arXiv subjects

Eugene Vasiliev

Publications and source records attributed to Eugene Vasiliev.

At least 55 records · Page 3Linked to original sources

arXiv2021

The Black Hole Mass of NGC 4151 from Stellar Dynamical Modeling

The mass of a supermassive black hole ($M_\mathrm{BH}$) is a fundamental property that can be obtained through observational methods. Constraining $M_\mathrm{BH}$ through multiple methods for an individual galaxy is important for verifying the accuracy of different techniques, and for investigating the assumptions inherent in each method. NGC 4151 is one of those rare galaxies for which multiple methods can be used: stellar and gas dynamical modeling because of its proximity ($D=15.8\pm0.4$ Mpc from Cepheids), and reverberation mapping because of its active accretion. In this work, we re-analyzed $H-$band integral field spectroscopy of the nucleus of NGC 4151 from Gemini NIFS, improving the analysis at several key steps. We then constructed a wide range of axisymmetric dynamical models with the new orbit-superposition code Forstand. One of our primary goals is to quantify the systematic uncertainties in $M_\mathrm{BH}$ arising from different combinations of the deprojected density profile, inclination, intrinsic flattening, and mass-to-light ratio. As a consequence of uncertainties on the stellar luminosity profile arising from the presence of the AGN, our constraints on \mbh are rather weak. Models with a steep central cusp are consistent with no black hole; however, in models with more moderate cusps, the black hole mass lies within the range of $0.25\times10^7\,M_\odot \lesssim M_\mathrm{BH} \lesssim 3\times10^7\,M_\odot$. This measurement is somewhat smaller than the earlier analysis presented by Onken et al., but agrees with previous $M_\mathrm{BH}$ values from gas dynamical modeling and reverberation mapping. Future dynamical modeling of reverberation data, as well as IFU observations with JWST, will aid in further constraining $M_\mathrm{BH}$ in NGC 4151.

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arXiv2021

Accurate distances to Galactic globular clusters through a combination of Gaia EDR3, HST and literature data

We have derived accurate distances to Galactic globular clusters by combining data from the Gaia Early Data Release 3 with distances based on Hubble Space telescope HST data and literature based distances. We determine distances either directly from the Gaia EDR3 parallaxes, or kinematically by combining line-of-sight velocity dispersion profiles with Gaia EDR3 and HST based proper motion velocity dispersion profiles. We furthermore calculate cluster distances from fitting nearby subdwarfs, whose absolute luminosities we determine from their Gaia EDR3 parallaxes, to globular cluster main-sequences. We finally use HST based stellar number counts to determine distances. We find good agreement in the average distances derived from the different methods down to a level of about 2%. Combining all available data, we are able to derive distances to 162 Galactic globular clusters, with the distances to about 20 nearby globular clusters determined with an accuracy of 1% or better. We finally discuss the implications of our distances for the value of the local Hubble constant.

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arXiv2021

Tango for three: Sagittarius, LMC, and the Milky Way

We assemble a catalogue of candidate Sagittarius stream members with 5d and 6d phase-space information, using astrometric data from Gaia DR2, distances estimated from RR Lyrae stars, and line-of-sight velocities from various spectroscopic surveys. We find a clear misalignment between the stream track and the direction of the reflex-corrected proper motions in the leading arm of the stream, which we interpret as a signature of a time-dependent perturbation of the gravitational potential. A likely cause of this perturbation is the recent passage of the most massive Milky Way satellite - the Large Magellanic Cloud (LMC). We develop novel methods for simulating the Sagittarius stream in the presence of the LMC, using specially tailored N-body simulations and a flexible parametrization of the Milky Way halo density profile. We find that while models without the LMC can fit most stream features rather well, they fail to reproduce the misalignment and overestimate the distance to the leading arm apocentre. On the other hand, models with an LMC mass in the range (1.3+-0.3)x10^11 Msun rectify these deficiencies. We demonstrate that the stream can not be modelled adequately in a static Milky Way. Instead, our Galaxy is required to lurch toward the massive in-falling Cloud, giving the Sgr stream its peculiar shape and kinematics. By exploring the parameter space of Milky Way potentials, we determine the enclosed mass within 100 kpc to be (5.6+-0.4)x10^11 Msun, and the virial mass to be (9.0+-1.3)x10^11 Msun, and find tentative evidence for a radially-varying shape and orientation of the Galactic halo.

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arXiv2020

Action-based distribution function modelling for constraining the shape of the Galactic dark matter halo

We estimate the 3D density profile of the Galactic dark matter (DM) halo within $r \lesssim 30$ kpc from the Galactic centre by using the astrometric data for halo RR Lyrae stars from Gaia DR2. We model both the stellar halo distribution function and the Galactic potential, fully taking into account the survey selection function, the observational errors, and the missing line-of-sight velocity data for RR Lyrae stars. With a Bayesian MCMC method, we infer the model parameters, including the density flattening of the DM halo $q$, which is assumed to be constant as a function of radius. We find that 99\% of the posterior distribution of $q$ is located at $q>0.963$, which strongly disfavours a flattened DM halo. We cannot draw any conclusions as to whether the Galactic DM halo at $r \lesssim 30$ kpc is prolate, because we restrict ourselves to axisymmetric oblate halo models with $q\leq1$. Our result is inconsistent with predictions from cosmological hydrodynamical simulations that advocate more oblate ($\langle{q}\rangle \sim0.8 \pm 0.15$) DM halos within $\sim 15\%$ of the virial radius for Milky-Way-sized galaxies. An alternative possibility, based on our validation tests with a cosmological simulation, is that the true value $q$ of the Galactic halo could be consistent with cosmological simulations but that disequilibrium in the Milky Way potential is inflating our measurement of $q$ by 0.1-0.2. As a by-product of our analysis, our model constrains the DM density in the Solar neighbourhood to be $ρ_{\mathrm{DM},\odot} = (9.01^{+0.18}_{-0.20})\times10^{-3}M_\odot \mathrm{pc}^{-3} = 0.342^{+0.007}_{-0.007}$ $\;\mathrm{GeV} \mathrm{cm}^{-3}$.

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arXiv2020

The last breath of the Sagittarius dSph

We use the astrometric and photometric data from Gaia Data Release 2 and line-of-sight velocities from various other surveys to study the 3d structure and kinematics of the Sagittarius dwarf galaxy. The combination of photometric and astrometric data makes it possible to obtain a very clean separation of Sgr member stars from the Milky Way foreground; our final catalogue contains ~2.6e5 candidate members with magnitudes G<18, more than half of them being red clump stars. We construct and analyze maps of the mean proper motion and its dispersion over the region ~30x12 degrees, which show a number of interesting features. The intrinsic 3d density distribution (orientation, thickness) is strongly constrained by kinematics; we find that the remnant is a prolate structure with the major axis pointing at 45deg from the orbital velocity and extending up to ~5 kpc, where it transitions into the stream. We perform a large suite of N-body simulations of a disrupting Sgr galaxy as it orbits the Milky Way over the past 2.5 Gyr, which are tailored to reproduce the observed properties of the remnant (not the stream). The richness of available constraints means that only a narrow range of parameters produce a final state consistent with observations. The total mass of the remnant is ~4e8 Msun, of which roughly a quarter resides in stars. The galaxy is significantly out of equilibrium, and even its central density is below the limit required to withstand tidal forces. We conclude that the Sgr galaxy will likely be disrupted over the next Gyr.

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arXiv2020

Rates of Stellar Tidal Disruption

Tidal disruption events occur rarely in any individual galaxy. Over the last decade, however, time-domain surveys have begun to accumulate statistical samples of these flares. What dynamical processes are responsible for feeding stars to supermassive black holes? At what rate are stars tidally disrupted in realistic galactic nuclei? What may we learn about supermassive black holes and broader astrophysical questions by estimating tidal disruption event rates from observational samples of flares? These are the questions we aim to address in this Chapter, which summarizes current theoretical knowledge about rates of stellar tidal disruption, and compares theoretical predictions to the current state of observations.

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arXiv2019

Agama reference documentation

Agama (Action-based Galaxy Modelling Architecture) is a software library intended for a broad range of tasks within the field of stellar dynamics. As the name suggests, it is centered around the use of action/angle formalism to describe the structure of stellar systems, but this is only one of its many facets. The library contains a powerful framework for dealing with arbitrary density/potential profiles and distribution functions (analytic, extracted from N-body models, or fitted to the data), a vast collection of general-purpose mathematical routines, and covers many aspects of galaxy dynamics up to the very high-level interface for constructing self-consistent galaxy models. This document serves two purposes. First of all, this is a detailed reference for the library itself. Second, it describes various mathematical and numerical methods that could be applicable in a broader context. These include: (1) one- and multidimensional interpolation using cubic and quintic splines; (2) penalized spline fitting of noisy data and penalized spline density estimation; (3) adaptive rejection sampling from multidimensional probability distributions; (4) computation of gravitational potentials using spherical- and azimuthal-harmonic expansions; (5) Staeckel fudge method for computing action/angle variables; (6) a general discussion about good programming practices.

astro-ph.IM↗

arXiv2019

A new implementation of the Schwarzschild method for constructing observationally-driven dynamical models of galaxies of all morphological types

We present Forstand, a new code for constructing dynamical models of galaxies with the Schwarzschild orbit-superposition method. These models are constrained by line-of-sight kinematic observations and applicable to galaxies of all morphological types, including disks and triaxial rotating bars. Our implementation has several novel and improved features, is computationally efficient, and made publicly available. Using mock datasets taken from N-body simulations, we demonstrate that the pattern speed of a bar can be recovered with an accuracy of 10-20%, regardless of orientation, if the 3D shape of the galaxy is known or inferred correctly.

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arXiv2019

Schwarzschild modeling of barred galaxies

We review the Schwarzschild orbit-superposition approach and present a new implementation of this method, which can deal with a large class of systems, including rotating barred disk galaxies. We discuss two conceptuals problems in this field: the intrinsic degeneracy of determining the potential from line-of-sight kinematics, and the non-uniqueness of deprojection and related biases in potential inference, especially acute for triaxial bars. When applied to mock datasets with known 3d shape, our method correctly recovers the pattern speed and other potential parameters. However, more work is needed to systematically address these two problems for real observational datasets.

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arXiv2019

Using Gaia for studying Milky Way star clusters

We review the implications of the Gaia Data Release 2 catalogue for studying the dynamics of Milky Way globular clusters, focusing on two separate topics. The first one is the analysis of the full 6-dimensional phase-space distribution of the entire population of Milky Way globular clusters: their mean proper motions (PM) can be measured with an exquisite precision (down to 0.05 mas/yr, including systematic errors). Using these data, and a suitable ansatz for the steady-state distribution function (DF) of the cluster population, we then determine simultaneously the best-fit parameters of this DF and the total Milky Way potential. We also discuss possible correlated structures in the space of integrals of motion. The second topic addresses the internal dynamics of a few dozen of the closest and richest globular clusters, again using the Gaia PM to measure the velocity dispersion and internal rotation, with a proper treatment of spatially correlated systematic errors. Clear rotation signatures are detected in 10 clusters, and a few more show weaker signatures at a level >=0.05$ mas/yr. PM dispersion profiles can be reliably measured down to 0.1 mas/yr, and agree well with the line-of-sight velocity dispersion profiles from the literature.

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arXiv2019

Systematic errors in Gaia DR2 astrometry and their impact on measurements of internal kinematics of star clusters

We use stellar proper motions (PM) from Gaia Data Release 2 for studying the internal kinematics of Milky Way globular clusters. In addition to statistical measurement errors, there are significant spatially correlated systematic errors, which cannot be ignored when studying the internal kinematics. We develop a mathematically consistent procedure for incorporating the spatial correlations in any model-fitting approach, and use it to determine rotation and velocity dispersion profiles of a few dozen clusters. We confirm detection of rotation in the sky plane for ~10 clusters reported in previous studies, and discover a few more clusters with rotation amplitudes exceeding ~0.05 mas/yr. However, in more than half of these cases the significance of this rotation signature is rather low when taking into account the systematic errors. We find that the PM dispersion is not sensitive to systematic errors in PM, however, it is quite sensitive to the selection criteria on the input sample, most importantly, in crowded central regions. When using the cleanest possible samples, PM dispersion can be reliably measured down to 0.1 mas/yr for ~60 clusters.

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arXiv2019

Proper motions and dynamics of the Milky Way globular cluster system from Gaia DR2

We use Gaia Data Release 2 to determine the mean proper motions for 150 Milky Way globular clusters (almost the entire known population), with a typical uncertainty of 0.05 mas/yr limited mainly by systematic errors. Combining them with distance and line-of-sight velocity measurements from the literature, we analyze the distribution of globular clusters in the 6d phase space, using both position/velocity and action/angle coordinates. The population of clusters in the central 10 kpc has a mean rotational velocity reaching 50-80 km/s, and a nearly isotropic velocity dispersion 100-120 km/s, while in the outer galaxy the cluster orbits are strongly radially anisotropic. We confirm a concentration of clusters at high radial action in the outer region of the Galaxy. Finally, we explore a range of equilibrium distribution function-based models for the entire globular cluster system, and the information they provide about the potential of the Milky Way. The dynamics of clusters is best described by models with the circular velocity between 10 and 50 kpc staying in the range 210-240 km/s.

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arXiv2019

Evolution of supermassive black hole binaries and tidal disruption rates in nonspherical galactic nuclei

Binary supermassive black holes (SMBH) are expected to form naturally during galaxy mergers. After the dynamical friction phase, when the two SMBHs become gravitationally bound to each other, and a brief stage of initial rapid hardening, the orbit gradually continues to shrink due to three-body interactions with stars that enter the loss cone of the binary. Using the stellar-dynamical Monte Carlo code Raga, we explore the co-evolution of the binary SMBH and the nuclear star cluster in this slow stage, for various combinations of parameters (geometry of the star cluster, primary/secondary SMBH mass, initial eccentricity, inclusion of stellar captures/tidal disruptions). We compare the rates of stellar captures in galactic nuclei containing a binary to those of galaxies with a single SMBH. At early times, the rates are substantially higher in the case of a binary SMBH, but subsequently they drop to lower levels. Only in triaxial systems both the binary hardening rates and the capture rates remain sufficiently high during the entire evolution. We find that the hardening rate is not influenced by star captures, nor does it depend on eccentricity; however, it is higher when the difference between the black hole masses is greater. We confirm that the eccentricity of the binary tends to grow, which may significantly shorten the coalescence time due to earlier onset of gravitational-wave emission. We also explore the properties of the orbits entering the loss cone, and demonstrate that it remains partially full throughout the evolution in the triaxial case, but significantly depleted in the axisymmetric case. Finally, we study the distribution of ejected hypervelocity stars and the corresponding mass deficits in the central parts of the galaxies hosting a binary, and argue that the missing mass is difficult to quantify observationally.

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arXiv2018

AGAMA: Action-based galaxy modelling architecture

Agama is a publicly available software library for a broad range of applications in the field of stellar dynamics. It provides methods for computing the gravitational potential of arbitrary analytic density profiles or N-body models; orbit integration and analysis; transformations between position/velocity and action/angle variables; distribution functions expressed in terms of actions and their moments; iterative construction of self-consistent multicomponent galaxy models. Applications include the inference about the structure of Milky Way or other galaxies from observations of stellar kinematics; preparation of equilibrium initial conditions for N-body simulations; analysis of snapshots from simulations. The library is written in C++, provides a Python interface, and can be coupled to other stellar-dynamical software: Amuse, Galpy and Nemo.

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arXiv2018

Internal dynamics of the Large Magellanic Cloud from Gaia DR2

We use the proper motions (PM) of half a million red giant stars in Large Magellanic Cloud measured by Gaia to construct a 2d kinematic map of mean PM and its dispersion across the galaxy, out to 7 Kpc from its centre. We then explore a range of dynamical models and measure the rotation curve, mean azimuthal velocity, velocity dispersion profiles, and the orientation of the galaxy. We find that the circular velocity reaches 90 km/s at 5 Kpc, and that the velocity dispersion ranges from 30-40 km/s in the galaxy centre to 15-20 km/s at 7 Kpc.

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arXiv2017

Primordial Black Holes as Dark Matter: Constraints From Compact Ultra-Faint Dwarfs

The ground-breaking detections of gravitational waves from black hole mergers by LIGO have rekindled interest in primordial black holes (PBHs) and the possibility of dark matter being composed of PBHs. It has been suggested that PBHs of tens of solar masses could serve as dark matter candidates. Recent analytical studies demonstrated that compact ultra-faint dwarf galaxies can serve as a sensitive test for the PBH dark matter hypothesis, since stars in such a halo-dominated system would be heated by the more massive PBHs, their present-day distribution can provide strong constraints on PBH mass. In this study, we further explore this scenario with more detailed calculations, using a combination of dynamical simulations and Bayesian inference methods. The joint evolution of stars and PBH dark matter is followed with a Fokker-Planck code PhaseFlow. We run a large suite of such simulations for different dark matter parameters, then use a Markov Chain Monte Carlo approach to constrain the PBH properties with observations of ultra-faint galaxies. We find that two-body relaxation between the stars and PBH drives up the stellar core size, and increases the central stellar velocity dispersion. Using the observed half-light radius and velocity dispersion of stars in the compact ultra-faint dwarf galaxies as joint constraints, we infer that these dwarfs may have a cored dark matter halo with the central density in the range of 1-2 $\rm{M_{\odot}/pc^3}$, and that the PBHs may have a mass range of 2-14 $\rm{M_{\odot}}$ if they constitute all or a substantial fraction of the dark matter.

astro-ph.CO↗

arXiv2017

A new Fokker-Planck approach for relaxation-driven evolution of galactic nuclei

We present an approach for simulating the collisional evolution of spherical isotropic stellar systems based on the one-dimensional Fokker-Planck equation. A novel aspect is that we use the phase volume as the argument of the distribution function, instead of the traditionally used energy, which facilitates the solution. The publicly available code, PhaseFlow, implements a high-accuracy finite-element method for the Fokker-Planck equation, and can handle multiple-component systems, optionally with the central black hole and taking into account loss-cone effects and star formation. We discuss the energy balance in the general setting, and in application to the Bahcall-Wolf cusp around a central black hole, for which we derive a perturbative solution. We stress that the cusp is not a steady-state structure, but rather evolves in amplitude while retaining an approximately $ρ\propto r^{-7/4}$ density profile. Finally, we apply the method to the nuclear star cluster of the Milky Way, and illustrate a possible evolutionary scenario in which a two-component system of lighter main-sequence stars and stellar-mass black holes develops a Bahcall-Wolf cusp in the heavier component and a weaker $ρ\propto r^{-3/2}$ cusp in the lighter, visible component, over the period of several Gyr. The present-day density profile is consistent with the recently detected mild cusp inside the central parsec, and is weakly sensitive to initial conditions.

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arXiv2017

The Delay Time Distribution of Tidal Disruption Flares

Recent observations suggest that stellar tidal disruption events (TDE) are strongly overrepresented in rare, post-starburst galaxies. Several dynamical mechanisms have been proposed to elevate their TDE rates, ranging from central stellar overdensities to the presence of supermassive black hole (SMBH) binaries. Another such mechanism, introduced here, is a radial velocity anisotropy in the nuclear star cluster produced during the starburst. These, and other, dynamical hypotheses can be disentangled by comparing observations to theoretical predictions for the TDE delay time distribution (DTD). We show that SMBH binaries are a less plausible solution for the post-starburst preference, as they can only reproduce the observed DTD with extensive fine-tuning. The overdensity hypothesis produces a reasonable match to the observed DTD (based on the limited data currently available), provided that the initial stellar density profile created during the starburst, $ρ(r)$, is exceptional in both steepness and normalization. In particular, explaining the post-starburst preference requires $ρ\propto r^{-γ}$ with $γ\gtrsim 2.5$, i.e. much steeper than the classic Bahcall-Wolf equilibrium profile of $γ= 7/4$. For "ultrasteep" density cusps ($γ\ge 9/4$), we show that the TDE rate decays with time measured since the starburst as $\dot{N} \propto t^{-(4γ-9)/(2γ-3)} / \ln t$. Radial anisotropies also represent a promising explanation, provided that initial anisotropy parameters of $β_0 \approx 0.5$ are sustainable against the radial orbit instability. TDE rates in initially anisotropic cusps will decay roughly as $\dot{N} \propto t^{-β_0}$. As the sample of TDEs with well-studied host galaxies grows, the DTD will become a powerful tool for constraining the exceptional dynamical properties of post-starburst galactic nuclei.

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