Nikolay Kacharov

Context . The central regions of galaxies pose challenges for observers due to their high density of stars, gas, and dust. The Large Magellanic Cloud (LMC) hosts a dense stellar bar, whereas the Small Magellanic Cloud (SMC) hosts a dense core. Both of these dense centres have eluded even the most sensitive large-scale wide-field surveys of individual stars from the ground and from space. Aims . This work describes deep imaging observations obtained with the Visual and Infrared Survey Telescope for Astronomy (VISTA) of the inner regions of the Clouds. They were obtained to resolve stars fainter than the red clump, which due to crowding were missed in previous observations, and to quantify features associated with the star formation, kinematics, and structure of the galaxies. These studies will allow us to establish how the dynamical interactions between the LMC and the SMC, and the interaction between the Clouds and the Milky Way, have influenced the most gravitationally bound regions. Methods . Multiple near-infrared images were obtained in the J and K _s filters during excellent weather conditions (e.g. with a seeing of ∼0.7 arcsec). The data were reduced using the VISTA Data Flow System, which provides single-filter and band-merged calibrated images and catalogues, with associated confidence maps, for individual observations and combined observations for deep tiles. Results . Approximately one million and 920 thousand sources, with only minor quality issues (i.e. by excluding sources with a problematic photometric calibration or location on the detectors) and about 40 epochs per filter, are detected in the central 1.77 deg ² region of the LMC and SMC, respectively. They provide a more contiguous spatial area coverage than sources in the VISTA survey of the Magellanic Clouds system (VMC), which covers the same regions by overlapping two VISTA tiles per galaxy. Two thirds of these sources are likely stars, whereas the other one third could be stellar blends or background galaxies. The catalogues, which are made available with this work, contain parameters to select unique good-quality detections for which we provide examples of ongoing scientific applications. Conclusions . These data represent valuable complementary information to deep imaging observations at other wavelengths and provide additional epochs for sources in common with the VMC survey, for proper motion and photometric variability studies extending the time baseline to ∼11 yr, as well as targets for spectroscopic observations.

Omega Centauri ( ω Cen) is the most massive and chemically complex star cluster in the Milky Way (MW) and is widely interpreted as the surviving nuclear star cluster of an accreted dwarf galaxy, although the identity of its parent remains uncertain. We test whether the merger remnants Sequoia, Thamnos, and Gaia–Enceladus (GE) originated in a single progenitor, the ω Dwarf, whose nucleus survives as ω Cen. Combining APOGEE and GALAH abundances with Gaia astrometry, we reconstruct the progenitor’s chemical structure using orbital energy as a proxy for premerger radius. The chemically evolved, younger, Al–N-rich population is strongly concentrated toward the center, consistent with formation during or after the merger. By contrast, the primordial population exhibits dwarf-galaxy-like chemistry across all components, supporting a common origin. Its metallicity profile follows an inverted U-shaped radial trend similar to those observed in present-day nucleated dwarf galaxies. The inner regions, traced by ω Cen and Thamnos, are also more α -enhanced than the outskirts, implying shorter and more efficient star formation and supporting a scenario in which the primordial nucleus assembled partly through the inspiral and merger of globular clusters. Although the chemical evidence permits GE to be associated with ω Dwarf, Sequoia and Thamnos alone form a coherent outside-in stripping sequence around ω Cen. Excluding GE yields a mass–metallicity-relation estimate of 6 . 8 − 3.7 + 8.3 × 1 0 8 M ⊙ for ω Dwarf, whereas including GE increases the inferred mass by a factor of ∼3.4, to 2 . 3 − 1.5 + 4.4 × 1 0 9 M ⊙ . Both estimates are consistent with the nuclear-star-cluster–host-galaxy mass relation, identifying Sequoia and Thamnos as the most robust surviving debris of the ω Cen progenitor.

Context. The Small Magellanic Cloud (SMC) exhibits significant kinematic disequilibrium due to interactions with the Large Magellanic Cloud (LMC). Aims. Here, we investigate the two-dimensional stellar kinematics of the SMC to understand the dynamical effects of these interactions by exploiting the increased time baseline of 6−11 years from the VISTA Survey of the Magellanic Clouds (VMC) data release 7. Methods. We derive proper motions with a threefold improvement in precision compared to previous studies based on VMC data. We used a geometric framework accounting for perspective effects from line-of-sight motion to model the systemic motion across the SMC and construct a residual proper motion map. We further introduce an anisotropic linear velocity gradient model to quantify the stretching of the galaxy. Results. For the first time across all stellar populations, the residual proper motion map reveals expansion along the south-east and north-west directions, consistent with LMC-induced tidal forces, detectable even in the central regions. The gradient-corrected residuals show predominantly radial motions towards the SMC centre with no evidence of rotation. Velocity maps for different stellar populations, without assuming a rotating-disk model, reveal a coherent northward motion away from the centre exclusively in older red giant branch stars, interpreted as a kinematic signature of a past (> 2 Gyr ago) interaction. Conclusions. This study highlights the inadequacy of simple rotating-disk models in capturing the internal kinematics of the galaxy.

We present a detailed analysis of the structural parameters of the globular cluster M15 using g- and i-band photometric data from Pan-STARRS1 DR2. The central coordinates (X_C,Y_C), ellipticity (ϵ), and position angle (PA) are derived via two independent methods: ellipse fitting of the two-dimensional stellar number isodensity distribution and Markov Chain Monte Carlo (MCMC) sampling. Our analysis of 38 stellar density intervals reveals a cluster center offset by only 4.′′1 ± 9.′′6 from the commonly accepted in the literature value, a good agreement on the order of our map resolution. We find a radial variation in the ellipticity, with a mean value of ϵ = 0.09 ± 0.02 for the inner region (R ≤ 4.′5) and ϵ = 0.04 ± 0.02 for the outer region (R > 4.′5), where the errors correspond to 1σ. The MCMC analysis of 75 datasets yields a mean ϵ = 0.022 ± 0.005 for the entire cluster. The PA remains constant with increasing distance from the cluster center, PA = 44.°4 ± 16.°2, and the MCMC method providing a consistent value of PA = 46.°6 ± 7.°1. Our results are in agreement with some recent studies but challenge others, suggesting that a single ϵ value may be insufficient to fully characterize the overall oblateness of M15 due to incompleteness and crowding effects in its core.

In this paper, we explore the kinematic properties of a sample of 19 young ( < 1 Gyr) and intermediate-age (1–2.5 Gyr) massive star clusters within the Large Magellanic Cloud (LMC). We analysed the proper motions of the clusters, which have been measured based on multi-epoch Hubble Space Telescope (HST) observations. Additionally, from the HST data we inferred homogeneous and robust estimates for the distances, ages, and metallicities of the clusters. This collection of information, in combination with literature line-of-sight velocities, allowed us to investigate the full 3D dynamics of our sample of clusters within the frame of the LMC in a self-consistent way. While most young clusters orbit the LMC close to the stellar disc plane, NGC 1850 (∼100 Myr old) depicts a peculiar case. Depending on the exact distance from the disc, it follows either a highly inclined retrograde orbit or an eccentric orbit along the bar structure. The orbits of young clusters that formed north of the LMC centre show signs that might be connected to the resettling motion of the LMC bar structure. Based on the dynamic properties in combination with the positions of the clusters in the age-metallicity space, we find no clear-cut evidence of clusters in our sample that could have been stripped from the Small Magellanic Cloud (SMC) onto the LMC. Finally, we compared the kinematics of the intermediate-age clusters with a suite of simple numerical simulations of the Magellanic system to interpret the cluster motions. A possible interaction history of the LMC with the SMC, where the SMC had two past crossings of the LMC disc plane (about 300 and 900 Myr ago), in combination with the recent SMC pericentre passage, can qualitatively explain the observed kinematic structure of the clusters analysed in this work.

In this study, we report conclusive evidence for an ancient star cluster that has been accreted by the Large Magellanic Cloud (LMC). By leveraging observations from the Hubble Space Telescope (HST), we investigate the chrono-dynamical structure of a sample of seven old star clusters within the LMC in a self-consistent way. The multi-epoch nature of the dataset allowed the determination of high-precision proper motions for the clusters. Employing an isochrone-fitting methodology, we additionally infer from the deep high-resolution HST data homogeneous and robust estimates for their distances, ages and metallicities. Supplementing these data with literature line-of-sight velocities, we investigate the full 3-dimensional dynamics of the clusters within the frame of the LMC. With respect to the other clusters in our sample, NGC 1841 depicts a peculiar case. Its position in the age-metallicity plane, that makes it about 1 Gyr younger than the other metal-poor LMC clusters, but also its dynamical properties with a radial orbit almost perpendicular to the LMC disc plane, clearly advocates for a different origin. We thus conclude that NGC 1841 has likely been accreted by the LMC from a smaller galaxy. The other clusters in our sample show disc-like kinematics, with the case of NGC 2210 being peculiar, based on its inclined orbit. Their coherent age-metallicity relation closely resembles that of Gaia-Sausage-Enceladus globular clusters, thus suggesting a similar early evolution for the two dwarf galaxies. We do not find clear-cut chrono-kinematic evidence that NGC 2005 has been accreted by the LMC as suggested by a previous study based on its chemical abundance pattern. Regardless of its nature, its very old age illustrates that peculiar chemical evolutions already emerge at very early times.

Context. The study of the internal kinematics of galaxies provides insights into their past evolution, current dynamics, and future trajectory. The Large Magellanic Cloud (LMC), as the largest and one of the nearest satellite galaxies of the Milky Way (MW), presents unique opportunities to investigate these phenomena in great detail. Aims. We aim to investigate the internal kinematics of the LMC by deriving precise stellar proper motions using data from the VISTA survey of the Magellanic Clouds system (VMC). The main objective is to refine the LMC’s dynamical parameters using improved proper motion measurements exploiting the additional epochs of observations from the VMC survey. Methods. We utilised high-precision proper motion measurements from the VMC survey, leveraging an extended time baseline from approximately 2 to 10 years. This extension significantly enhanced the precision of the proper motion data, reducing uncertainties from 6 mas yr⁻¹ in prior studies using the VMC dataset to 1.5 mas yr⁻¹. Using this data, we derived geometrical and kinematic parameters, and generated velocity maps and rotation curves in the LMC disc plane and the sky plane, for both young and old stellar populations. Finally, we compared a suite of dynamical models that simulate the interaction of the LMC with the MW and Small Magellanic Cloud (SMC), against the observations. Results. The tangential rotation curve reveals an asymmetric drift between young and old stars, while the radial velocity curve for the young population shows an increasing trend within the inner bar region, suggesting non-circular orbits. The internal rotation map confirms the clockwise rotation around the dynamical centre of the LMC, which is consistent with previous predictions. A significant residual motion was detected towards the north-east of the LMC, directed away from the centre. This feature observed in the inner disc region is kinematically connected with a substructure identified in the periphery known as Eastern Substructure 1. This motion of the LMC sources suggests a possible tidal influence from the MW, combined with the effects of the recent close pericentre passage of the SMC ∼150 Myr ago.

Constraining the helium enhancement in stars is critical for understanding the formation mechanisms of multiple populations in star clusters. However, measuring helium variations for many stars within a cluster remains observationally challenging. We use Hubble Space Telescope photometry combined with MUSE spectroscopic data for over 7200 red giant branch stars in ωCen to measure helium differences between distinct groups of stars as a function of metallicity, separating the impact of helium enhancements from other abundance variations on the pseudo-color (chromosome) diagram. Our results show that stars at all metallicities have subpopulations with significant helium enhancement (ΔY _min ≳ 0.11). We find a rapid increase in helium enhancement across low metallicities ([Fe/H] ≃ −2.05 to [Fe/H] ≃ −1.92), with this enhancement leveling out at ΔY = 0.15 at higher metallicities. The fraction of helium-enhanced stars steadily increases with metallicity from 10% at [Fe/H] ≃ −2.04 to over 90% at [Fe/H] ≃ −1.04. From these measurements, we calculate the total mass in helium produced over the cluster’s lifetime, finding M He tot ∼ 2.7 × 1 0 5 M ⊙ . This study is the first to examine helium enhancement across the full range of metallicities in ωCen, providing new insight into its formation history and additional constraints on enrichment mechanisms.

Omega Centauri (ω Cen) is the Milky Way’s most massive globular cluster and is likely the stripped nucleus of an accreted dwarf galaxy. In this paper, we analyze ω Cen’s kinematics using data from oMEGACat, a comprehensive catalog of ω Cen’s central regions, including 1.4 million proper motion measurements and 300,000 spectroscopic radial velocities. Our velocity dispersion profiles and kinematic maps are consistent with previous work but improve on their resolution, precision, and spatial coverage. The cluster’s 3D dispersion is isotropic in the core, with increasing radial anisotropy at larger radii. The 2D kinematic maps show an elongation of the velocity dispersion field comparable to the flattening observed photometrically. We find good agreement between proper motions and line-of-sight velocity dispersion, and we measure a kinematic distance of 5494 ± 61 pc, the most precise kinematic distance to ω Cen available. The subset of data with precise metallicity measurements shows no correlation between metallicity and kinematics, supporting the picture of well-mixed stellar populations within the half-light radius of ω Cen. Finally, we study the degree of energy equipartition using a large range of stellar masses. We find partial energy equipartition in the center that decreases towards large radii. The spatial dependence of the radial energy equipartition is stronger than the tangential energy equipartition. Our kinematic observations can serve as a new reference for future dynamical modeling efforts that will help to further disentangle the complex mass distribution within ω Cen.

We introduce a novel orbit superposition method designed to reconstruct the stellar density structure, kinematics, and chemical abundance distribution of the entire Milky Way by leveraging 6D phase-space information from its resolved stellar populations, limited by the spatial coverage of APOGEE DR17.