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We present the first steps toward applying causal representation learning to astronomy. Following up on previous work that introduced causal discovery to the field for the first time, here we solve a long standing conundrum by identifying the direction of the causal relation between supermassive black hole (SMBH) mass and their host galaxy properties. This leverages a score-based causal discovery approach with an exact posterior calculation. Causal relations between SMBHs and their host galaxies are further clarified by Independent Component Analysis (ICA). The astrophysical problem we focus on is one of the most important open issues in the field and one that has not seen a definitive resolution in decades. We consider the space of six physical properties of galaxies, subdivided by morphology: elliptical, lenticular, and spiral, plus SMBH mass. We calculate an exact posterior over the space of directed acyclic graphs for these variables based on a flat prior and the Bayesian Gaussian equivalent score. The nature of the causal relation between galaxy properties and SMBH mass is found to vary smoothly with morphology, with galaxy properties determining SMBH mass in ellipticals and vice versa in spirals. This settles a long-standing debate and is compatible with our theoretical understanding of galaxy evolution. ICA reveals a decreasing number of meaningful Independent Components (ICs) from ellipticals and lenticular to spiral. Moreover, we find that only one IC correlates with SMBH mass in spirals while multiple ones do in ellipticals, further confirming our finding that SMBH mass causes galaxy properties in spirals, but the reverse holds in ellipticals., Comment: Accepted by NeurIPS 2024 Causal Representation Learning Workshop. arXiv admin note: substantial text overlap with arXiv:2410.00965

Correlations between galaxies and their supermassive black holes (SMBHs) have been observed, but the causal mechanisms remained unclear. The emerging field of causal inference now enables examining these relationships using observational data. This study, using advanced causal discovery techniques and a state-of-the-art dataset, reveals a causal link between galaxy properties and SMBH masses. In elliptical galaxies, bulge properties influence SMBH growth, while in spiral galaxies, SMBHs affect host galaxy properties, potentially through feedback in gas-rich environments. For spiral galaxies, SMBHs progressively quench star formation, whereas in elliptical galaxies, quenching is complete, and the causal connection has reversed. These findings support theoretical models of active galactic nuclei feedback regulating galaxy evolution and suggest further exploration of causal links in astrophysical and cosmological scaling relations., Comment: 34 pages, 20 figures, submitted for peer-review

Context. Computational astronomy has reached the stage where running a gravitational N-body simulation of a stellar system, such as a Milky Way star cluster, is computationally feasible, but a major limiting factor that remains is the ability to set up physically realistic initial conditions. Aims. We aim to obtain realistic initial conditions for N-body simulations by taking advantage of machine learning, with emphasis on reproducing small-scale interstellar distance distributions. Methods. The computational bottleneck for obtaining such distance distributions is the hydrodynamics of star formation, which ultimately determine the features of the stars, including positions, velocities, and masses. To mitigate this issue, we introduce a new method for sampling physically realistic initial conditions from a limited set of simulations using Gaussian processes. Results. We evaluated the resulting sets of initial conditions based on whether they meet tests for physical realism. We find that direct sampling based on the learned distribution of the star features fails to reproduce binary systems. Consequently, we show that physics-informed sampling algorithms solve this issue, as they are capable of generating realisations closer to reality., Comment: Accepted to A&A (4th Sep 2024)

We present the first application of data-driven techniques for dynamical system analysis based on Koopman theory to variable stars. We focus on light curves of RRLyrae type variables, in the Galactic globular cluster $\omega$ Centauri. Light curves are thus summarized by a handful of complex eigenvalues, corresponding to oscillatory or fading dynamical modes. We find that variable stars of the RRc subclass can be summarized in terms of fewer ($\approx 8$) eigenvalues, while RRab need comparatively more ($\approx 12$). This result can be leveraged for classification and reflects the simpler structure of RRc light curves. We then consider variable stars displaying secular variations due to the Tseraskaya-Blazhko effect and find a change in relevant eigenvalues with time, with possible implications for the physical interpretation of the effect., Comment: Accepted (spotlight) at the Scaling AI for Science workshop at ICML 2024

Third-generation (3G) gravitational-wave detectors such as the Einstein Telescope (ET) will observe binary black hole (BBH) mergers at redshifts up to $z\sim 100$. However, an unequivocal determination of the origin of high-redshift sources will remain uncertain because of the low signal-to-noise ratio (S/N) and poor estimate of their luminosity distance. This study proposes a machine-learning approach to infer the origins of high-redshift BBHs. We specifically differentiate those arising from Population III (Pop. III) stars, which probably are the first progenitors of star-born BBH mergers in the Universe, and those originated from Population I-II (Pop. I-II) stars. We considered a wide range of models that encompass the current uncertainties on Pop. III BBH mergers. We then estimated the parameter errors of the detected sources with ET using the Fisher information-matrix formalism, followed by a classification using XGBoost, which is a machine-learning algorithm based on decision trees. For a set of mock observed BBHs, we provide the probability that they belong to the Pop. III class while considering the parameter errors of each source. In our fiducial model, we accurately identify $\gtrsim 10\%$ of the detected BBHs that originate from Pop. III stars with a precision $>90\%$. Our study demonstrates that machine-learning enables us to achieve some pivotal aspects of the ET science case by exploring the origin of individual high-redshift GW observations. We set the basis for further studies, which will integrate additional simulated populations and account for further uncertainties in the population modeling., Comment: Published in Astronomy & Astrophysics. 15 pages, 9 Figures and 6 tables. Comments are welcome

In astronomy, spectroscopy consists of observing an astrophysical source and extracting its spectrum of electromagnetic radiation. Once extracted, a model is fit to the spectra to measure the observables, leading to an understanding of the underlying physics of the emission mechanism. One crucial, and often overlooked, aspect of this model is the background emission, which contains foreground and background astrophysical sources, intervening atmospheric emission, and artifacts related to the instrument such as noise. This paper proposes an algorithmic approach to constructing a background model for SITELLE observations using statistical tools and supervised machine learning algorithms. We apply a segmentation algorithm implemented in photutils to divide the data cube into background and source spaxels. After applying a principal component analysis (PCA) on the background spaxels, we train an artificial neural network to interpolate from the background to the source spaxels in the PCA coefficient space, which allows us to generate a local background model over the entire data cube. We highlight the performance of this methodology by applying it to SITELLE observations obtained of a SIGNALS galaxy, \NGC4449, and the Perseus galaxy cluster of galaxies, NGC 1275. We discuss the physical interpretation of the principal components and noise reduction in the resulting PCA-based reconstructions. Additionally, we compare the fit results using our new background modeling approach to standard methods used in the literature and find that our method better captures the emission from HII regions in NGC 4449 and the faint emission regions in NGC 1275. These methods also demonstrate that the background does change as a function of the position of the datacube., Comment: Accepted to RASTI

Cosmological galaxy formation simulations are powerful tools to understand the complex processes that govern the formation and evolution of galaxies. However, evaluating the realism of these simulations remains a challenge. The two common approaches for evaluating galaxy simulations is either through scaling relations based on a few key physical galaxy properties, or through a set of pre-defined morphological parameters based on galaxy images. This paper proposes a novel image-based method for evaluating the quality of galaxy simulations using unsupervised deep learning anomaly detection techniques. By comparing full galaxy images, our approach can identify and quantify discrepancies between simulated and observed galaxies. As a demonstration, we apply this method to SDSS imaging and NIHAO simulations with different physics models, parameters, and resolution. We further compare the metric of our method to scaling relations as well as morphological parameters. We show that anomaly detection is able to capture similarities and differences between real and simulated objects that scaling relations and morphological parameters are unable to cover, thus indeed providing a new point of view to validate and calibrate cosmological simulations against observed data., Comment: 13 pages, 16 figures, published in MNRAS

Future gravitational wave observatories can probe dark matter by detecting the dephasing in the waveform of binary black hole mergers induced by dark matter overdensities. Such a detection hinges on the accurate modelling of the dynamical friction, induced by dark matter on the secondary compact object in intermediate and extreme mass ratio inspirals. In this paper, we introduce NbodyIMRI, a new publicly available code designed for simulating binary systems within cold dark matter `spikes'. Leveraging higher particle counts and finer timesteps, we validate the applicability of the standard dynamical friction formalism and provide an accurate determination of the maximum impact parameter of particles which can effectively scatter with a compact object, across various mass ratios. We also show that in addition to feedback due to dynamical friction, the dark matter also evolves through a `stirring' effect driven by the time-dependent potential of the binary. We introduce a simple semi-analytical scheme to account for this effect and demonstrate that including stirring tends to slow the rate of dark matter depletion and therefore enhances the impact of dark matter on the dynamics of the binary., Comment: 11 pages, 9 figures + appendices. Companion paper to "Sharpening the dark matter signature in gravitational waveforms I: Accretion and eccentricity evolution" (arXiv:2402.13053). NbodyIMRI code available here: https://github.com/bradkav/NbodyIMRI

Definitive evidence that globular clusters (GCs) host intermediate-mass black holes (IMBHs) is elusive. Machine learning (ML) models trained on GC simulations can in principle predict IMBH host candidates based on observable features. This approach has two limitations: first, an accurate ML model is expected to be a black box due to complexity; second, despite our efforts to realistically simulate GCs, the simulation physics or initial conditions may fail to fully reflect reality. Therefore our training data may be biased, leading to a failure in generalization on observational data. Both the first issue -- explainability/interpretability -- and the second -- out of distribution generalization and fairness -- are active areas of research in ML. Here we employ techniques from these fields to address them: we use the anchors method to explain an XGBoost classifier; we also independently train a natively interpretable model using Certifiably Optimal RulE ListS (CORELS). The resulting model has a clear physical meaning, but loses some performance with respect to XGBoost. We evaluate potential candidates in real data based not only on classifier predictions but also on their similarity to the training data, measured by the likelihood of a kernel density estimation model. This measures the realism of our simulated data and mitigates the risk that our models may produce biased predictions by working in extrapolation. We apply our classifiers to real GCs, obtaining a predicted classification, a measure of the confidence of the prediction, an out-of-distribution flag, a local rule explaining the prediction of XGBoost and a global rule from CORELS., Comment: ApJ accepted

The upcoming facilities like the Vera C. Rubin Observatory will provide extremely deep photometry of thousands of star clusters to the edge of the Galaxy and beyond, which will require adequate tools for automatic analysis, capable of performing tasks such as the characterization of a star cluster through the analysis of color-magnitude diagrams (CMDs). The latter are essentially point clouds in N-dimensional space, with the number of dimensions corresponding to the photometric bands employed. In this context, machine learning techniques suitable for tabular data are not immediately applicable to CMDs because the number of stars included in a given CMD is variable, and equivariance for permutations is required. To address this issue without introducing ad-hoc manipulations that would require human oversight, here we present a new CMD featurization procedure that summarizes a CMD by means of a quadtree-like structure through iterative partitions of the color-magnitude plane, extracting a fixed number of meaningful features of the relevant subregion from any given CMD. The present approach is robust to photometric noise and contamination and it shows that a simple linear regression on our features predicts distance modulus (metallicity) with a scatter of 0.33 dex (0.16 dex) in cross-validation., Comment: 10 pages, 6 figures. Submitted to the International Conference on Machine Learning 2023

RR Lyrae (RRL) are old, low-mass radially pulsating variable stars in their core helium burning phase. They are popular stellar tracers and primary distance indicators, since they obey to well defined period-luminosity relations in the near-infrared regime. Their photometric identification is not trivial, indeed, RRL samples can be contaminated by eclipsing binaries, especially in large datasets produced by fully automatic pipelines. Interpretable machine-learning approaches for separating eclipsing binaries from RRL are thus needed. Ideally, they should be able to achieve high precision in identifying RRL while generalizing to new data from different instruments. In this paper, we train a simple logistic regression classifier on Catalina Sky Survey (CSS) light curves. It achieves a precision of 87% at 78% recall for the RRL class on unseen CSS light curves. It generalizes on out-of-sample data (ASAS/ASAS-SN light curves) with a precision of 85% at 96% recall. We also considered a L1-regularized version of our classifier, which reaches 90% sparsity in the light-curve features with a limited trade-off in accuracy on our CSS validation set and -- remarkably -- also on the ASAS/ASAS-SN light curve test set. Logistic regression is natively interpretable, and regularization allows us to point out the parts of the light curves that matter the most in classification. We thus achieved both good generalization and full interpretability., Comment: To appear on The Astrophysical Journal. 13 pages, 8 figures, 3 tables

We recently introduced a new method for simulating collisional gravitational N-body systems with approximately linear time scaling with $N$, based on the Multi-Particle Collision (MPC) scheme, previously applied in Plasma Physics. We simulate globular clusters with a realistic number of stellar particles (at least up to several times $10^6$) on a standard workstation. We simulate clusters hosting an intermediate mass black hole (IMBH), probing a broad range of BH-cluster and BH-average-star mass ratios, unrestricted by the computational constraints affecting direct N-body codes. We use either single mass models or models with a Salpeter mass function, with the IMBH initially sitting at the centre. The force exerted by and on the IMBH is evaluated with a direct scheme. We measure the evolution of the Lagrangian radii and core density and velocity dispersion over time. In addition, we study the evolution of the velocity anisotropy profiles. We find that models with an IMBH undergo core collapse at earlier times, the larger the IMBH mass the shallower, with an approximately constant central density at core collapse. The presence of an IMBH tends to lower the central velocity dispersion. These results hold independently of the mass function. For the models with Salpeter MF we observe that equipartition of kinetic energies is never achieved. Orbital anisotropy at large radii appears driven by energetic escapers on radial orbits. We measure the wander radius. Among the results we obtained, which mostly confirm or extend previously known trends that had been established over the range of parameters accessible to direct N-body simulations, we underline that the leptokurtic nature of the IMBH wander radius distribution might lead to IMBHs presenting as off-center more frequently than expected, with implications on observational IMBH detection., Comment: 11 pages, 10 figures. Matching the version of the ms accepted in A&A

We use Gaia DR3 data to study the Collinder 132-Gulliver 21 region via the machine learning algorithm StarGO, and find eight subgroups of stars (ASCC 32, Collinder 132 gp 1--6, Gulliver 21) located in close proximity. Three co-moving populations were identified among these eight subgroups: (i) a coeval 25 Myr-old moving group (Collinder 132); (ii) an intermediate-age (50--100 Myr) group; and (iii) the 275 Myr-old dissolving cluster Gulliver 21. These three populations form parallel diagonal stripe-shape over-densities in the U--V distribution, which differ from open clusters and stellar groups in the solar neighborhood. We name this kinematic structure the Collinder 132-Gulliver 21 stream, as it extends over 270 pc in the 3D space. The oldest population Gulliver21 is spatially surrounded by the Collinder 132 moving group and the intermediate-age group. Stars in the Collinder 132-Gulliver 21 stream have an age difference up to 250 Myr. Metallicity information shows a variation of 0.3 dex between the youngest and oldest populations. The formation of the Collinder132-Gulliver 21 stream involves both star formation and dynamical heating. The youngest population (Collinder 132 moving group) with homogeneous metallicity is probably formed through filamentary star formation. The intermediate-age and the oldest population were then scatted by the Galactic bar or spiral structure resonance to intercept Collinder 132's orbit. Without mutual interaction between each population, the three populations are flying by each other currently and will become distinct three groups again in approximately ~50Myr., Comment: Accepted to ApJL, 15 pages, 7 figures

Although active galactic nuclei (AGN) feedback is required in simulations of galaxies to regulate star formation, further downstream effects on the dark matter distribution of the halo and stellar kinematics of the central galaxy can be expected. We combine simulations of galaxies with and without AGN physics from the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) to investigate the effect of AGN on the dark matter profile and central stellar rotation of the host galaxies. Specifically, we study how the concentration-halo mass ($c-M$) relation and the stellar spin parameter ($\lambda_R$) are affected by AGN feedback. We find that AGN physics is crucial to reduce the central density of simulated massive ($\gtrsim 10^{12}$ M$_\odot$) galaxies and bring their concentration to agreement with results from the Spitzer Photometry & Accurate Rotation Curves (SPARC) sample. Similarly, AGN feedback has a key role in reproducing the dichotomy between slow and fast rotators as observed by the ATLAS$^{3\text{D}}$ survey. Without star formation suppression due to AGN feedback, the number of fast rotators strongly exceeds the observational constraints. Our study shows that there are several collateral effects that support the importance of AGN feedback in galaxy formation, and these effects can be used to constrain its implementation in numerical simulations., Comment: Accepted for publication in MNRAS. 15 pages, 10 figures

We identify members of 65 open clusters in the solar neighborhood using the machine-learning algorithm StarGO based on Gaia EDR3 data. After adding members of twenty clusters from previous studies (Pang et al. 2021a,b; Li et al. 2021) we obtain 85 clusters, and study their morphology and kinematics. We classify the substructures outside the tidal radius into four categories: filamentary (f1) and fractal (f2) for clusters $<100$ Myr, and halo (h) and tidal-tail (t) for clusters $>100$ Myr. The kinematical substructures of f1-type clusters are elongated; these resemble the disrupted cluster Group X. Kinematic tails are distinct in t-type clusters, especially Pleiades. We identify 29 hierarchical groups in four young regions (Alessi 20, IC 348, LP 2373, LP 2442); ten among these are new. The hierarchical groups form filament networks. Two regions (Alessi 20, LP 2373) exhibit global "orthogonal" expansion (stellar motion perpendicular to the filament), which might cause complete dispersal. Infalling-like flows (stellar motion along the filament) are found in UBC 31 and related hierarchical groups in the IC 348 region. Stellar groups in the LP 2442 region (LP 2442 gp 1-5) are spatially well-mixed but kinematically coherent. A merging process might be ongoing in the LP 2442 subgroups. For younger systems ($\lesssim30$ Myr), the mean axis ratio, cluster mass and half-mass radius tend to increase with age values. These correlations between structural parameters may imply two dynamical processes occurring in the hierarchical formation scenario in young stellar groups: (1) filament dissolution and (2) sub-group mergers., Comment: Accepted to ApJ, 33 pages, 14 figures. OC 3D interactive visualization included on http://3doc-morphology.lowell.edu

Dynamical interactions in dense star clusters are considered one of the most effective formation channels of binary black holes (BBHs). Here, we present direct $N-$body simulations of two different star cluster families: low-mass ($\sim{500-800}$ M$_\odot$) and relatively high-mass star clusters ($\ge{5000}$ M$_\odot$). We show that the formation channels of BBHs in low- and high-mass star clusters are extremely different and lead to two completely distinct populations of BBH mergers. Low-mass clusters host mainly low-mass BBHs born from binary evolution, while BBHs in high-mass clusters are relatively massive (chirp mass up to $\sim{100}$ M$_\odot$) and driven by dynamical exchanges. Tidal disruption dramatically quenches the formation and dynamical evolution of BBHs in low-mass clusters on a very short timescale ($\lesssim{100}$ Myr), while BBHs in high-mass clusters undergo effective dynamical hardening until the end of our simulations (1.5 Gyr). In high-mass clusters we find that 8\% of BBHs have primary mass in the pair-instability mass gap, all of them born via stellar collisions, while only one BBH with primary mass in the mass gap forms in low-mass clusters. These differences are crucial for the interpretation of the formation channels of gravitational-wave sources., Comment: 14 pages, 9 figures, 2 tables. Revised version submitted after fixing typos. Comments welcome

XMM-Newton provides unprecedented insight into the X-ray Universe, recording variability information for hundreds of thousands of sources. Manually searching for interesting patterns in light curves is impractical, requiring an automated data-mining approach for the characterization of sources. Straightforward fitting of temporal models to light curves is not a sure way to identify them, especially with noisy data. We used unsupervised machine learning to distill a large data set of light-curve parameters, revealing its clustering structure in preparation for anomaly detection and subsequent searches for specific source behaviors (e.g., flares, eclipses). Self-organizing maps (SOMs) achieve dimensionality reduction and clustering within a single framework. They are a type of artificial neural network trained to approximate the data with a two-dimensional grid of discrete interconnected units, which can later be visualized on the plane. We trained our SOM on temporal-only parameters computed from more than 100,000 detections from the EXTraS catalog. The resulting map reveals that about 2500 most variable sources are clustered based on temporal characteristics. We find distinctive regions of the SOM map associated with flares, eclipses, dips, linear light curves, and others. Each group contains sources that appear similar by eye. We single out a handful of interesting sources for further study. The condensed view of our dataset provided by SOMs allowed us to identify groups of similar sources, speeding up manual characterization by orders of magnitude. Our method also highlights problems with fitting simple temporal models to light curves and can be used to mitigate them to an extent. This will be crucial for fully exploiting the high data volume expected from upcoming X-ray surveys, and may also help with interpreting supervised classification models., Comment: Accepted in Astronomy & Astrophysics (A&A). Main part: 12 pages, 12 figures, 1 video (appendix: 5 pages and 4 figures)

We identify hierarchical structures in the Vela OB2 complex and the cluster pair Collinder 135 and UBC 7 with Gaia EDR3 using the neural network machine learning algorithm StarGO. Five second-level substructures are disentangled in Vela OB2, which are referred to as Huluwa 1 (Gamma Velorum), Huluwa 2, Huluwa 3, Huluwa 4 and Huluwa 5. For the first time, Collinder 135 and UBC 7 are simultaneously identified as constituent clusters of the pair with minimal manual intervention. We propose an alternative scenario in which Huluwa 1-5 have originated from sequential star formation. The older clusters Huluwa 1-3 with an age of 10-22 Myr, generated stellar feedback to cause turbulence that fostered the formation of the younger-generation Huluwa 4-5 (7-20 Myr). A supernova explosion located inside the Vela IRAS shell quenched star formation in Huluwa 4-5 and rapidly expelled the remaining gas from the clusters. This resulted in global mass stratification across the shell, which is confirmed by the regression discontinuity method. The stellar mass in the lower rim of the shell is $0.32\pm0.14$ $\rm M_\odot$ higher than in the upper rim. Local, cluster-scale mass segregation is observed in the lowest-mass cluster Huluwa 5. Huluwa 1-5 (in Vela OB2) are experiencing significant expansion, while the cluster pair suffers from moderate expansion. The velocity dispersions suggest that all five groups (including Huluwa 1A and Huluwa 1B) in Vela OB2 and the cluster pair are supervirial and are undergoing disruption, and also that Huluwa 1A and Huluwa 1B may be a coeval young cluster pair. N-body simulations predict that Huluwa 1-5 in Vela OB2 and the cluster pair will continue to expand in the future 100 Myr and eventually dissolve., Comment: 28 pages, 16 figures. Accepted to ApJ

Star formation in molecular clouds is clumpy, hierarchically subclustered. Fractal structure also emerges in hydro-dynamical simulations of star-forming clouds. Simulating the formation of realistic star clusters with hydro-dynamical simulations is a computational challenge, considering that only the statistically averaged results of large batches of simulations are reliable, due to the chaotic nature of the gravitational N-body problem. While large sets of initial conditions for N-body runs can be produced by hydro-dynamical simulations of star formation, this is prohibitively expensive in terms of computational time. Here we address this issue by introducing a new technique for generating many sets of new initial conditions from a given set of star masses, positions and velocities from a hydro-dynamical simulation. We use hierarchical clustering in phase space to learn a tree representation of the spatial and kinematic relations between stars. This constitutes the basis for the random generation of new sets of stars which share the same clustering structure of the original ones but have individually different masses, positions, and velocities. We apply this method to the output of a number of hydro-dynamical star-formation simulations, comparing the generated initial conditions to the original ones through a series of quantitative tests, including comparing mass and velocity distributions and fractal dimension. Finally, we evolve both the original and the generated star clusters using a direct N-body code, obtaining a qualitatively similar evolution., Comment: Updated version of the manuscript: "Seeing the forest for the trees: hierarchical generative models for star clusters from hydro-dynamical simulations". 15 pages, 15 figures, 2 tables. Comments welcome

Intermediate mass black holes (IMBHs) in the mass range $10^2-10^5\,\mathrm{M_{\odot}}$ bridge the gap between stellar black holes (BHs) and supermassive BHs. Here, we investigate the possibility that IMBHs form in young star clusters via runaway collisions and BH mergers. We analyze $10^4$ simulations of dense young star clusters, featuring up-to-date stellar wind models and prescriptions for core collapse and (pulsational) pair instability. In our simulations, only 9 IMBHs out of 218 form via binary BH mergers, with a mass $\sim{}100-140$ M$_\odot$. This channel is strongly suppressed by the low escape velocity of our star clusters. In contrast, IMBHs with masses up to $\sim{}438$ M$_{\odot}$ efficiently form via runaway stellar collisions, especially at low metallicity. Up to $\sim{}0.2$~% of all the simulated BHs are IMBHs, depending on progenitor's metallicity. The runaway formation channel is strongly suppressed in metal-rich ($Z=0.02$) star clusters, because of stellar winds. IMBHs are extremely efficient in pairing with other BHs: $\sim{}70$% of them are members of a binary BH at the end of the simulations. However, we do not find any IMBH-BH merger. More massive star clusters are more efficient in forming IMBHs: $\sim{}8$% ($\sim{}1$%) of the simulated clusters with initial mass $10^4-3\times{}10^4$ M$_\odot$ ($10^3-5\times{}10^3$ M$_\odot$) host at least one IMBH., Comment: 12 pages, 16 figures, 2 tables. Comments welcome

Most of massive stars form in binary or higher-order systems in clumpy, sub-structured clusters. In the very first phases of their life, these stars are expected to interact with the surrounding environment, before being released to the field when the cluster is tidally disrupted by the host galaxy. We present a set of N-body simulations to describe the evolution of young stellar clusters and their binary content in the first phases of their life. To do this, we have developed a method that generates realistic initial conditions for binary stars in star clusters from hydrodynamical simulations. We considered different evolutionary cases to quantify the impact of binary and stellar evolution. Also, we compared their evolution to that of King and fractal models with different length scales. Our results indicate that the global expansion of the cluster from hydrodynamical simulations is initially balanced by the sub-clump motion and accelerates when a monolithic shape is reached, as in a post-core collapse evolution. Compared to the spherical initial conditions, the ratio of the 50% to 10% Lagrangian radius shows a very distinctive trend, explained by the formation of a hot core of massive stars triggered by the high initial degree of mass segregation. As for its binary population, each cluster shows a self-regulating behaviour by creating interacting binaries with binding energies of the order of its energy scales. Also, in absence of original binaries, the dynamically formed binaries present a mass dependent binary fraction, that mimics the trend of the observed one., Comment: 15 pages, 12 figures, 1 table. Comments welcome

In a previous paper we introduced a new method for simulating collisional gravitational $N$-body systems with linear time scaling on $N$, based on the Multi-Particle Collision (MPC) approach. This allows us to simulate globular clusters with a realistic number of stellar particles in a matter of hours on a typical workstation. We evolve star clusters containing up to $10^6$ stars to core collapse and beyond. We quantify several aspects of core collapse over multiple realizations and different parameters, while always resolving the cluster core with a realistic number of particles. We run a large set of N-body simulations with our new code. The cluster mass function is a power-law with no stellar evolution, allowing us to clearly measure the effects of the mass spectrum. Leading up to core collapse, we find a power-law relation between the size of the core and the time left to core collapse. Our simulations thus confirm the theoretical self-similar contraction picture but with a dependence on the slope of the mass function. The time of core collapse has a non-monotonic dependence on the slope, which is well fit by a parabola. This holds also for the depth of core collapse and for the dynamical friction timescale of heavy particles. Cluster density profiles at core collapse show a broken power law structure, suggesting that central cusps are a genuine feature of collapsed cores. The core bounces back after collapse, and the inner density slope evolves to an asymptotic value. The presence of an intermediate-mass black hole inhibits core collapse. We confirm and expand on several predictions of star cluster evolution before, during, and after core collapse. Such predictions were based on theoretical calculations or small-size direct $N$-body simulations. Here we put them to the test on MPC simulations with a much larger number of particles, allowing us to resolve the collapsing core., Comment: 8 pages, 9 figs., 1 tab. Version accepted for publication in A&A

We analyze the 3D morphology and kinematics of 13 open clusters (OCs) located within 500 pc of the Sun, using Gaia EDR3 and kinematic data from literature. Members of OCs are identified using the unsupervised machine learning method StarGO, using 5D parameters (X, Y, Z, $\mu_\alpha \cos\delta, \mu_\delta$). The OC sample covers an age range of 25Myr--2.65Gyr. We correct the asymmetric distance distribution due to the parallax error using Bayesian inversion. The uncertainty in the corrected distance for a cluster at 500~pc is 3.0--6.3~pc, depending on the intrinsic spatial distribution of its members. We determine the 3D morphology of the OCs in our sample and fit the spatial distribution of stars within the tidal radius in each cluster with an ellipsoid model. The shapes of the OCs are well-described with oblate spheroids (NGC2547, NGC2516, NGC2451A, NGC2451B, NGC2232), prolate spheroids (IC2602, IC4665, NGC2422, Blanco1, Coma Berenices), or triaxial ellipsoids (IC2391, NGC6633, NGC6774). The semi-major axis of the fitted ellipsoid is parallel to the Galactic plane for most clusters. Elongated filament-like substructures are detected in three young clusters (NGC2232, NGC2547, NGC2451B), while tidal-tail-like substructures (tidal tails) are found in older clusters (NGC2516, NGC6633, NGC6774, Blanco1, Coma Berenices). Most clusters may be super-virial and expanding. $N$-body models of rapid gas expulsion with an SFE of $\approx 1/3$ are consistent with clusters more massive than $250\rm M_\odot$, while clusters less massive than 250$\rm M_\odot$ tend to agree with adiabatic gas expulsion models. Only six OCs (NGC2422, NGC6633, and NGC6774, NGC2232, Blanco1, Coma Berenices) show clear signs of mass segregation., Comment: 35 pages, 17 figures, accepted by ApJ

Turbulence plays a key role in star formation in molecular clouds, affecting star cluster primordial properties. As modelling present-day objects hinges on our understanding of their initial conditions, better constraints on turbulence can result in windfalls in Galactic archaeology, star cluster dynamics and star formation. Observationally, constraining the spectral index of turbulent gas usually involves computing spectra from velocity maps. Here we suggest that information on the spectral index might be directly inferred from column density maps (possibly obtained by dust emission/absorption) through deep learning. We generate mock density maps from a large set of adaptive mesh refinement turbulent gas simulations using the hydro-simulation code RAMSES. We train a convolutional neural network (CNN) on the resulting images to predict the turbulence index, optimize hyper-parameters in validation and test on a holdout set. Our adopted CNN model achieves a mean squared error of 0.024 in its predictions on our holdout set, over underlying spectral indexes ranging from 3 to 4.5. We also perform robustness tests by applying our model to altered holdout set images, and to images obtained by running simulations at different resolutions. This preliminary result on simulated density maps encourages further developments on real data, where observational biases and other issues need to be taken into account., Comment: 7 pages, 7 figures, 1 table

We identify structures of the young star cluster NGC 2232 in the solar neighborhood (323.0 pc), and a newly discovered star cluster LP 2439 (289.1 pc). Member candidates are identified using the Gaia DR2 sky position, parallax and proper motion data, by an unsupervised machine learning method, \textsc{StarGO}. Member contamination from the Galactic disk is further removed using the color magnitude diagram. The four identified groups (NGC 2232, LP 2439 and two filamentary structures) of stars are coeval with an age of 25 Myr and were likely formed in the same giant molecular cloud. We correct the distance asymmetry from the parallax error with a Bayesian method. The 3D morphology shows the two spherical distributions of clusters NGC 2232 and LP 2439. Two filamentary structures are spatially and kinematically connected to NGC 2232. Both NGC 2232 and LP 2439 are expanding. The expansion is more significant in LP 2439, generating a loose spatial distribution with shallow volume number and mass density profiles. The expansion is suggested to be mainly driven by gas expulsion. NGC 2232, with 73~percent of the cluster mass bound, is currently experiencing a process of re-virialization, However, LP 2439, with 52 percent cluster mass being unbound, may fully dissolve in the near future. The different survivability traces different dynamical states of NGC 2232 and LP 2439 prior to the onset of gas expulsion. NGC 2232 may have been substructured and subvirial, while LP 2439 may either have been virial/supervirial, or it has experienced a much faster rate of gas removal., Comment: 14 pages, 6 figures, Accepted by ApJL

Stellar systems are broadly divided into collisional and non-collisional. The latter are large-N systems with long relaxation timescales and can be simulated disregarding two-body interactions, while either computationally expensive direct N-body simulations or approximate schemes are required to properly model the former. Large globular clusters and nuclear star clusters, with relaxation timescales of the order of a Hubble time, are small enough to display some collisional behaviour and big enough to be impossible to simulate with direct $N$-body codes and current hardware. We introduce a new method to simulate collisional stellar systems, and validate it by comparison with direct $N$-body codes on small-$N$ simulations. The Multi-Particle collision for Dense stellar systems Code (MPCDSS) is a new code for evolving stellar systems with the Multi-Particle Collision method. Such method amounts to a stochastic collision rule that allows to conserve exactly the energy and momentum over a cluster of particles experiencing the collision. The code complexity scales with $N \log N$ in the number of particles. Unlike Monte-Carlo codes, MPCDSS can easily model asymmetric, non-homogeneous, unrelaxed and rotating systems, while allowing us to follow the orbits of individual stars. We evolve small ($N = 3.2 \times 10^4$) star clusters with MPCDSS and with the direct-summation code NBODY6, finding a similar evolution of key indicators. We then simulate different initial conditions in the $10^4 - 10^6$ star range. MPCDSS bridges the gap between small, collisional systems that can be simulated with direct $N$-body codes and large noncollisional systems. MPCDSS in principle allows us to simulate globular clusters such as Omega Cen and M54 and even the nuclear star cluster, beyond the limits of current direct N-body codes in terms of the number of particles., Comment: 12 pages, 13 figures. Matching the version accepted for publication in A&A

In globular clusters, blue straggler stars are heavier than the average star, so dynamical friction strongly affects them. The radial distribution of BSS, normalized to a reference population, appears bimodal in a fraction of Galactic GCs, with a density peak in the core, a prominent zone of avoidance at intermediate radii, and again higher density in the outskirts. The zone of avoidance appears to be located at larger radii the more relaxed the host cluster, acting as a sort of dynamical clock. We use a new method to compute the evolution of the BSS radial distribution under dynamical friction and diffusion. We evolve our BSS in the mean cluster potential under dynamical friction plus a random fluctuating force, solving the Langevin equation with the Mannella quasi symplectic scheme. This amounts to a new simulation method which is much faster and simpler than direct N-body codes but retains their main feature: diffusion powered by strong, if infrequent, kicks. We compute the radial distribution of initially unsegregated BSS normalized to a reference population as a function of time. We trace the evolution of its minimum, corresponding to the zone of avoidance. We compare the evolution under kicks extracted from a Gaussian distribution to that obtained using a Holtsmark distribution. The latter is a fat tailed distribution which correctly models the effects of close gravitational encounters. We find that the zone of avoidance moves outwards over time, as expected based on observations, only when using the Holtsmark distribution. Thus the correct representation of near encounters is crucial to reproduce the dynamics of the system. We confirm and extend earlier results that showed how the dynamical clock indicator depends both on dynamical friction and effective diffusion powered by dynamical encounters., Comment: 8 pages, 6 figures. Version accepted in Astronomy & Astrophysics

Young star clusters are the most common birth-place of massive stars and are dynamically active environments. Here, we study the formation of black holes (BHs) and binary black holes (BBHs) in young star clusters, by means of 6000 N-body simulations coupled with binary population synthesis. We probe three different stellar metallicities (Z=0.02, 0.002 and 0.0002) and two initial density regimes (density at the half-mass radius $\rho_{\rm h}\ge{}3.4\times10^4$ and $\ge{1.5\times10^2}$ M$_\odot$ pc$^{-3}$ in dense and loose star clusters, respectively). Metal-poor clusters tend to form more massive BHs than metal-rich ones. We find $\sim{}6$, $\sim{}2$, and $<1$% of BHs with mass $m_{\rm BH}>60$ M$_\odot$ at Z=0.0002, 0.002 and 0.02, respectively. In metal-poor clusters, we form intermediate-mass BHs with mass up to $\sim{}320$ M$_\odot$. BBH mergers born via dynamical exchanges (exchanged BBHs) can be more massive than BBH mergers formed from binary evolution: the former (latter) reach total mass up to $\sim{}140$ M$_\odot$ ($\sim{}80$ M$_\odot$). The most massive BBH merger in our simulations has primary mass $\sim{}88$ M$_\odot$, inside the pair-instability mass gap, and a mass ratio of $\sim{}0.5$. Only BBHs born in young star clusters from metal-poor progenitors can match the masses of GW170729, the most massive event in O1 and O2, and those of GW190412, the first unequal-mass merger. We estimate a local BBH merger rate density $\sim{}110$ and $\sim{}55$ Gpc$^{-3}$ yr$^{-1}$, if we assume that all stars form in loose and dense star clusters, respectively., Comment: 14 pages, 10 figures, 5 tables, accepted for publication in MNRAS

We investigate the properties of the host galaxies of compact binary mergers across cosmic time. To this end, we combine population synthesis simulations together with galaxy catalogues from the hydrodynamical cosmological simulation EAGLE to derive the properties of the host galaxies of binary neutron star (BNS), black hole-neutron star (BHNS) and binary black hole (BBH) mergers. Within this framework, we derive the host galaxy probability, i.e., the probability that a galaxy hosts a compact binary coalescence as a function of its stellar mass, star formation rate, $K_s$ magnitude and $B$ magnitude. This quantity is particularly important for low-latency searches of gravitational wave (GW) sources as it provides a way to rank galaxies lying inside the credible region in the sky of a given GW detection, hence reducing the number of viable host candidates. Furthermore, even if no electromagnetic counterpart is detected, the proposed ranking criterion can still be used to classify the galaxies contained in the error box. Our results show that massive galaxies (or equivalently galaxies with a high luminosity in $K_s$ band) have a higher probability of hosting BNS, BHNS, and BBH mergers. We provide the probabilities in a suitable format to be implemented in future low-latency searches., Comment: 13 pages, 6 figures. Accepted in MNRAS

We investigate the properties of the host galaxies of compact binary mergers across cosmic time, by means of population-synthesis simulations combined with galaxy catalogues from the EAGLE suite. We analyze the merger rate per galaxy of binary neutron stars (BNSs), black hole--neutron star binaries (BHNSs) and binary black holes (BBHs) from redshift zero up to redshift six. The binary merger rate per galaxy strongly correlates with the stellar mass of the host galaxy at any redshift considered here. This correlation is significantly steeper for BNSs than for both BHNSs and BBHs. Moreover, we find that the merger rate per galaxy depends also on host galaxy's star formation rate and metallicity. We derive a robust fitting formula that relates the merger rate per galaxy with galaxy's star formation rate, stellar mass and metallicity at different redshifts. The typical masses of the host galaxies increase significantly as redshift decreases, as a consequence of the interplay between delay time distribution of compact binaries and cosmic assembly of galaxies. Finally, we study the evolution of the merger rate density with redshift. At low redshift ($z\leq{}0.1$) early-type galaxies give a larger contribution to the merger rate density than late-type galaxies. This trend reverts at $z\ge{}1$., Comment: 15 pages, 8 figures. Submitted to MNRAS. Comments are welcome!

Recent interpretation of the color$-$magnitude diagrams of the Milky Way (MW) bulge has suggested that the observed double red-clump feature can be a natural consequence of He-enhanced stellar populations in the MW bulge. This implies that globular clusters (GCs), where the He-enhanced second-generation (SG) stars can be efficiently created, are the most likely candidate contributors of He-rich stars to the MW bulge. We extend this idea to the Galactic inner halo and investigate the fraction of the SG stars as a function of the Galactocentric distance. We use bluer blue-horizontal branch (bBHB) stars, which are assumed to be originated from He-rich SG populations, as proxies of SG stars, and find that the fraction of bBHB stars increases with decreasing Galactocentric distance. Simulations of the GC evolution in the MW tidal field qualitatively support the observed trend of bBHB enhancement in the inner halo. In these simulations, the increasing tidal force with decreasing Galactocentric distance leads to stripping of stars not only from the outskirts but also from the central regions of GCs, where SG stars are more abundant. We discuss the implication and prospect of our findings concerning the formation history of the bulge and inner halo of the MW., Comment: 11 pages, 4 figures, accepted for publication in the ApJL

Sizeable number of stellar-mass black holes (BHs) in globular clusters (GCs) can strongly influence the dynamical evolution and observational properties of their host cluster. Using results from a large set of numerical simulations, we identify the key ingredients needed to sustain a sizeable population of BHs in GCs up to a Hubble time. We find that while BH natal kick prescriptions are essential in determining the initial retention fraction of BHs in GCs, the long-term survival of BHs is determined by the size, initial central density and half-mass relaxation time of the GC. Simulated GC models that contain many BHs are characterized by relatively low central surface brightness, large half-light and core radii values. We also discuss novel ways to compare simulated results with available observational data to identify GCs that are most likely to contain many BHs., Comment: 4 pages, 2 figures, 1 table. Contribution to the proceedings of the IAU Symposium No. 351: "Star Clusters: from the Milky Way to the Early Universe"

The Alessandrini A$+$ indicator is a quantitative measure of star cluster dynamical evolution based on the mass-segregation of blue straggler stars. A$+$ is defined as the integral of the cumulative distribution of blue straggler stars with the radius measured in logarithmic scale, minus a term related to the reference population used. In a companion paper I introduced a simplified model of dynamical friction and calculated the A$+$ indicator analytically. Here I show further properties of the time evolution of A$+$, focusing in particular on the physical interpretation of its time derivative $d{A^+}/dt$. I find that $d{A^+}/dt$ is the mean of the reciprocal dyamical friction timescale, weighted by the density of blue straggler stars. I show that it is non-negative (as expected based on monotonicity) due to the density of blue-straggler stars being non-negative and that, for a dynamical friction timescale that is non-decreasing with radius, $d{A^+}/dt$ is also non-decreasing with time, making the A$+$ indicator a convex function., Comment: No figures, submitted to RevMexAA

Blue straggler stars are more massive than the average star in globular clusters, as they originate from the merger of two stars. Consequently, they experience dynamical friction, progressively sinking to the cluster center. Recently, several indicators of the degree of dynamical relaxation of a globular cluster have been proposed, based on the observed radial distribution of blue straggler stars. The most successful is the Alessandrini indicator, or A+ for short, which is the integral of the cumulative distribution of the blue straggler stars minus that of a lighter reference population. A+ correlates with the dynamical age of a cluster both in realistic simulations and in observations. Here I calculate the temporal dependence of the A+ indicator analytically in a simplified model of the evolution of the blue straggler star distribution under dynamical friction only., Comment: No figures, Submitted to RevMexAA

In the massive globular cluster NGC 2808, RGB stars form at least five distinct groups in the so-called chromosome map photometric plane, arguably corresponding to different stellar populations. While a human expert can separate the groups by eye relatively easily, algorithmic approaches are desirable for reproducibility and for handling a larger sample of globular clusters. Unfortunately, cluster analysis algorithms often produced unsatisfactory results. Here we apply a range of non-parametric clustering algorithms to the NGC 2808 RGB dataset: partitioning (k-means, Partitioning Around Medoids - PAM), hierarchical (AGglomerative NESting - AGNES, DIvisive ANAlysis - DIANA), and density based (Density-Based Spatial Clustering of Applications with Noise - DBSCAN, Ordering Points To Identify the Clustering Struture - OPTICS). For each algorithm we discuss different choices of the relevant hyperparameters and their impact on the resulting clustering. We find that AGNES produces results that are most similar to the expectations of a human expert, depending on the prescription used for joining adjacent groups - linkage. Among the linkage prescriptions we tested, Ward's method performs best, and average linkage obtains comparable results only if outliers are removed beforehand. We recommend using AGNES with Ward's method or similar linkages in future studies to automatically identify stellar populations in the chromosome map plane., Comment: submitted to MNRAS

Globular Clusters (GCs) have historically been subdivided in either two (disk/halo) or three (disk/inner-halo/outer-halo) groups based on their orbital, chemical and internal physical properties. The qualitative nature of this subdivision makes it impossible to determine whether the natural number of groups is actually two, three, or more. In this paper we use cluster analysis on the $(\log M, \log \sigma_0, \log R_e, [Fe/H], \log | Z |)$ space to show that the intrinsic number of GC groups is actually either $k=2$ or $k=3$, with the latter being favored albeit non-significantly. In the $k=2$ case, the Partitioning Around Medoids (PAM) clustering algorithm recovers a metal-poor halo GC group and a metal-rich disk GC group. With $k=3$ the three groups can be interpreted as disk/inner-halo/outer-halo families. For each group we obtain a medoid, i.e. a representative element (NGC $6352$, NGC $5986$, and NGC $5466$ for the disk, inner halo, and outer halo respectively), and a measure of how strongly each GC is associated to its group, the so-called silhouette width. Using the latter, we find a correlation with age for both disk and outer halo GCs where the stronger the association of a GC with the disk (outer halo) group, the younger (older) it is., Comment: 17 figures, MNRAS submitted

Searching for distinctive signatures, which characterize different formation channels of binary black holes (BBHs), is a crucial step towards the interpretation of current and future gravitational wave detections. Here, we investigate the demography of merging BBHs in young star clusters (SCs), which are the nursery of massive stars. We performed $4\times{} 10^3$ N-body simulations of SCs with metallicity $Z=0.002$, initial binary fraction $0.4$ and fractal initial conditions, to mimic the clumpiness of star forming regions. Our simulations include a novel population-synthesis approach based on the code MOBSE. We find that SC dynamics does not affect the merger rate significantly, but leaves a strong fingerprint on the properties of merging BBHs. More than 50 % of merging BBHs in young SCs form by dynamical exchanges in the first few Myr. Dynamically formed merging BBHs are significantly heavier than merging BBHs in isolated binaries: merging BBHs with total mass up to $\sim{}120$ M$_\odot$ form in young SCs, while the maximum total mass of merging BBHs in isolated binaries with the same metallicity is only $\sim{}70$ M$_\odot$. Merging BBHs born via dynamical exchanges tend to have smaller mass ratios than BBHs in isolated binaries. Furthermore, SC dynamics speeds up the merger: the delay time between star formation and coalescence is significantly shorter in young SCs. In our simulations, massive systems such as GW170729 form only via dynamical exchanges. Finally $\sim{}2$ % of merging BBHs in young SCs have mass in the pair-instability mass gap ($\sim{}60-120$ M$_\odot$). This represents a unique fingerprint of merging BBHs in SCs., Comment: 15 pages, 11 figures, 5 tables

Machine learning is a powerful technique, becoming increasingly popular in astrophysics. In this paper, we apply machine learning to more than a thousand globular cluster (GC) models simulated as part of the 'MOCCA-Survey Database I' project in order to correlate present-day observable properties with the presence of a subsystem of stellar mass black holes (BHs). The machine learning model is then applied to available observed parameters for Galactic GCs to identify which of them that are most likely to be hosting a sizeable number of BHs and reveal insights into what properties lead to the formation of BH subsystems. With our machine learning model, we were able to shortlist 21 Galactic GCs that are most likely to contain a BH subsystem. We show that the clusters shortlisted by the machine learning classifier include those in which BH candidates have been observed (M22, M10 and NGC 3201) and that our results line up well with independent simulations and previous studies that manually compared simulated GC models with observed properties of Galactic GCs. These results can be useful for observers searching for elusive stellar mass BH candidates in GCs and further our understanding of the role BHs play in GC evolution. In addition, we have released an online tool that allows one to get predictions from our model after they input observable properties., Comment: 20 pages, 9 figures, 7 tables. Accepted for publication in MNRAS. Source code available at https://github.com/ammaraskar/black-holes-black-boxes

The shape of the radial distribution of Blue Straggler Stars (BSS), when normalized to a reference population of Horizontal Branch (HB) stars, has been found to be a powerful indicator of the dynamical evolution reached by a Globular Cluster (GC). In particular, observations suggest that the BSS distribution bimodality is modulated by the dynamical age of the host GC, with dynamically unrelaxed GCs showing a flat BSS distribution, and more relaxed GCs showing a minimum at a radius that increases for increasing dynamical age, resulting in a natural dynamical clock. While direct N-body simulations are able to reproduce the general trend, thus supporting its dynamical origin, the migration of the minimum of the distribution appears to be noisy and not well defined. Here we show that a simple unidimensional model based on dynamical friction (drift) and Brownian motion (diffusion) correctly reproduces the qualitative motion of the minimum, without adjustable parameters except for the BSS to HB stars mass-ratio. Differential dynamical friction effects combine with diffusion in creating a bimodality in the BSS distribution and determining its evolution, driving the migration of the minimum to larger radii over time. The diffusion coefficient is strongly constrained by the need to reproduce the migratory behaviour of the minimum, and the radial dependence of diffusion set by fundamental physical arguments automatically satisfies this constraint. Therefore, our model appears to capture the fluctuation-dissipation dynamics that underpins the dynamical clock., Comment: 6 pages, 4 figures

The high velocity gradient observed in the compact cloud CO-0.40-0.22, at a projected distance of 60 pc from the centre of the Milky Way, has led its discoverers to identify the closeby mm continuum emitter, CO-0.40-0.22*, with an intermediate mass black hole (IMBH) candidate. We describe the interaction between CO-0.40-0.22 and the IMBH, by means of a simple analytical model and of hydrodynamical simulations. Through such calculation, we obtain a lower limit to the mass of CO-0.40-0.22* of few $10^4 \times \; M_{\odot}$. This result tends to exclude the formation of such massive black hole in the proximity of the Galactic Centre. On the other hand, CO-0.40-0.22* might have been brought to such distances in cosmological timescales, if it was born in a dark matter halo or globular cluster around the Milky Way., Comment: 9 pages, 4 figures. To be published on MNRAS

Most globular clusters (GCs) are now known to host multiple stellar populations with different light element abundances. Here we use narrow-band photometry and low-resolution spectroscopy for NGC 362 and NGC 6723 to investigate their chemical properties and radial distributions of subpopulations. We confirm that NGC 362 and NGC 6723 are among the GCs with multiple populations showing bimodal CN distribution and CN-CH anti-correlation without a significant spread in calcium abundance. These two GCs show more centrally concentrated CN-weak earlier generation stars compared to the later generation CN-strong stars. These trends are reversed with respect to those found in previous studies for many other GCs. Our findings, therefore, seem contradictory to the current scenario for the formation of multiple stellar populations, but mass segregation acting on the two subpopulations might be a possible solution to explain this reversed radial trend., Comment: 12 pages, 8 figures, 2 tables, accepted for publication in ApJ

Mergers of stellar-mass black holes were recently observed in the gravitational wave window opened by LIGO. This puts the spotlight on dense stellar systems and their ability to create intermediate-mass black holes (IMBHs) through repeated merging. Unfortunately, attempts at direct and indirect IMBH detection in star clusters in the nearby universe have proven inconclusive as of now. Indirect detection methods attempt to constrain IMBHs through their effect on star cluster photometric and kinematic observables. They are usually based on looking for a specific, physically motivated signature. While this approach is justified, it may be suboptimal in its usage of the available data. Here I present a new indirect detection method, based on machine learning, that is unaffected by these restrictions. I reduce the scientific question whether a star cluster hosts an IMBH to a classification problem in the machine learning framework. I present preliminary results to illustrate how machine learning models are trained on simulated datasets and measure their performance on previously unseen, simulated data., Comment: Four pages, two figures, proceedings of the MODEST-16 meeting (Apr. 2016 Bologna, Italy)

Recently, both stellar mass-segregation and binary-fractions were uniformly measured on relatively large samples of Galactic Globular Clusters (GCs). Simulations show that both sizeable binary-star populations and Intermediate-Mass Black Holes (IMBHs) quench mass-segregation in relaxed GCs. Thus mass-segregation in GCs with a reliable binary-fraction measurement is a valuable probe to constrain IMBHs. In this paper we combine mass-segregation and binary-fraction measurements from the literature to build a sample of 33 GCs (with measured core-binary fractions), and a sample of 43 GCs (with a binary fraction measurement in the area between the core radius and the half-mass radius). Within both samples we try to identify IMBH-host candidates. These should have relatively low mass-segregation, a low binary fraction (< 5%), and short (< 1 Gyr) relaxation time. Considering the core binary fraction sample, no suitable candidates emerge. If the binary fraction between the core and the half-mass radius is considered, two candidates are found, but this is likely due to statistical fluctuations. We also consider a larger sample of 54 GCs where we obtained an estimate of the core binary fraction using a predictive relation based on metallicity and integrated absolute magnitude. Also in this case no suitable candidates are found. Finally, we consider the GC core- to half-mass radius ratio, that is expected to be larger for GCs containing either an IMBH or binaries. We find that GCs with large core- to half-mass radius ratios are less mass-segregated (and show a larger binary fraction), confirming the theoretical expectation that the energy sources responsible for the large core are also quenching mass-segregation, Comment: 5 pages, 7 figures, ApJ accepted

Context. Machine-Learning (ML) solves problems by learning patterns from data, with limited or no human guidance. In Astronomy, it is mainly applied to large observational datasets, e.g. for morphological galaxy classification. Aims. We apply ML to gravitational N-body simulations of star clusters that are either formed by merging two progenitors or evolved in isolation, planning to later identify Globular Clusters (GCs) that may have a history of merging from observational data. Methods. We create mock-observations from simulated GCs, from which we measure a set of parameters (also called features in the machine-learning field). After dimensionality reduction on the feature space, the resulting datapoints are fed to various classification algorithms. Using repeated random subsampling validation we check whether the groups identified by the algorithms correspond to the underlying physical distinction between mergers and monolithically evolved simulations. Results. The three algorithms we considered (C5.0 trees, k-nearest neighbour, and support-vector machines) all achieve a test misclassification rate of about 10% without parameter tuning, with support-vector machines slightly outperforming the others. The first principal component of feature space correlates with cluster concentration. If we exclude it from the regression, the performance of the algorithms is only slightly reduced., Comment: 7 pages, 5 figures, A&A accepted (research note)

The formation of the Milky Way stellar halo is thought to be the result of merging and accretion of building blocks such as dwarf galaxies and massive globular clusters. Recently, Deason et al. (2015) suggested that the Milky Way outer halo formed mostly from big building blocks, such as dwarf spheroidal galaxies, based on the similar number ratio of blue straggler (BS) stars to blue horizontal-branch (BHB) stars. Here we demonstrate, however, that this result is seriously biased by not taking into detailed consideration on the formation mechanism of BHB stars from helium enhanced second-generation population. In particular, the high BS-to-BHB ratio observed in the outer halo fields is most likely due to a small number of BHB stars provided by GCs rather than to a large number of BS stars. This is supported by our dynamical evolution model of GCs which shows preferential removal of first generation stars in GCs. Moreover, there are a sufficient number of outer halo GCs which show very high BS-to-BHB ratio. Therefore, the BS-to-BHB number ratio is not a good indicator to use in arguing that more massive dwarf galaxies are the main building blocks of the Milky Way outer halo. Several lines of evidence still suggest that GCs can contribute a significant fraction of the outer halo stars., Comment: 5 pages, 3 figures, 1 table, accepted for publication in MNRAS

Context. Stellar collision rates in globular clusters (GCs) do not appear to correlate with horizontal branch (HB) morphology, sug- gesting that dynamics does not play a role in the second-parameter problem. However, core densities and collision rates derived from surface-brightness may be significantly underestimated as the surface-brightness profile of GCs is not necessarily a good indicator of the dynamical state of GC cores. Core-collapse may go unnoticed if high central densities of dark remnants are present. Aims. We test whether GC HB morphology data supports a dynamical contribution to the so-called second-parameter effect. Methods. To remove first-parameter dependence we fitted the maximum effective temperature along the HB as a function of metal- licity in a sample of 54 Milky Way GCs. We plotted the residuals to the fit as a function of second-parameter candidates, namely dynamical age and total luminosity. We considered dynamical age (i.e. the ratio between age and half-light relaxation time) among possible second-parameters. We used a set of direct N-body simulations, including ones with dark remnants to illustrate how core density peaks, due to core collapse, in a dynamical-age range similar to that in which blue HBs are overabundant with respect to the metallicity expectation, especially for low-concentration initial conditions. Results. GC total luminosity shows nonlinear behavior compatible with the self-enrichment picture. However, the data are amenable to a different interpretation based on a dynamical origin of the second-parameter effect. Enhanced mass-stripping in the late red-giant- branch phase due to stellar interactions in collapsing cores is a viable candidate mechanism. In this picture, GCs with HBs bluer than expected based on metallicity are those undergoing core-collapse., Comment: 7 pages, 7 figures, A&A accepted