arXiv subjects

Eugene Vasiliev

Publications and source records attributed to Eugene Vasiliev.

At least 37 records · Page 2Linked to original sources

arXiv2023

Dear Magellanic Clouds, welcome back!

We propose a scenario in which the Large Magellanic Cloud (LMC) is on its second passage around the Milky Way. Using a series of tailored N-body simulations, we demonstrate that such orbits are consistent with current observational constraints on the mass distribution and relative velocity of both galaxies. The previous pericentre passage of the LMC could have occurred 5-10 Gyr ago at a distance >~100 kpc, large enough to retain its current population of satellites. The perturbations of the Milky Way halo induced by the LMC look nearly identical to the first-passage scenario, however, the distribution of LMC debris is considerably broader in the second-passage model. We examine the likelihood of current and past association with the Magellanic system for dwarf galaxies in the Local Group, and find that in addition to 10-11 current LMC satellites, it could have brought a further 4-6 galaxies that have been lost after the first pericentre passage. In particular, four of the classical dwarfs - Carina, Draco, Fornax and Ursa Minor - each have a ~50% probability of once belonging to the Magellanic system, thus providing a possible explanation for the ``plane of satellites'' conundrum.

astro-ph.GA↗

arXiv2023

Chemodynamical models of our Galaxy

A chemodynamical model of our galaxy is fitted to data from DR17 of the APOGEE survey supplemented with data from the StarHorse catalogue and gaia DR3. Dynamically, the model is defined by action-based distribution functions for dark matter and six stellar components plus a gas disc. The gravitational potential jointly generated by the model's components is used to examine the galaxy's chemical composition within action space. The observational data probably cover all parts of action space that are populated by stars. The overwhelming majority of stars have angular momentum J_ϕ>0 implying that they were born in the Galactic disc. High-alpha stars dominate in a region that is sharply bounded by J_ϕ\la J_ϕ(solar). Chemically the model is defined by giving each stellar component a Gaussian distribution in ([Fe/H],[Mg/Fe]) space about a mean that is a linear function of the actions. The model's 47 dynamical parameters are chosen to maximise the likelihood of the data given the model in 72 three-dimensional velocity spaces while its 70 chemical parameters are similarly chosen in five-dimensional chemo-dynamical space. The circular speed falls steadily from 237\kms at R=4\kpc to 218\kms at R=20\kpc. Dark matter contributes half the radial force on the Sun and has local density 0.011\msun\pc^{-3}, there being 24.5\msun\pc^{-2} in dark matter and 26.5\msun\pc^{-2} in stars within 1.1\kpc of the plane.

astro-ph.GA↗

arXiv2023

Deprojection and stellar dynamical modelling of boxy/peanut bars in edge-on discs

We present a new method to infer the 3D dimensional luminosity distributions of edge-on barred galaxies with boxy-peanut/X (BP/X) shaped structures from their 2D surface brightness distributions. Our method relies on forward modeling of newly introduced parametric 3D density distributions for the BP/X bar, disc and other components using an existing image fitting software package (IMFIT). We validate our method using an N-body simulation of a barred disc galaxy with a moderately strong BP/X shape. For fixed orientation angles the derived 3D BP/X shaped density distribution is shown to yield a gravitational potential that is accurate to at least 5% and forces that are accurate to at least 15%, with average errors being ~1.5% for both. When additional quantities of interest, such as the orientation of the bar to the line-of-sight, its pattern speed, and the stellar mass-to-light ratio are unknown they can be recovered to high accuracy by providing the parametric density distribution to the Schwarzschild modelling code FORSTAND. We also explore the ability of our models to recover the mass of the central supermassive black hole. This method is the first to be able to accurately recover both the orientation of the bar to the line-of-sight and its pattern speed even when the disc is perfectly edge-on.

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arXiv2023

The ALMA Interferometric Pipeline Heuristics

We describe the calibration and imaging heuristics developed and deployed in the ALMA interferometric data processing pipeline, as of ALMA Cycle 9. The pipeline software framework is written in Python, with each data reduction stage layered on top of tasks and toolkit functions provided by the Common Astronomy Software Applications package. This framework supports a variety of tasks for observatory operations, including science data quality assurance, observing mode commissioning, and user reprocessing. It supports ALMA and VLA interferometric data along with ALMA and NRO45m single dish data, via different stages and heuristics. In addition to producing calibration tables, calibrated measurement sets, and cleaned images, the pipeline creates a WebLog which serves as the primary interface for verifying the data quality assurance by the observatory and for examining the contents of the data by the user. Following the adoption of the pipeline by ALMA Operations in 2014, the heuristics have been refined through annual development cycles, culminating in a new pipeline release aligned with the start of each ALMA Cycle of observations. Initial development focused on basic calibration and flagging heuristics (Cycles 2-3), followed by imaging heuristics (Cycles 4-5), refinement of the flagging and imaging heuristics with parallel processing (Cycles 6-7), addition of the moment difference analysis to improve continuum channel identification (2020 release), addition of a spectral renormalization stage (Cycle 8), and improvement in low SNR calibration heuristics (Cycle 9). In the two most recent Cycles, 97% of ALMA datasets were calibrated and imaged with the pipeline, ensuring long-term automated reproducibility. We conclude with a brief description of plans for future additions, including self-calibration, multi-configuration imaging, and calibration and imaging of full polarization data.

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arXiv2023

The fragility of thin discs in galaxies -- II. Thin discs as tracers of the assembly history of galaxies

Thin galactic discs and nuclear stellar discs (NSDs) are fragile structures that can be easily disturbed by merger events. By studying the age of the stellar populations in present-day discs, we can learn about the assembly history of galaxies and place constraints on their past merger events. Following on the steps of our initial work, we explore the fragility of such disc structures in intermediate-mass-ratio dry encounters using the previously constructed $N$-body model of the Fornax galaxy NGC 1381 (FCC 170), which hosts both a thin galactic disc and a NSD. We dismiss major and minor encounters, as the former were previously shown to easily destroy thin-disc structures, whereas the latter take several Hubble times to complete in the specific case of FCC 170. The kinematics and structure of the thin galactic disc are dramatically altered by the mergers, whereas the NSD shows a remarkable resilience, exhibiting only a smooth increase of its size when compared to the model evolved in isolation. Our results suggest that thin galactic discs are better tracers for intermediate-mass-ratio mergers, while NSDs may be more useful for major encounters. Based on our simulations and previous analysis of the stellar populations, we concluded that FCC 170 has not experienced any intermediate-mass-ratio dry encounters for at least $\sim$10 Gyr, as indicated by the age of its thin-disc stellar populations.

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arXiv2023

The effect of the LMC on the Milky Way system

We review the recent theoretical and observational developments concerning the interaction of the Large Magellanic Cloud (LMC) with the Milky Way and its neighbourhood. An emerging picture is that the LMC is a fairly massive companion (10-20% of the Milky Way mass) and just passed the pericentre of its orbit, likely for the first time. The gravitational perturbation caused by the LMC is manifested at different levels. The most immediate effect is the deflection of orbits of stars, stellar streams or satellite galaxies passing in the vicinity of the LMC. Less well known but equally important is the displacement (reflex motion) of central regions of the Milky Way about the centre of mass of both galaxies. Since the Milky Way is not a rigid body, this displacement varies with the distance from the LMC, and as a result, the Galaxy is deformed and its outer regions (beyond a few tens kpc) acquire a net velocity with respect to its centre. These phenomena need to be taken into account at the level of precision warranted by current and future observational data, and improvements on the modelling side are also necessary for an adequate interpretation of these data.

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arXiv2023

Accelerated phase-mixing in the stellar halo due to a rotating bar

In a galaxy merger, the stars tidally stripped from the satellite and accreted onto the host galaxy undergo phase mixing and form finely-grained structures in the phase space. However, these fragile structures may be destroyed in the subsequent galaxy evolution, in particular, by a rotating bar that appears well after the merger is completed. In this work, we investigate the survivability of phase-space structures in the presence of a bar. We find that a bar with amplitude and pattern speed similar to those of the Milky Way would blur and destroy a substantial amount of the substructure that consists of particles with pericentre radii comparable to the bar length. While this appears to be in tension with the recent discovery of phase-space chevrons in \textit{Gaia} DR3 data, the most prominent chevrons in our simulations can still be recovered when applying the same analysis procedure as in observations. Moreover, the smoothing effect is less pronounced in the population of stars whose angular momenta have the opposite sign to the bar pattern speed.

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arXiv2023

The fragility of thin discs in galaxies -- I. Building tailored N-body galaxy models

Thin stellar discs on both galactic and nuclear, sub-kpc scales are believed to be fragile structures that would be easily destroyed in major mergers. In turn, this makes the age-dating of their stellar populations a useful diagnostics for the assembly history of galaxies. We aim at carefully exploring the fragility of such stellar discs in intermediate- and low- mass encounters, using high-resolution N-body simulations of galaxy models with structural and kinematic properties tailored to actually observed galaxies. As a first but challenging step, we create a dynamical model of FCC 170, a nearly edge-on galaxy in the Fornax cluster with multiple galactic components and including both a galactic scale and nuclear stellar disc (NSD), using detailed kinematic data from the Multi Unit Spectroscopic Explorer and a novel method for constructing distribution function-based self-consistent galaxy models. We then create N-body realisations of this model and demonstrate that it remains in equilibrium and preserves its properties over many Gyr, when evolved with a sufficiently high particle number. However, the NSD is more prone to numerical heating, which gradually increases its thickness by up to 22 per cent in 10 Gyr even in our highest-resolution runs. Nevertheless, these N-body models can serve as realistic representations of actual galaxies in merger simulations.

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arXiv2022

Ironing the folds: The phase space chevrons of a GSE-like merger as a dark matter subhalo detector

Recent work uncovered features in the phase space of the Milky Way's stellar halo which may be attributed to the last major merger. When stellar material from a satellite is accreted onto its host, it phase mixes and appears finely substructured in phase space. For a high-eccentricity merger, this substructure most clearly manifests as numerous wrapping chevrons in $(v_r, r)$ space, corresponding to stripes in $(E, θ_r)$ space. We introduce the idea of using this substructure as an alternative subhalo detector to cold stellar streams. We simulate an N-body merger akin to the GSE and assess the impact of subhaloes on these chevrons. We examine how their deformation depends on the mass, pericentre, and number of subhaloes. To quantify the impact of perturbers, we utilise the appearance of chevrons in $(E, θ_r)$ space to introduce a new quantity -- the ironing parameter. We show that: (1) a single flyby of a massive ($\sim 10^{10}$ M$_{\odot}$) subhalo with pericentre comparable to, or within, the shell's apocentre smooths out the substructure, (2) a single flyby of a low mass ($\lesssim 10^8$ M$_{\odot}$) has negligible effect, (3) multiple flybys of subhalos derived from a subhalo mass function between $10^7-10^{10}$ M$_{\odot}$ cause significant damage if deep within the potential, (4) the effects of known perturbers (e.g. Sagittarius) should be detectable and offer constraints on their initial mass. The sensitivity to the populations of subhaloes suggests that we should be able to place an upper limit on the Milky Way's subhalo mass function.

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arXiv2022

The Mass of the Black Hole in NGC 5273 from Stellar Dynamical Modeling

We present a new constraint on the mass of the black hole in the active S0 galaxy NGC 5273. Due to the proximity of the galaxy at $16.6 \pm 2.1$ Mpc, we were able to resolve and extract the bulk motions of stars near the central black hole using AO-assisted observations with Gemini NIFS, as well as constrain the large-scale kinematics using re-reduced archival SAURON spectroscopy. High resolution HST imaging allowed us to generate a surface brightness decomposition, determine approximate mass-to-light ratios for the bulge and disk, and obtain an estimate for the disk inclination. We constructed an extensive library of dynamical models using the Schwarzschild orbit-superposition code FORSTAND, exploring a range of disk and bulge shapes, halo masses, etc. We determined a black hole mass of $M_{\bullet} = [0.5 - 2] \times 10^{7}$ $M_{\odot}$, where the low side of the range is in agreement with the reverberation mapping measurement of $M_{\bullet} = [4.7 \pm 1.6] \times 10^{6}$ $M_{\odot}$. NGC 5273 is one of only a small number of nearby galaxies hosting broad-lined AGN, allowing crucial comparison of the black hole masses derived from different mass measurement techniques.

astro-ph.GA↗

arXiv2022

Energy wrinkles and phase-space folds of the last major merger

Relying on the dramatic increase in the number of stars with full 6D phase-space information provided by the Gaia Data Release 3, we discover unambiguous signatures of phase-mixing in the stellar halo around the Sun. We show that for the stars likely belonging to the last massive merger, the (v_r,r) distribution contains a series of long and thin chevron-like overdensities. These phase-space sub-structures are predicted to emerge following the dissolution of a satellite, when its tidal debris is given time to wind up, thin out and fold. Additionally, the observed energy and angular momentum (E, L_z) distribution appears more prograde at high energies, possibly revealing the original orbital angular momentum of the in-falling galaxy. The energy distribution of the debris is strongly asymmetric with a peak at low E -- which, we surmise, may be evidence of the dwarf's rapid sinking -- and riddled with wrinkles and bumps. If these small-scale energy inhomogeneities have been seeded during or immediately after the interaction with the Milky Way, and are not due to the spatial restriction of our study, then making use of the (v_r,r) chevrons to constrain the time of the merger becomes cumbersome. Nonetheless, we demonstrate that similar phase-space and (E,L_z) sub-structures are present in numerical simulations of galaxy interactions, both in bespoke N-body runs and in cosmological hydrodynamical zoom-in suites. The remnant traces of the progenitor's disruption and the signatures of the on-going phase-mixing discovered here will not only help to constrain the properties of our Galaxy's most important interaction, but also can be used as a novel tool to map out the Milky Way's current gravitational potential and its perturbations.

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arXiv2022

Self-consistent models of our Galaxy

A new class of models of stellar discs is introduced and used to build a self-consistent model of our Galaxy. The model is defined by the parameters that specify the action-based distribution functions (DFs) f(J) of four stellar discs (three thin-disc age cohorts and a thick disc), spheroidal bulge and spheroidal stellar and dark haloes. From these DFs plus a specified distribution of gas, we solve for the densities of stars and dark matter and the potential they generate. The principal observational constraints are the kinematics of stars with Gaia RVS data and the density of stars in the column above the Sun. The model predicts the density and kinematics of stars and dark matter throughout the Galaxy. We determine the structure of the dark halo prior to the infall of baryons. A simple extension of the DFs of stellar components to include chemistry allows the model to reproduce the way the Galaxy's chemistry is observed to vary in the (R,z) plane. Surprisingly, the data indicate that high-alpha stars are confined to orbits with J_z >~ 50 kpc km/s. The code used to create the model is available on Github.

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arXiv2022

The stellar mass distribution of the Milky Way's bar: an analytic model

We present an analytic model of the stellar mass distribution of the Milky Way bar. The model is obtained by fitting a multi-component parametric density distribution to a made-to-measure N-body model of Portail et al., constructed to match a variety of density and kinematics observational data. The analytic model reproduces in detail the 3D density distribution of the N-body bar including the X-shape. The model and the gravitational potential it generates are available as part of the software package AGAMA for galactic dynamics, and can be readily used for orbit integrations, hydrodynamical simulations or other applications.

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arXiv2022

The Local Group Mass in the light of Gaia

High accuracy proper motions (PMs) of M31 and other Local Group satellites have now been provided by the {\it Gaia} satellite. We revisit the Timing Argument to compute the total mass $M$ of the Local Group from the orbit of the Milky Way and M31, allowing for the Cosmological Constant. We rectify for a systematic effect caused by the presence of the Large Magellanic Cloud (LMC). The interaction of the LMC with the Milky Way induces a motion towards the LMC. This contribution to the measured velocity of approach of the Milky Way and M31 must be removed. We allow for cosmic bias and scatter by extracting correction factors tailored to the accretion history of the Local Group. The distribution of correction factors is centered around $0.63$ with a scatter $\pm 0.2$, indicating that the Timing Argument significantly overestimates the true mass. Adjusting for all these effects, the estimated mass of the Local Group is $ M = 3.4^{+1.4}_{-1.1} \times 10^{12} M_{\odot}$ (68 % CL) when using the M31 tangential velocity $ 82^{+38}_{-35}$ km/s. Lower tangential velocity models with $59^{+42}_{-38}$ km/s (derived from the same PM data with a flat prior on the tangential velocity) lead to an estimated mass of $ M = 3.1^{+1.3}_{-1.0} \times 10^{12} M_{\odot}$ (68 % CL). By making an inventory of the total mass associated with the 4 most substantial LG members (the Milky Way, M31, M33 and the LMC), we estimate the known mass is in the range $3.7^{+0.5}_{-0.5} \times 10^{12} \, M_{\odot}$.

astro-ph.GA↗

arXiv2022

Measuring the Milky Way mass distribution in the presence of the LMC

The ongoing interaction between the Milky Way (MW) and its largest satellite - the Large Magellanic Cloud (LMC) - creates a significant perturbation in the distribution and kinematics of distant halo stars, globular clusters and satellite galaxies, and leads to biases in MW mass estimates from these tracer populations. We present a method for compensating these perturbations for any choice of MW potential by computing the past trajectory of LMC and MW and then integrating the orbits of tracer objects back in time until the influence of the LMC is negligible, at which point the equilibrium approximation can be used with any standard dynamical modelling approach. We add this orbit-rewinding step to the mass estimation approach based on simultaneous fitting of the potential and the distribution function of tracers, and apply it to two datasets with the latest Gaia EDR3 measurements of 6d phase-space coordinates: globular clusters and satellite galaxies. We find that models with LMC mass in the range (1-2)x10^11 Msun better fit the observed distribution of tracers, and measure MW mass within 100 kpc to be (0.75+-0.1)x10^12 Msun, while neglecting the LMC perturbation increases it by ~15%.

astro-ph.GA↗

arXiv2022

Radialization of satellite orbits in galaxy mergers

We consider the orbital evolution of satellites in galaxy mergers, focusing on the evolution of eccentricity. Using a large suite of N-body simulations, we study the phenomenon of satellite orbital radialization -- a profound increase in the eccentricity of its orbit as it decays under dynamical friction. While radialization is detected in a variety of different setups, it is most efficient in the cases of high satellite mass, not very steep host density profiles, and high initial eccentricity. To understand the origin of this phenomenon, we run additional simulations with various physical factors selectively turned off: satellite mass loss, reflex motion and distortion of the host, etc. We find that all these factors are important for radialization, since it does not occur for point-mass satellites or when the host potential is replaced with an unperturbed initial profile. The analysis of forces and torques acting on both galaxies confirms the major role of self-gravity of both host and satellite in the reduction of orbital angular momentum. The classical Chandrasekhar dynamical friction formula, which accounts only for the forces between the host and the satellite, but not for internal distortions of both galaxies, does not match the evolution of eccentricity observed in N-body simulations.

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arXiv2022

Non-Parametric Spherical Jeans Mass Estimation with B-splines

Spherical Jeans modeling is widely used to estimate mass profiles of systems from star clusters to galactic stellar haloes to clusters of galaxies. It derives the cumulative mass profile, M(<r), from kinematics of tracers of the potential under the assumptions of spherical symmetry and dynamical equilibrium. We consider the application of Jeans modeling to mapping the dark matter distribution in the outer reaches of the Milky Way using field halo stars. We present a novel non-parametric routine for solving the spherical Jeans equation by fitting B-splines to the velocity and density profiles of halo stars. While most implementations assume parametric forms for these profiles, B-splines provide non-parametric fitting curves with analytical derivatives. Our routine recovers the mass profiles of equilibrium systems with flattened haloes or a stellar disc and bulge excellently (<~ 10% error at most radii). Tests with non-equilibrium, Milky Way-like galaxies from the Latte suite of FIRE-2 simulations perform quite well (<~ 15% error for r <~ 100 kpc). We also create observationally motivated datasets for the Latte suite by imposing selection functions and errors on phase space coordinates characteristic of Gaia and the DESI Milky Way Survey. The resulting imprecise and incomplete data require us to introduce an MCMC-based subroutine to obtain deconvolved density and velocity dispersion profiles from the tracer population. With these observational effects taken into account, the accuracy of the Jeans mass estimate remains at the level 20% or better.

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arXiv2021

Gaia EDR3 view on Galactic globular clusters

We use the data from Gaia Early Data Release 3 (EDR3) to study the kinematic properties of Milky Way globular clusters. We measure the mean parallaxes and proper motions (PM) for 170 clusters, determine the PM dispersion profiles for more than 100 clusters, uncover rotation signatures in more than 20 objects, and find evidence for radial or tangential PM anisotropy in a dozen richest clusters. At the same time, we use the selection of cluster members to explore the reliability and limitations of the Gaia catalogue itself. We find that the formal uncertainties on parallax and PM are underestimated by 10-20% in dense central regions even for stars that pass numerous quality filters. We explore the the spatial covariance function of systematic errors, and determine a lower limit on the uncertainty of average parallaxes and PM at the level 0.01 mas and 0.025 mas/yr, respectively. Finally, a comparison of mean parallaxes of clusters with distances from various literature sources suggests that the parallaxes (after applying the zero-point correction suggested by Lindegren et al. 2021) are overestimated by 0.01+-0.003 mas. Despite these caveats, the quality of Gaia astrometry has been significantly improved in EDR3 and provides valuable insights into the properties of star clusters.

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