MercÈ Romero-GÓMez

Context . Several theories have attempted to explain the formation, evolution, and overall nature of spiral arms in barred galaxies. However, although some are supported by theoretical arguments or numerical simulations, none has been fully conclusive. One such framework is invariant manifolds theory, which links spiral structure to the orbital dynamics associated with the saddle-unstable Lagrangian points. In particular, it has been shown that the fraction of particles trapped by the exterior unstable manifold branches closely follows the temporal evolution of spiral-arm strength. Aims . We investigated the origin of the weakening of strong spiral arms, seeking to determine the physical mechanism responsible for the decline in the fraction of trapped particles over time. Methods . We reconstructed the gravitational potential of a fully self-consistent N -body simulation of a barred galaxy using AGAMA , computed the Lagrangian points, and quantified how their spiral-induced angular displacement affects the geometry of the invariant manifolds and the associated orbital flows. Results . We find that sufficiently strong, self-gravitating spiral arms significantly reshape the gravitational potential of a barred galaxy, displacing the equilibrium points up to 20–50° from the bar major axis, where they are expected to lie in the standard invariant-manifold framework. This shift alters the configuration of the manifold branches and disrupts the coherent stellar flows that sustain the spiral structure. As the spiral weakens, the equilibrium points gradually return to their standard configuration, restoring the conditions for manifold-driven spiral arm formation on a timescale of ∼200 Gyr. Conclusions . We show for the first time that invariant manifolds and the spiral structures they generate are coupled through a self-regulating feedback mechanism, allowing the spiral pattern to recur over time. Our results therefore indicate that, although barred galaxies may sustain spiral activity over long timescales, individual episodes of strong spiral arms are intrinsically transient.

Context. Many barred galaxies, including the Large Magellanic Cloud (LMC), display strong lopsidedness and off-centre bars. The dynamical connection between bar–disc misalignments, internal-mass asymmetries, and arm morphology has not yet been fully characterised. Aims. We aim to investigate how internal-mass imbalances within the bar and global offsets between the bar and the centre of mass (CM) of the system modify the equilibrium-point structure and the invariant manifolds that organise arms. Methods. We constructed a barred galaxy model that includes an off-centred and asymmetrically shaped bar. Using numerical continuation, we tracked the position, stability, and bifurcations of the Lagrangian equilibrium points as functions of the displacement of the asymmetric mass component along the bar and of the offset between the bar and the system’s CM. For representative configurations we computed the invariant manifolds of planar Lyapunov orbits around unstable points and analysed the resulting arm structures. Results. Internal bar lopsidedness and modest bar–disc offsets that keep the CM inside the bar preserve the classical configuration with five equilibrium points, but strongly distort the associated invariant manifolds, producing two arms with different densities and shapes. The bar–disc offset reaches a threshold at the point at which the galactic CM exits the bar ellipsoid, in which a pitchfork bifurcation removes the collinear unstable points and the system transitions to a three-equilibrium-point configuration; there, a single unstable point and its associated manifold support one arm. Conclusions. Internal inhomogeneities in the bar’s density and modest bar–disc offsets dynamically support two asymmetric arms, while larger offsets drive a transition to one-armed structures. This framework is compatible with the observed correlation between off-centre bars and photometric lopsidedness, and it provides a dynamical explanation for the strongly asymmetric arm morphology of galaxies such as the LMC.

Understanding the nature of spiral arms in disc galaxies remains an open problem. Invariant manifolds associated with bars provide a framework in which particles with specific kinematic properties (manifold-compatible) are naturally able to support spiral structure, although their contribution to the observed total overdensities is yet to be quantified. The main goal of this work is to quantify, through a robust methodology, the contribution of invariant manifolds to the formation of spiral arms in a pure N-body simulation, setting up a machinery to perform similar tests in other and more complex simulations. We computed the invariant manifolds associated with the hyperbolic equilibrium points of the potential and quantified the fraction of particles whose motion is governed by these phase-space structures. We then compared the temporal evolution of this trapped fraction with the strength of the spiral arms, traced by the A_2 Fourier amplitude. We find a correlation between the fraction of trapped particles in the unstable exterior branches of the invariant manifolds and the strength of spiral arms. In particular, we determine that up to 50% of all the particles located on the spiral arms region (and up to 90% from the manifold-compatible population) are trapped by the manifolds, with oscillations over a period of ∼ 100 Myr. Invariant manifolds provide a dynamically relevant framework for understanding the formation of spiral structure in pure N-body simulations of barred galaxies. We present the first quantitative evidence --based on a fully self-consistent N-body model-- that a significant fraction of spiral-arm particles are governed by manifold-driven dynamics. These particles act as seeds of overdensities that subsequently evolve into fully developed spiral arms through the delayed gravitational response of the disc to the self-gravity of the manifold-trapped material. The influence of the invariant manifolds remains non-negligible at all times, and phases of stronger spiral structure are associated with higher trapped fractions.

Context. The origin and nature of spiral arms in barred galaxies are still under debate. Among others, the theory of invariant manifolds provides a framework capable of explaining spiral and ringed structures in these systems. Aims. We investigated the dynamical origin and kinematic signatures of spiral structure in an N -body simulation of an isolated barred galaxy to assess whether invariant manifold theory provides a consistent dynamical framework to disentangle the disc particle populations and identify those that genuinely build, trace, and sustain the spiral arms. Methods. For each simulation snapshot, we measured the bar and spiral properties and reconstructed the corresponding gravitational potential using AGAMA . We computed the Jacobi energy of disc particles and classified them relative to the energies of the equilibrium points, thereby isolating manifold-compatible orbits. We analysed their spatial distribution and velocity structure to characterise spiral-related streaming motions. Results. The Jacobi constant provides a physically motivated dynamical separator that reveals three distinct kinematic populations: (i) low-energy particles on nearly circular orbits populating most of the disc (∼60%), (ii) high-energy particles associated with banana orbits (≲10%), and (iii) manifold-compatible particles (∼30–40%) originating near the bar and following transit orbits along the spiral arms. Only the manifold-compatible population generates the prominent outward-migrating ridge observed in the R − v _φ plane and reproduces the characteristic spiral streaming pattern. In contrast, the low-energy population exhibits a global quasi-circular motion with small perturbations induced by the self-gravity of the spiral structure. Conclusions. Our results demonstrate that the spiral arms are dynamically traced by the manifold-compatible population, which forms the backbone of the structure and drives effective radial transport. The bulk of low-energy disc particles respond to the spiral perturbation similarly to the traditional density wave picture, enhancing the density contrast caused by the invariant-manifold compatible particles. In this framework, barred spiral arms emerge as material structures sustained by manifold-guided transport, with the surrounding disc behaving as a system of material density waves.

The bar pattern speed of the Large Magellanic Cloud (LMC) has been measured using Gaia data, suggesting the presence of a slow pattern that is perhaps not rotating at all. Numerical simulations of interacting LMC-SMC systems were able to reproduce a bar stoppage. Here, we report on the first measurement of the bar pattern speed of the LMC as a function of the evolutionary phase of its stellar populations. Astrometric and photometric data of ∼11 million LMC stars from Gaia DR3 were used to build five evolutionary phases, from less to more evolved stars. The Dehnen method, a new procedure to derive bar pattern speeds from kinematics of particles in N-body simulations, was applied to the LMC stellar populations. We observe a modulation of the bar pattern speed with the evolutionary phase, meaning that different LMC stellar populations exhibit different pattern speeds, ranging from −0.9 to 6.6 km s ⁻¹ kpc ⁻¹ . Moreover, less evolved stars have a lower pattern speed, while the bar of more evolved phases tends to rotate faster. The LMC bar is thus extremely slow, ruling out the presence of bar corotation within the disc, in agreement with a previous claim, but this time observed with various stellar populations. It is the first time that a pattern speed is measured separately for different stellar populations in any galaxy. The LMC pattern speed cannot be simply resumed to a singular value, but instead is an overlay of different patterns depending on the evolutionary phase of the stars. Future Gaia releases will be crucial to investigate more deeply the relations of the pattern speed with important astrophysical parameters of stars, such as their age and metallicity, which will be helpful to constrain the chemo-dynamical evolution of the LMC bar.

Context. The analysis of precise Gaia DR3 astrometry in the LMC region has revealed asymmetric patterns in the bar quadrupole and the disc outskirts of the LMC in-plane velocity maps. Aims. We aim to quantify the asymmetries detected in the LMC radial and residual tangential velocity maps, and determine whether they are generated naturally due to the LMC’s interaction with the SMC. Methods. We analyse the velocity maps of different simulations from the KRATOS suite of N-body simulations of the LMC–SMC– MW system, proposing a new methodology to quantify the kinematic asymmetry in the bar and the outskirts of the disc. We also transform the KRATOS simulations into mock catalogues with G magnitudes and Gaia observational errors, to confirm that the asymmetric signature in the LMC is not an effect of observational uncertainties. In addition, we investigate the possibility of a classification bias in the neural network classifier of the Gaia optimal sample. Results. In the KRATOS simulations of the LMC and SMC interaction, the dynamical effect of the SMC passages produces a displacement of the bar and asymmetries in the LMC velocity maps. Individual stars of the SMC do not have a substantial effect on the kinematics of the LMC. By comparing the velocity maps of mock catalogues of the future Gaia data releases DR4, DR5, and GaiaNIR, we find that the asymmetric signature in the bar quadrupole is independent of observational errors. We thereby confirm that it is a consequence of the interaction of the LMC with the SMC. We also find a classification bias in the neural network classifier, indicating that the outer disc asymmetry observed in the optimal sample is artificial. Conclusions. The analysis of the KRATOS simulations reveals that the interaction of the LMC with the SMC can generate asymmetric patterns in the velocity field. In the case of the Gaia DR3 LMC velocity maps we conclude that the bar quadrupole asymmetry is directly correlated with the SMC interaction, while the outer disc asymmetry is an artefact of the classifier for the optimal sample.

Context. The coupling between the dark matter (DM) halo and the stellar disc is a key factor in galactic evolution. While the interaction between structures like the Galactic bar and DM halos has been explored (e.g. slowing down of the bar due to dynamical friction), the effect of spiral arms on the DM halo distribution has received limited attention. Aims. We aim to detect and characterize the interaction between the stellar spiral arms and the DM halo. Methods. We analysed a suite of simulations featuring strong stellar spiral arms, ranging in complexity from test-particle models to fully cosmological hydrodynamical simulations. Using Fourier transforms, we mapped the phase and amplitude of the stellar spirals at different times and radii. We then applied the same methodology to DM particles near the stellar disc and compared trends in Fourier coefficients and phases between the two components. Results. We detect a clear spiral arm signal in the DM distribution, correlated with the stellar spirals, confirming the reaction of the halo. The strength of the DM spirals consistently measures around 10% of that of the stellar spiral arms. In the N-body simulation, the DM spiral persistently trails the stellar spiral arm by approximately 10^∘. A strong spiral signal of a few kilometres per second appears in the radial, azimuthal, and vertical velocities of halo particles, distinct from the stellar kinematic signature. In a test-particle simulation with an analytical spiral potential (omitting self-gravity), we reproduce a similar density and kinematic response, showing that the test-particle halo responds in the same way as the N-body halo. This similarity confirms that we are observing the forced response of the halo to the stellar spiral arms potential. Finally, we also find the presence of DM spiral arms in a pure N-body simulation with an external perturber, and isolated and cosmological hydrodynamical simulations, indicating that the dynamical signatures of the forced response in the DM halo are independent of the dynamical origin of the stellar spiral arms. Conclusions. We reveal the ubiquitous presence of DM spiral arms in Milky Way-like galaxies, driven by a forced response to the stellar spiral potential.

The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015–2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14 000 deg² of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

Context. The Large and Small Magellanic Clouds (LMC and SMC, respectively) are the brightest satellites of the Milky Way (MW), and for the last thousand million years they have been interacting with one another. As observations only provide a static picture of the entire process, numerical simulations are used to interpret the present-day observational properties of these kinds of systems, and most of them have been focused on attempting to recreate the neutral gas distribution and characteristics through hydrodynamical simulations. Aims. We present KRATOS, a comprehensive suite of 28 open-access pure N-body simulations of isolated and interacting LMC-like galaxies designed for studying the formation of substructures in their discs after interaction with an SMC-mass galaxy. The primary objective of this paper is to provide theoretical models that help us to interpret the formation of general structures in an LMC-like galaxy under various tidal interaction scenarios. This is the first paper of a series dedicated to the analysis of this complex interaction. Methods. Simulations are grouped into 11 sets of up to three configurations, with each set containing (1) a control model of an isolated LMC-like galaxy; (2) a model that contains the interaction with an SMC-mass galaxy, and (3) a model where both an SMC-mass and a MW-mass galaxy may interact with the LMC-like galaxy (the most realistic model). In each simulation, we analysed the orbital history between the three galaxies and examined the morphological and kinematic features of the LMC-like disc galaxy throughout the interaction. This includes investigating the disc scale height and velocity maps. When a bar was found to develop, we characterised its strength, length, off-centredness, and pattern speed. Results. The diverse outcomes found in the KRATOS simulations, including the presence of bars, warped discs, and various spiral arm shapes, demonstrate the opportunities they offer to explore a range of LMC-like galaxy morphologies. These morphologies directly correspond to distinct disc kinematic maps, making them well-suited for a first-order interpretation of the LMC’s kinematic maps. From the simulations, we note that tidal interactions can: boost the disc scale height; both destroy and create bars; and naturally explain the off-centre stellar bars. The bar length and pattern speed of long-lived bars are not appreciably altered by the interaction. Conclusions. The high spatial, temporal, and mass resolution used in the KRATOS simulations has been shown to be appropriate for the purpose of interpreting the internal kinematics of LMC-like discs, as evidenced by the first scientific results presented in this work.

Context. Gravitational waves from black-hole (BH) merging events have revealed a population of extra-galactic BHs residing in short-period binaries with masses that are higher than expected based on most stellar evolution models – and also higher than known stellar-origin black holes in our Galaxy. It has been proposed that those high-mass BHs are the remnants of massive metal-poor stars. Aims. Gaia astrometry is expected to uncover many Galactic wide-binary systems containing dormant BHs, which may not have been detected before. The study of this population will provide new information on the BH-mass distribution in binaries and shed light on their formation mechanisms and progenitors. Methods. As part of the validation efforts in preparation for the fourth Gaia data release (DR4), we analysed the preliminary astrometric binary solutions, obtained by the Gaia Non-Single Star pipeline, to verify their significance and to minimise false-detection rates in high-mass-function orbital solutions. Results. The astrometric binary solution of one source, Gaia BH3, implies the presence of a 32.70 ± 0.82 M_⊙ BH in a binary system with a period of 11.6 yr. Gaia radial velocities independently validate the astrometric orbit. Broad-band photometric and spectroscopic data show that the visible component is an old, very metal-poor giant of the Galactic halo, at a distance of 590 pc. Conclusions. The BH in the Gaia BH3 system is more massive than any other Galactic stellar-origin BH known thus far. The low metallicity of the star companion supports the scenario that metal-poor massive stars are progenitors of the high-mass BHs detected by gravitational-wave telescopes. The Galactic orbit of the system and its metallicity indicate that it might belong to the Sequoia halo substructure. Alternatively, and more plausibly, it could belong to the ED-2 stream, which likely originated from a globular cluster that had been disrupted by the Milky Way.