arXiv subjects

A. Just

Publications and source records attributed to A. Just.

At least 19 recordsLinked to original sources

arXiv2025

Dynamical model of Praesepe and its tidal tails

Context. The dynamical evolution of open clusters in the tidal field of the Milky Way and the feeding of the disc field star population depend strongly on the initial conditions at the time of gas removal. Detailed dynamical models tailored to individual clusters help us understand the role of open clusters in the Galactic disc evolution. Aims. We present a detailed dynamical model of Praesepe, which reproduces the mass profile, the stellar mass function, and the mass segregation observed with the help of Gaia EDR3 data. Based on this model, we investigate the kinematic properties of the tidal tail stars in detail. Methods. We used direct N-body simulations along the eccentric orbit of Praesepe in the tidal field of the Milky Way, where each particle represents one star. The initial mass and size of the cluster, the dynamical state, and the initial mass function were adapted to reach the best-fitting model. Based on this model and a comparison model on a circular orbit, we analysed the stars in the tidal tails in terms of density, angular momentum, and orbit shapes. Results. Praesepe can be well reproduced by a cluster model with concentrated star formation in a supervirial state after instantaneous gas expulsion, adopting a global star formation efficiency of 17%. About 75% of the initially 7500 MSol are lost in the violent relaxation phase, and the observed mass segregation can be understood by two-body relaxation. We find that the self-gravity of the tail stars is the dominant force altering the angular momentum of the tail stars. For a typical star, the total change after escaping is about 1.6 kpc km/s. This corresponds to an offset in guiding radius of 7 pc, where tail stars contribute up to 70% to the alteration. The total radial shift of the orbit of the cluster in the Galactic plane can exceed 50 pc. This effect is not a result of the eccentricity of the orbit.

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arXiv2023

Evolution of open clusters with or without black holes

Binary black holes (BHs) can be formed dynamically in the centers of star clusters. The high natal kicks for stellar-mass BHs used in previous works made it hard to retain BHs in star clusters. Recent studies of massive star evolution and supernovae (SN) propose kick velocities that are lower due to the fallback of the SN ejecta. We study the impact of these updates by performing $N$-body simulations following instantaneous gas expulsion. For comparison, we simulate two additional model sets with the previous treatment of stars: one with high kicks and another with artificial removal of the kicks. Our model clusters initially consist of about one hundred thousand stars, formed with centrally-peaked efficiency. We find that the updated treatment of stars, due to the fallback-scaled lower natal kicks, allows clusters to retain SN remnants after violent relaxation. The mass contribution of the retained remnants does not exceed a few percent of the total bound cluster mass during the early evolution. For this reason, the first giga year of evolution is not affected significantly by this effect. Nevertheless, during the subsequent long-term evolution, the retained BHs accelerate mass segregation, leading to the faster dissolution of the clusters.

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arXiv2022

Towards a fully consistent Milky Way disk model V. The disk model for 4 $-$ 14 kpc

We present a generalised version of the semi-analytic Just-Jahreiss (JJ) model of the Galactic disk that incorporates our findings for the solar neighbourhood and is applicable to a wide range of galactocentric distances, 4 kpc $\lesssim R \lesssim $ 14 kpc. The JJ model is a flexible tool for stellar population synthesis with a fine age resolution of 25 Myr. It includes six Milky-Way (MW) components: the four flattened (exponential thin and thick disk, atomic and molecular gas) and two spheroidal (spherical stellar halo and a cored isothermal dark matter sphere). The overall thin-disk thickness is assumed to be constant at all radii, though flaring can also be tested. The adopted radial variation in the thin-disk star-formation rate (SFR) reflects the inside-out disk growth scenario. We allow a smooth power-law SFR continuum to be modified by an arbitrary number of Gaussian peaks. We present a public code of the JJ model complemented by the three sets of isochrones (PARSEC, MIST, and BaSTI). Using metallicity distributions of the red clump giants from APOGEE, we constrain the radial variation of the JJ-model age-metallicity relation (AMR) and propose a new analytic form for the AMR function. The generalised JJ model is a publicly available tool for studying different stellar populations across the MW disk. With its fine age resolution and flexibility, it can be particularly useful for reconstructing the thin-disk SFR, as a variety of different SFR shapes can be constructed within its framework.

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arXiv2021

The bound mass of Dehnen models with centrally peaked star formation efficiency

Understanding the formation of star clusters with low star-formation efficiency (SFE) is very important to know about the star-formation history. In N-body models of star cluster evolution after gas expulsion, the Plummer model with outer power law density profile has been used massively. We study the impact of the density profile slopes on the survivability of the low-SFE star clusters after instantaneous gas expulsion. We compare cases when stellar cluster has Plummer profile and Dehnen profiles with cusp of different slopes at the time of formation. We determine the corresponding density profile of the residual gas for a given global SFE, assuming that our model clusters formed with a constant efficiency per free-fall time and hence have shallower density profile of gas than that of stars. We perform direct $N$-body simulations of evolution of clusters initially in virial equilibrium within gas potential after gas removal. We find that the violent relaxation lasts no longer than 20~Myr independently of the density profile power law slopes. Dehnen model clusters survive after violent relaxation with significantly lower SFEs when the global SFE measured within the Jacobi radius or within a half-mass radius. Dehnen $\gamma=0$ model clusters show similar final bound fraction with the Plummer model clusters if global SFE is measured within 10 scale radii. The final bound fraction increases with $\gamma$ values for a given global SFE. We conclude that Dehnen clusters better resist the consequences of the violent relaxation followed the instantaneous gas expulsion than the Plummer clusters. Thus the shallower the outer density slope of the low-SFE clusters, the better for their survivability after gas expulsion. Among Dehnen clusters we find that the steeper the inner slope (cusp) the higher the bound mass fraction is retained after violent relaxation for a given global SFE.

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arXiv2021

Towards a fully consistent Milky Way disk model -- IV. The impact of Gaia DR2 and APOGEE

We present an updated version of the semi-analytic Just-Jahrei{\ss} (JJ) model of the Galactic disk and constrain its parameters in the Solar neighbourhood. The new features of the JJ model include a simple two-component gaseous disk, a star-formation rate (SFR) function of the thick disk that has been extended in time, and a correlation between the kinematics of molecular gas and thin-disk populations. Here, we study the vertical number density profiles and W-velocity distributions determined from ~2 million local stars of the Gaia DR2. We also investigate an apparent Hess diagram of the Gaia stars selected in a conic volume towards the Galactic poles. Using a stellar evolution library, we synthesise stellar populations with a four-slope broken power-law initial mass function (IMF), the SFR, and an age-metallicity relation (AMR). The latter is consistently derived with the observed metallicity distribution of the local Red Clump (RC) giants from the APOGEE. Working within a Bayesian approach, we sample the posterior probability distribution in a multidimensional parameter space using the MCMC method. We find that the spatial distribution and motion of the Gaia stars imply two recent SF bursts centered at ages of ~0.5 Gyr and ~3 Gyr and characterised by a ~30% and ~55% SF enhancement, respectively, relative to a monotonously declining SFR continuum. The stellar populations associated with this SF excess are found to be dynamically hot for their age: they have W-velocity dispersions of ~12.5 km/s and ~26 km/s. The new JJ model is able to reproduce the local star counts with an accuracy of ~5 %. Using Gaia DR2 data, we self-consistently constrained 22 parameters of the updated JJ model. Our optimised model predicts two SF bursts within the last ~4 Gyr, which may point to recent episodes of gas infall.

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arXiv2019

Simulation of the loss-cone instability in spherical systems. II. Dominating Keplerian potential

A new so-called `gravitational loss-cone instability' in stellar systems has recently been investigated theoretically in the framework of linear perturbation theory and proved to be potentially important in understanding the physical processes in centres of galaxies, star clusters, and the Oort comet cloud. Using N-body simulations, we confirm previous findings and go beyond the linear theory. Unlike the well-known instabilities, the new one shows no notable change in spherical geometry of the cluster, but it significantly accelerates the speed of diffusion of particles in phase space leading to a repopulation of the loss cone and early instability saturation.

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arXiv2019

Simulation of the loss-cone instability in spherical systems. I. Dominating harmonic potential

A new so-called `gravitational loss-cone instability' in stellar systems has recently been investigated theoretically in the framework of linear perturbation theory and proved to be potentially important in understanding the physical processes in centres of galaxies, star clusters, and the Oort comet cloud. Using N-body simulations of a toy model, we confirm previous findings for the harmonic dominating potential and go beyond the linear theory. Unlike the well-known instabilities, the new one shows no notable change in the spherical geometry of the cluster, but it significantly accelerates the speed of diffusion of particles in phase space leading to an early instability saturation.

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arXiv2019

Peculiar motions of the gas at the centre of the barred galaxy UGC 4056

We derive the circular velocity curves of the gaseous and stellar discs of UGC 4056, a giant barred galaxy with an active galactic nucleus (AGN). We analyse UGC 4056 using the 2D spectroscopy obtained within the framework of the Mapping Nearby Galaxies at APO (MaNGA) survey. Using images and the colour index g-r from the Sloan Digital Sky Survey (SDSS), we determined the tilt of the galaxy, which allows us to conclude that the galaxy rotates clockwise with trailing spiral arms. We found that the gas motion at the central part of the UGC 4056 shows peculiar features. The rotation velocity of the gaseous disc shows a bump within around three kiloparsecs while the rotation velocity of the stellar disc falls smoothly to zero with decreasing galactocentric distance. We demonstrate that the peculiar radial velocities in the central part of the galaxy may be caused by the inflow of the gas towards the nucleus of the galaxy. The unusual motion of the gas takes place at the region with the AGN-like radiation and can be explained by the gas response to the bar potential.

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arXiv2019

The star cluster survivability after gas expulsion is independent of the impact of the Galactic tidal field

We study the impact of the tidal field on the survivability of star clusters following instantaneous gas expulsion. Our model clusters are formed with a centrally-peaked star-formation efficiency profile as a result of star-formation taking place with a constant efficiency per free-fall time. We define the impact of the tidal field as the ratio of the cluster half-mass radius to its Jacobi radius immediately after gas expulsion, $\lambda = r_{h}/R_{J}$. We vary $\lambda$ by varying either the Galactocentric distance, or the size (hence volume density) of star clusters. We propose a new method to measure the violent relaxation duration, in which we compare the total mass-loss rate of star clusters with their stellar evolutionary mass-loss rate. That way, we can robustly estimate the bound mass fraction of our model clusters at the end of violent relaxation. The duration of violent relaxation correlates linearly with the Jacobi radius, when considering identical clusters at different Galactocentric distances. In contrast, it is nearly constant for the solar neighbourhood clusters, slightly decreasing with $\lambda$. The violent relaxation does not last longer than 50 Myr in our simulations. Identical model clusters placed at different Galactocentric distances have the same final bound fraction, despite experiencing different impacts of the tidal field. The solar neighbourhood clusters with different densities experience only limited variations of their final bound fraction. In general, we conclude that the cluster survivability after instantaneous gas expulsion, as measured by their bound mass fraction at the end of violent relaxation, $F_{bound}$, is independent of the impact of the tidal field, $\lambda$.

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arXiv2018

Characterizing the radial oxygen abundance distribution in disk galaxies

We examine the possible dependence of the radial oxygen abundance distribution on non-axisymmetrical structures (bar/spirals) and other macroscopic parameters such as the mass, the optical radius R25, the color g-r, and the surface brightness of the galaxy. A sample of disk galaxies from the CALIFA DR3 is considered. We adopted the Fourier amplitude A2 of the surface brightness as a quantitative characteristic of the strength of non-axisymmetric structures in a galactic disk, in addition to the commonly used morphologic division for A, AB, and B types based on the Hubble classification. To distinguish changes in local oxygen abundance caused by the non-axisymmetrical structures, the multiparametric mass--metallicity relation was constructed as a function of parameters such as the bar/spiral pattern strength, the disk size, color index g-r in the SDSS bands, and central surface brightness of the disk. The gas-phase oxygen abundance gradient is determined by using the R calibration. We find that there is no significant impact of the non-axisymmetric structures such as a bar and/or spiral patterns on the local oxygen abundance and radial oxygen abundance gradient of disk galaxies. Galaxies with higher mass, however, exhibit flatter oxygen abundance gradients in units of dex/kpc, but this effect is significantly less prominent for the oxygen abundance gradients in units of dex/R25 and almost disappears when the inner parts are avoided. We show that the oxygen abundance in the central part of the galaxy depends neither on the optical radius R25 nor on the color g-r or the surface brightness of the galaxy. Instead, outside the central part of the galaxy, the oxygen abundance increases with g-r value and central surface brightness of the disk.

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arXiv2018

Local disc model in view of Gaia DR1 and RAVE data

We test the performance of the semi-analytic self-consistent Just-Jahrei{\ss} disc model (JJ model) with the astrometric data from the Tycho-Gaia Astrometric Solution (TGAS) sub-catalogue of the first Gaia data release (Gaia DR1), as well as the radial velocities from the fifth data release of the Radial Velocity Experiment survey (RAVE DR5). We use a sample of 19,746 thin disc stars from the TGAS$\times$RAVE cross-match selected in the local solar cylinder of 300 pc radius and 1 kpc height below the Galactic plane and simulate this sample via the forward modelling technique. First, we convert the predicted vertical density laws of the thin disc populations into a mock sample. Then the obtained mock populations are reddened with a 3D dust map and are subjected to the selection criteria corresponding to the RAVE and TGAS observational limitations as well as to additional cuts applied to the data sample. We calculate the quantities of interest separately at different heights above the Galactic plane taking into account the distance error effects separately in horizontal and vertical directions. We investigate the simulated sample in terms of the vertical number density profiles, Hess diagrams and velocity distribution functions. Basing on a good agreement of our simulations with the data, we conclude that our fiducial disc model confidently reproduces the vertical trends in the thin disc stellar population properties. Thus, it can serve as a starting point for the future extension of the JJ model to other Galactocentric distances.

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arXiv2018

The local rotation curve of the Milky Way based on SEGUE and RAVE data

We construct the rotation curve of the Milky Way in the extended solar neighbourhood using a sample of SEGUE (Sloan Extension for Galactic Understanding and Exploration) G-dwarfs. We investigate the rotation curve shape for the presence of any peculiarities just outside the solar radius as has been reported by some authors. We approach the problem in a framework of classical Jeans analysis. Using the most recent data from RAVE (RAdial Velocity Experiment), we determine the solar peculiar velocity and the radial scalelengths for the three populations of different metallicities representing the Galactic thin disc. Then with the same binning in metallicity for the SEGUE G-dwarfs, we construct the rotation curve in the range of Galactocentric distances 7-10 kpc. We derive the circular velocity by correcting the mean tangential velocity for the asymmetric drift in each distance bin. With SEGUE data we also calculate the radial scalelength of the thick disc taking as known the derived peculiar motion of the Sun and the slope of the rotation curve. The rotation curve constructed through SEGUE G-dwarfs appears to be smooth in the selected radial range. The local kinematics of the thin disc rotation as determined in the framework of our new careful analysis does not favour the presence of a massive overdensity ring just outside the solar radius.

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arXiv2018

Global survey of star clusters in the Milky Way VI. Age distribution and cluster formation history

The all-sky Milky Way Star Clusters (MWSC) survey provides uniform and precise ages and other parameters for a variety of clusters in the Solar Neighbourhood. We construct the cluster age distribution, investigate its spatial variations, and discuss constraints on cluster formation scenarios of the Galactic disk during the last 5 Gyrs. Due to the spatial extent of the MWSC, we consider spatial variations of the age distribution along galactocentric radius $R_G$, and along $Z$-axis. For the analysis of the age distribution we use 2242 clusters, which all lie within roughly 2.5 kpc of the Sun. To connect the observed age distribution to the cluster formation history we build an analytical model based on simple assumptions on the cluster initial mass function and on the cluster mass-lifetime relation, fit it to the observations, and determine the parameters of the cluster formation law. Comparison with the literature shows that earlier results strongly underestimated the number of evolved clusters with ages $t\gtrsim 100$ Myr. Recent studies based on all-sky catalogues agree better with our data, but still lack the oldest clusters with ages $t\gtrsim 1$ Gyr. We do not observe a strong variation in the age distribution along $R_G$, though we find an enhanced fraction of older clusters ($t>1$ Gyr) in the inner disk. In contrast, the distribution strongly varies along $Z$. The high altitude distribution practically does not contain clusters with $t<1$ Gyr. With simple assumptions on the cluster formation history, cluster initial mass function and cluster lifetime we can reproduce the observations. Cluster formation rate and cluster lifetime are strongly degenerate, which does not allow us to disentangle different formation scenarios. In all cases the cluster formation rate is strongly declining with time, and the cluster initial mass function is very shallow at the high mass end. (abridged)

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arXiv2017

Spiral arms in CALIFA galaxies traced by non-circular velocities, abundances, and extinctions

We derive maps of the observed velocity of ionized gas, the oxygen abundance, and the extinction (Balmer decrement) across the area of the four spiral galaxies NGC36, NGC180, NGC6063, and NGC7653 from integral field spectroscopy obtained by the Calar Alto Legacy Integral Field Area (CALIFA) survey. We searched for spiral arms through Fourier analysis of the spatial distribution of three tracers (non-circular motion, enhancement of the oxygen abundance, and of the extinction) in the discs of our target galaxies. The spiral arms (two-armed logarithmic spirals in the deprojected map) are shown in each target galaxy for each tracer considered. The pitch angles of the spiral arms in a given galaxy obtained with the three different tracers are close to each other. The enhancement of the oxygen abundance in the spiral arms as compared to the abundance in the interarm regions at a given galactocentric distance is small; within a few per cent. We identified a metallicity gradient in our target galaxies. Both barred galaxies in our sample show flatter gradients than the two galaxies without bars. Galactic inclination, position angle of the major axis, and the rotation curve were also obtained for each target galaxy using the Fourier analysis of the two-dimensional velocity map.

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arXiv2017

Is the Milky Way still breathing? RAVE-Gaia streaming motions

We use data from the Radial Velocity Experiment (RAVE) and the Tycho-Gaia astrometric solution catalogue (TGAS) to compute the velocity fields yielded by the radial (VR), azimuthal (Vphi) and vertical (Vz) components of associated Galactocentric velocity. We search in particular for variation in all three velocity components with distance above and below the disc midplane, as well as how each component of Vz (line-of-sight and tangential velocity projections) modifies the obtained vertical structure. To study the dependence of velocity on proper motion and distance we use two main samples: a RAVE sample including proper motions from the Tycho-2, PPMXL and UCAC4 catalogues, and a RAVE-TGAS sample with inferred distances and proper motions from the TGAS and UCAC5 catalogues. In both samples, we identify asymmetries in VR and Vz. Below the plane we find the largest radial gradient to be dVR / dR = -7.01+- 0.61 km\s kpc, in agreement with recent studies. Above the plane we find a similar gradient with dVR / dR= -9.42+- 1.77 km\s kpc. By comparing our results with previous studies, we find that the structure in Vz is strongly dependent on the adopted proper motions. Using the Galaxia Milky Way model, we demonstrate that distance uncertainties can create artificial wave-like patterns. In contrast to previous suggestions of a breathing mode seen in RAVE data, our results support a combination of bending and breathing modes, likely generated by a combination of external or internal and external mechanisms.

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arXiv2017

Effects of galaxy--satellite interactions on bar formation

Aims. We aim to show how encounters with low-mass satellite galaxies may alter the bar formation in a Milky Way-like disc galaxy. Methods. We use high-resolution N-body simulations of a disc galaxy prone to mild bar instability. For realistic initial conditions of satellites, we take advantage of cosmological simulations of Milky Way-like dark matter haloes. Results. The satellites may have a significant impact on the time of bar formation. Some runs with satellites demonstrate a delay, while others show an advancement in bar formation compared to the isolated run, with such time differences reaching $\sim$ 1 Gyr. Meanwhile, the final bar configuration, including its very appearance and the bar characteristics such as the pattern speed and the exponential growth rate of its amplitude are independent of the number of encounters and their orbits. The contribution of satellites with masses below $10^9 M_{\odot}$ is insignificant, unless their pericentre distances are small. We suggest that the encounters act indirectly via inducing perturbations across the disc that evolve to delayed waves in the central part and interfere with an emerging seed bar. The predicted effect for the present-day host galaxy is expected to be even more significant at redshifts $z \gtrsim 0.5$.

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arXiv2017

Bar formation in the Milky Way type galaxies

Many barred galaxies, possibly including the Milky Way, have cusps in the centres. There is a widespread belief, however, that usual bar instability taking place in bulgeless galaxy models is impossible for the cuspy models, because of the presence of the inner Lindblad resonance for any pattern speed. At the same time there are numerical evidences that the bar instability can form a bar. We analyse this discrepancy, by accurate and diverse N-body simulations and using the calculation of normal modes. We show that bar formation in cuspy galaxies can be explained by taking into account the disc thickness. The exponential growth time is moderate for typical current disc masses (about 250 Myr), but considerably increases (factor 2 or more) upon substitution of the live halo and bulge with a rigid halo/bulge potential; meanwhile pattern speeds remain almost the same. Normal mode analysis with different disc mass favours a young bar hypothesis, according to which the bar instability saturated only recently.

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arXiv2016

Impact of cosmological satellites on the vertical heating of the Milky Way disc

We present a high resolution study of the impact of realistic satellite galaxies, extracted from cosmological simulations of Milky Way haloes including 6 Aquarius suites and Via Lactea \rom{2}, on the dynamics of the galactic disc. The initial conditions for the multi-component Milky Way galaxy were generated using the GalIC code, to ensure a system in dynamical equilibrium state prior to addition of satellites. Candidate subhaloes that came closer than 25\,kpc to the centre of the host DM haloes with initial mass enclosed within the tidal radius, $M_\textrm{tid}$ $\ge$ 10$^{8} M_{\odot}$\,=\,0.003 $M_\textrm{disc}$, were identified, inserted into our high resolution N-body simulations and evolved for 2 Gyr. We quantified the vertical heating due to such impacts by measuring the disc thickness and squared vertical velocity dispersion $σ_{z}^{2}$ across the disc. According to our analysis the strength of heating is strongly dependent on the high mass end of the subhalo distribution from cosmological simulations. The mean increase of the vertical dispersion is $\sim$ 20\,km$^{2}$\,s$^{-2}$\,Gyr$^{-1}$ for R $>$ 4\,kpc with a flat radial profile while, excluding Aq-F2 results, the mean heating is $<$ 12\,km$^{2}$\,s$^{-2}$\,Gyr$^{-1}$, corresponding to 28\% and 17\% of the observed vertical heating rate in the solar neighbourhood. Taking into account the statistical dispersion around the mean we miss the observed heating rate by more than 3$σ$. We observed a general flaring of the disc height in the case of all 7 simulations in the outer disc.

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