arXiv subjects

Óscar Jiménez-Arranz

Publications and source records attributed to Óscar Jiménez-Arranz.

6 recordsLinked to original sources

arXiv2026

Characterisation of the Clouds' young stellar Bridge using Gaia DR3

The interaction between the LMC and SMC (the Clouds) has resulted in prominent tidal features, including an extended bridge of gas and stars connecting the two galaxies. This Bridge has likely formed during the most recent interaction between the Clouds, about 150-250 Myr ago. While some young stars observed in the Bridge have formed in-situ from the tidally stripped gas, stellar populations may also have been drawn out of the SMC during the tidal interaction. We aim to identify a clean sample of likely Bridge stars in the region between the LMC and SMC using Gaia DR3 astrometric and photometric data combined with machine-learning techniques. We use the dimensionality-reduction algorithm UMAP to construct a training sample of young stars in the outskirts of the SMC and LMC. A neural network trained on this sample is then applied to Gaia sources in the inter-Cloud region to classify the stars and identify candidate Bridge members. We present and characterise a new sample of young candidate Bridge stars, selected from Gaia DR3. We investigate its spatial distribution, kinematic properties and colour-magnitude diagram and validate it using existing Bridge samples. The young stellar Bridge aligns well with HI gas, clusters, and cepheid samples, apart from a small offset near the LMC outer disc. We measure a Bridge length of ~15 kpc and the stars are travelling from the SMC to the LMC at a median tangential velocity of ~114 km/s. This implies a crossing time of ~125 Myr, which is within the timeframe of the last interaction of the Clouds and therefore supports tidal stripping as a possible formation scenario of the Bridge.

astro-ph.GA↗

arXiv2026

Survival or Destruction: Effects of Spheroidal Satellite Collisions on Bars in Milky Way-Like Galaxies

Although stellar bars are prevalent in local galaxies, unbarred galaxies constitute a significant fraction, particularly at high redshifts. While some galaxies are unbarred by nature due to stability against the bar instability, several mechanisms capable of transforming barred galaxies into unbarred systems have also been proposed, such as central mass concentration, specific dark matter halo morphologies and tidal interactions. Regarding galactic interactions, mergers can undoubtedly disrupt bars while potentially destroying the entire disk. However, the effects of pure collisions (non-merging scenarios) on bars remain unclear, with limited existing studies yielding contradictory conclusions. Here we aim to systematically investigate the disruptive effects of collisions on bars hosted by Milky Way-like galaxies using N-body/SPH simulations. We model collisions between the barred galaxy and a spherical intruder, conducting multiple simulations by varying interaction parameters, with mass ratios set at 1:3, 1:5, and 1:15. We find that bars are remarkably robust, with most interactions failing to significantly reduce their strength or pattern speed. Only off-center high-inclination retrograde collisions can effectively destroy bars, while central high-inclination collisions can substantially decrease the pattern speed. Such destruction and deceleration primarily arise from gravitational forces rather than gas-related processes. Notably, compared to collisions occurring along the bar's major axis, those along the minor axis cause greater weakening but can slow the bar's natural deceleration. Furthermore, changes in mass resolution do not significantly affect the results when the resolution is better than ~10^5 Solar mass.

astro-ph.GA↗

arXiv2025

Tidally-induced radial migration waves in LMC-like galaxies

Stellar radial migration has predominantly been examined in isolated disc galaxies where non-axisymmetric structures drive the process. By contrast, while tidal interactions are known for having an influence, their contribution remains comparatively under explored. The LMC, the nearest disc galaxy to the Milky Way (MW) and currently interacting with the SMC, provides a unique laboratory to investigate this interplay. We aim to quantify the impact of tidal interactions on radial migration and metallicity distribution in high-resolution simulations of LMC-like disc galaxies. We leverage a subsample of KRATOS, a suite of 28 pure $N$-body simulations of the LMC-SMC-MW system. Specifically, we use 6 simulations of both isolated and interacting LMC-like galaxies, exploring different values of the Toomre stellar parameter $Q$. These simulations allow to map the evolution of the stars' guiding radii $R_g(t)$ and compute radial migration fluxes in interacting systems and compare with their isolated counterparts, allowing to quantify the link between tidal interactions, radial migration, non-axisymmetric patterns, disc internal stability, and radial metallicity distribution. We present tidally-triggered wave-like radial migration fluxes reaching up to $\sim40\%$ of disc stellar mass per Gyr. This wave-like migration appears during the satellite's pericentre passages, almost independently of $Q$ and induces a metallicity drop of $\sim$3-5\% of the isolated galaxy's maximum metallicity in the inner disc. Additionally, in the isolated simulations, the extent of variation in the bar's resonance region coincides with the mixing zones in the metallicity distribution. We propose a novel description of a wave-like radial migration flux as a dynamical response of a galaxy undergoing tidal interactions and sketch its impact on the galaxy's metallicity distribution.

astro-ph.GA↗

arXiv2025

Asymmetries in the LMC velocity maps

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. 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. 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 Gaia mock catalogues 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. 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. 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. 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.

astro-ph.GA↗

arXiv2025

Dark matter spiral arms in Milky Way-like halos

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. We analyze 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 characterize the phase and amplitude of the stellar spirals at different times and radii. We then apply the same methodology to DM particles near the stellar disc and compare trends in Fourier coefficients and phases between the two components. 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 $10^\circ$. A strong spiral signal of a few km\,s$^{-1}$ 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. Finally, we also find the rest of the 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. We reveal the ubiquitous presence of DM spiral arms in Milky Way-like galaxies, driven by a forced response to the stellar spiral potential. (ABR)

astro-ph.GA↗

arXiv2025

Tidal interaction can stop galactic bars: on the LMC non-rotating bar

Context: Using Gaia DR3 data, Jiménez-Arranz et al. (2024a) computed the LMC bar pattern speed using three different methods. One of them suggested that the LMC might be hosting a bar that barely rotates, and is slightly counter-rotating with respect to the LMC disc, with a pattern speed of $Ω_p = -1.0 \pm 0.5$ km/s/kpc. Aims: To confirm that tidal interactions can trigger the LMC hosting a non-rotating bar due to its interaction with the SMC, which could cause the LMC bar to slow down significantly until it (momentarily) stops. Methods: We analyse a subset of models (K9 and K21) from the KRATOS suite (Jiménez-Arranz et al. 2024b) where we detected non-rotating bars. We make use of two different methods to track the evolution of the bar pattern speed: the program patternSpeed.py (Dehnen et al. 2023), and temporal finite-differences of the change rate the bar major axis' phase angle. Results: In the second LMC-SMC-like pericenter passage of K9, the bar of the LMC-like galaxy weakens to almost disappear and regenerates with a pattern speed that suffers a slowdown from $Ω_p \sim 20$ km/s/kpc to $Ω_p \sim 0$ km/s/kpc in less than 75 Myr. Then, the bar rotates at less than $Ω_p \sim 3-5$ km/s/kpc for around 100 Myr, until it recovers the initial (before interaction) pattern speed of $Ω_p \sim 10$ km/s/kpc. The results for the K21 simulation are comparable. Conclusions: This work is the first direct evidence that galactic bars can be slowed down or even stopped by tidal interaction, which strengthens the possibility of the LMC hosting a non-rotating bar, and can add an alternative formation scenario for observed slow-rotating bars.

astro-ph.GA↗