself, questioning its previous estimates. Ram-pressure stripping and tidal shocks may contribute to the dwarf velocity dispersion excess. It predicts the presence of numerous stars in their outskirts and a few young stars in their cores.