arXiv subjects

Ewa L. Lokas

Publications and source records attributed to Ewa L. Lokas.

At least 55 records · Page 3Linked to original sources

arXiv2016

On the tidal radius of satellites on prograde and retrograde orbits

A tidal radius is a distance from a satellite orbiting in a host potential beyond which its material is stripped by the tidal force. We derive a revised expression for the tidal radius of a rotating satellite which properly takes into account the possibility of prograde and retrograde orbits of stars. Besides the eccentricity of the satellite orbit, the tidal radius depends also on the ratio of the satellite internal angular velocity to the orbital angular velocity. We compare our formula to the results of two $N$-body simulations of dwarf galaxies orbiting a Milky Way-like host on a prograde and retrograde orbit. The tidal radius for the retrograde case is larger than for the prograde. We introduce a kinematic radius separating stars still orbiting the dwarf galaxy from those already stripped and following the potential of the host galaxy. We find that the tidal radius matches very well the kinematic radius. Our results provide a connection between the formalism of the tidal radius derivation and the theory of resonant stripping.

astro-ph.GA↗

arXiv2015

Orbit anisotropy of dark matter haloes with Schwarzschild modelling

We apply the Schwarzschild orbit superposition method to mock data in order to investigate the accuracy of recovering the profile of the orbit anisotropy. The mock data come from four numerical realizations of dark matter haloes with well defined anisotropy profiles. We show that when assuming a correct mass distribution we are able to determine the anisotropy with high precision and clearly distinguish between the models.

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arXiv2015

Simulations of tidally induced spiral arms

The origin of grand design spiral structure in galaxies is still under debate but one of promising scenarios involves tidal interactions. We use N-body simulations to study the evolution of a Milky Way-size galaxy in a Virgo-like cluster. The galaxy is placed on a typical eccentric orbit and evolved for 10 Gyr. We find that grand design spiral arms are triggered by pericenter passages and later on they wind up and dissipate. The arms formed in the simulations are approximately logarithmic, but are also dynamic, transient and recurrent.

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arXiv2015

Spectral analysis of stellar orbits in a tidally induced bar

Using numerical analysis of fundamental frequencies we study the orbital structure of a tidally induced bar formed in a simulated dwarf galaxy orbiting a Milky Way-like host. We find that only about 10% of stars have frequencies compatible with x1 orbits, the classical periodic orbits in a barred potential. The rest of the stars follows box orbits parallel to the bar, with varying degree of elongation.

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arXiv2015

The origin of prolate rotation in dwarf spheroidal galaxies formed by mergers of disky dwarfs

Motivated by the discovery of prolate rotation of stars in Andromeda II, a dwarf spheroidal companion of M31, we study its origin via mergers of disky dwarf galaxies. We simulate merger events between two identical dwarfs changing the initial inclination of their disks with respect to the orbit and the amount of orbital angular momentum. On radial orbits the amount of prolate rotation in the merger remnants correlates strongly with the inclination of the disks and is well understood as due to the conservation of the angular momentum component of the disks along the merger axis. For non-radial orbits prolate rotation may still be produced if the orbital angular momentum is initially not much larger than the intrinsic angular momentum of the disks. The orbital structure of the remnants with significant rotation is dominated by box orbits in the center and long-axis tubes in the outer parts. The frequency analysis of stellar orbits in the plane perpendicular to the major axis reveals the presence of two families roughly corresponding to inner and outer long-axis tubes. The fraction of inner tubes is largest in the remnant forming from disks oriented most vertically initially and is responsible for the boxy shape of the galaxy. We conclude that prolate rotation results from mergers with a variety of initial conditions and no fine tuning is necessary to reproduce this feature. We compare the properties of our merger remnants to those of dwarfs resulting from the tidal stirring scenario and the data for Andromeda II.

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arXiv2015

The resonant nature of tidal stirring of disky dwarf galaxies orbiting the Milky Way

Using N-body simulations we study the tidal evolution of initially disky dwarf galaxies orbiting a Milky Way-like host, a process known to lead to the formation of dwarf spheroidal galaxies. We focus on the effect of the orientation of the dwarf galaxy disk's angular momentum with respect to the orbital one and find very strong dependence of the evolution on this parameter. We consider four different orientations: the exactly prograde, the exactly retrograde and two intermediate ones. Tidal evolution is strongest for the exactly prograde and weakest for the exactly retrograde orbit. In the prograde case the stellar component forms a strong bar and remains prolate until the end of the simulation, while its rotation is very quickly replaced by random motions of the stars. In the retrograde case the dwarf remains oblate, does not form a bar and loses rotation very slowly. In the two cases of intermediate orientation of the disk, the evolution is between the two extremes, suggesting a monotonic dependence on the inclination. We interpret the results in terms of the resonance between the angular velocity of the stars in the dwarf and its orbital motion by comparing the measurements from simulations to semi-analytic predictions. We conclude that resonant effects are the most important mechanism underlying the tidal evolution of disky dwarf galaxies.

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arXiv2014

Dark matter in elliptical galaxies: I. Is the total mass density profile of the NFW form or even steeper?

Elliptical galaxies are modelled as Sersic luminosity distributions with density profiles (DPs) for the total mass adopted from the DPs of haloes within dissipationless LambdaCDM N-body simulations. Ellipticals turn out to be inconsistent with cuspy low-concentration NFW models representing the total mass, nor are they consistent with a steeper -1.5 inner slope, nor with the shallower models proposed by Navarro et al. 04, nor with NFW models 10 times more concentrated than predicted, as deduced from several X-ray observations: the mass models, extrapolated inwards, lead to local mass-to-light ratios that are smaller than the stellar value inside an effective radius (R_e), and to central aperture velocity dispersions that are much smaller than observed. This conclusion remains true as long as there is no sharp steepening (slope < -2) of the dark matter (DM) DPs just inside 0.01 virial radii. The too low total mass and velocity dispersion produced within R_e by an NFW-like total mass profile suggests that the stellar component should dominate the DM one out to at least R_e. It should then be difficult to kinematically constrain the inner slope of the dark matter DP of ellipticals. The high concentration parameters deduced from X-ray observations appear to be a consequence of fitting an NFW model to the total mass DP made up of a stellar component that dominates inside and a DM component that dominates outwards. An appendix gives the virial mass dependence of the concentration parameter, central density, and total mass of the Navarro et al. model. In a 2nd appendix are given single integral expressions for the velocity dispersions averaged along the line-of-sight, in circular apertures and in thin slits, for general luminosity density and mass distributions, with isotropic orbits.

astro-ph↗

arXiv2014

Dark matter in elliptical galaxies: II. Estimating the mass within the virial radius

Elliptical galaxies are modelled with a a 4-component model: Sersic stars, LCDM dark matter (DM), hot gas and central black hole. DM is negligible in the inner regions, which are dominated by stars and the central black hole. This prevents any kinematical estimate (using a Jeans analysis) of the inner slope of the DM density profile. The gas fraction rises, but the baryon fraction decreases with radius, at least out to 10 effective radii (R_e). Even with line-of-sight velocity dispersion (VD) measurements at 4 to 6 R_e with 20 km/s accuracy and perfectly known velocity anisotropy, the total mass within the virial radius (r_v) is uncertain by a factor over 3. The DM distributions found in LCDM simulations are consistent with the stellar VD profiles, but appear inconsistent with the low VDs measured by Romanowsky et al. (2003) of planetary nebulae between 2 and 5 R_e, which imply such low M/Ls that the baryon fraction within r_v must be greater than the universal value. Replacing the NFW DM model by the new model of Navarro et al. (2004) decreases slightly the VD at a given radius. So, given the observed VD measured at 5 R_e, the inferred M/L within r_v is 40% larger than predicted with the NFW model. Folding in the slight (strong) radial anisotropy found in LCDM (merger) simulations, which is well modelled (much better than with the Osipkov-Merritt formula) with beta(r) = 1/2 r/(r+a), the inferred M/L within r_v is another 1.6 (2.4) times higher than for the isotropic NFW model. Thus, the DM model and radial anisotropy can partly explain the low PN VDs, but not in full. In an appendix, single integral expressions are derived for the VDs in terms of the tracer density and total mass profiles, for 3 anisotropic models: radial, Osipkov-Merritt, and the model above, for general radial profiles of luminosity density and mass.

astro-ph↗

arXiv2014

Radial orbit instability in dwarf dark matter haloes

Using N-body simulations we study the phenomenon of radial orbit instability occurring in dark matter haloes of the size of a dwarf galaxy. We carried out simulations of seven spherical models, with the same standard NFW density profile but different anisotropy profiles of particle orbits. Four of them underwent instability: two with a constant positive anisotropy, one with an anisotropic core and an isotropic envelope and one with a very small isotropic core and an anisotropic envelope. Haloes affected by the instability become approximately axisymmetric and prolate, with the profile of the shortest-to-longest axis ratio increasing with radius. The lower limit for the central value of this axis ratio is 0.3 for an NFW halo. The density profiles of the haloes did not change significantly, whereas the velocity distributions became axisymmetric. The total angular momentum rose due to large-amplitude oscillations of its components perpendicular to the symmetry axis of the halo. We also studied orbits of individual particles assigning them to classical orbit families in triaxial potentials. We find that the membership of a given particle in a family depends on its initial total angular momentum and its components along the principal axes of matter distribution.

astro-ph.GA↗

arXiv2014

Estimating masses of dwarf spheroidal galaxies

Precise measurements of mass in dark matter dominated dwarf spheroidal galaxies are of great importance for testing the theories of structure formation. We use $N$-body simulations of the tidal evolution of a dwarf galaxy orbiting the Milky Way to generate mock kinematical data sets and use them to test the reliability of a simple mass estimator proposed by Wolf et al. The evolution of the initially disky dwarf galaxy embedded in a dark matter halo was traced for 10 Gyr on a rather tight orbit. After about half of the time a dwarf spheroidal galaxy is formed that retains some remnant rotation and a non-spherical shape. Observing the triaxial galaxy along each of its principal axes we measure its half-light radius and the line-of-sight velocity dispersion and use them to estimate the mass. We find that the mass is significantly overestimated when the dwarf is seen along the longest axis of the stellar component and underestimated when observed along the shortest axis. We provide a formula that quantifies the systematic error in the estimated mass with respect to the true one as a function of the galaxy shape and line of sight.

astro-ph.GA↗

arXiv2014

Tidal evolution of disky dwarf galaxies: prograde versus retrograde orbits

The formation of dwarf spheroidal galaxies in the Local Group from disky progenitors via tidal interaction with a bigger host is one of the most promising scenarios of their origin. Using N-body simulations we study the process by following the evolution of a disky dwarf orbiting a Milky Way-like host. We focus on the effect of the orientation of the dwarf galaxy disk's angular momentum with respect to the orbital one. We find a strong dependence of the efficiency of the transformation from a disk to a spheroid on the disk orientation. The effect is strongest for the exactly prograde and weakest for the exactly retrograde orbit. In the prograde case the stellar component forms a strong bar and remains prolate until the end of the evolution, while its rotation is very quickly replaced by random motions of the stars. In the retrograde case the dwarf remains oblate, does not form a bar and loses rotation very slowly. Our results suggest that resonant effects are the most important mechanism underlying the evolution while tidal shocking plays only a minor role.

astro-ph.GA↗

arXiv2014

Andromeda II as a merger remnant

Using N-body simulations we study the origin of prolate rotation recently detected in the kinematic data for And II, a dSph satellite of M31. We propose an evolutionary model for the origin of And II involving a merger between two disky dwarf galaxies whose structural parameters differ only in their disk scale lengths. The dwarfs are placed on a radial orbit towards each other with their angular momenta inclined by 45 deg to the orbital plane and by 90 deg with respect to each other. After 5 Gyr of evolution the merger remnant forms a stable triaxial galaxy with rotation only around the longest axis. The origin of this rotation is naturally explained as due to the symmetry of the initial configuration which leads to the conservation of angular momentum components along the direction of the merger. The stars originating from the two dwarfs show significantly different surface density profiles while having very similar kinematics in agreement with the properties of separate stellar populations in And II. We also study an alternative scenario for the formation of And II, via tidal stirring of a disky dwarf galaxy. While intrinsic rotation occurs naturally in this model as a remnant of the initial rotation of the disk, it is mostly around the shortest axis of the stellar component. The rotation around the longest axis is induced only occasionally and remains much smaller that the system's velocity dispersion. We conclude that although tidal origin of the velocity distribution in And II cannot be excluded, it is much more naturally explained within the scenario involving a past merger event. Thus, in principle, the presence of prolate rotation in dSph galaxies of the Local Group and beyond may be used as an indicator of major mergers in their history or even as a way to distinguish between the two scenarios of their formation.

astro-ph.GA↗

arXiv2013

Tidal tails of dwarf galaxies on different orbits around the Milky Way

We present a phenomenological description of the properties of tidal tails forming around dwarf galaxies orbiting the Milky Way. For this purpose we use collisionless N-body simulations of dwarfs initially composed of a disk embedded in an NFW dark matter halo. The dwarfs are placed on seven orbits around the Milky Way-like host, differing in size and eccentricity, and their evolution is followed for 10 Gyr. In addition to the well-studied morphological and dynamical transformation of the dwarf's main body, the tidal stripping causes them to lose a substantial fraction of mass both in dark matter and stars which form pronounced tidal tails. We focus on the properties of the stellar component of the tidal tails thus formed. We first discuss the break radii in the stellar density profile defining the transition to tidal tails as the radii where the profile becomes shallower and relate them to the classically defined tidal radii. We then calculate the relative density and velocity of the tails at a few break radii as a function of the orbital phase. Next, we measure the orientation of the tails with respect to an observer placed at the centre of the Milky Way. The tails are perpendicular to this line of sight only for a short period of time near the pericentre. For most of the time the angles between the tails and this line of sight are low, with orbit-averaged medians below 42 degrees for all, even the almost circular orbit. The median angle is typically lower while the maximum relative density higher for more eccentric orbits. The combined effects of relative density and orientation of the tails suggest that they should be easiest to detect for dwarf galaxies soon after their pericentre passage.

astro-ph.GA↗

arXiv2013

Can we measure the slopes of density profiles in dwarf spheroidal galaxies?

Using collisionless N-body simulations of dwarf galaxies orbiting the Milky Way (MW) we construct realistic models of dwarf spheroidal (dSph) galaxies of the Local Group. The dwarfs are initially composed of stellar disks embedded in dark matter haloes with different inner density slopes and are placed on an eccentric orbit typical for MW subhaloes. After a few Gyr of evolution the stellar component is triaxial as a result of bar instability induced by tidal forces. Observing the simulated dwarfs along three principal axes of the stellar component we create mock data sets and determine the their half-light radii and line-of-sight velocity dispersions. Using the estimator proposed by Wolf et al. we calculate masses within half-light radii. The masses obtained this way are over(under)estimated by up to a factor of two when the line of sight is along the longest (shortest) axis of the stellar component. We then divide the initial stellar distribution into an inner and outer population and trace their evolution in time. The two populations, although affected by tidal forces, retain different density profiles even after a few Gyr. We measure the half-light radii and velocity dispersions of the stars in the two populations along different lines of sight and use them to estimate the slope of the mass distribution in the dwarfs following the method proposed by Walker & Penarrubia. The inferred slopes are systematically over- or underestimated, depending on the line of sight. In particular, when the dwarf is seen along the longest axis of the stellar component, a significantly shallower density profile is inferred than the real one measured from the simulations. Since most dSphs are non-spherical and their orientation with respect to our line of sight is unknown, the method can be reliably applied only to a large sample of dwarfs when these systematic errors are expected to be diminished.

astro-ph.CO↗

arXiv2013

Tidal Stirring of Disky Dwarfs with Shallow Dark Matter Density Profiles: Enhanced Transformation into Dwarf Spheroidals

(Abridged) The origin of dSphs in the Local Group (LG) remains an enigma. The tidal stirring model posits that late-type, rotationally-supported dwarfs resembling present-day dwarf irregular (dIrr) galaxies can transform into dSphs via interactions with Milky Way-sized hosts. Using collisionless N-body simulations, we investigate for the first time how tidal stirring depends on the dark matter (DM) density distribution in the central stellar region of the progenitor disky dwarf. Specifically, we explore various asymptotic inner slopes gamma of the dwarf DM density profiles (rho \propto r^{-gamma} as r -> 0). For a given orbit inside the primary, rotationally-supported dwarfs embedded in DM halos with core-like density distributions (gamma = 0.2) and mild density cusps (gamma = 0.6) demonstrate a substantially enhanced likelihood and efficiency of transformation into dSphs compared to their counterparts with steeper DM density profiles (gamma = 1). Such shallow DM distributions are akin to those of observed dIrrs, highlighting tidal stirring as a plausible model for the LG morphology-density relation. When gamma <1, a single pericentric passage can induce dSph formation and disky dwarfs on low-eccentricity or large-pericenter orbits are able to transform into dSphs; these new results allow the tidal stirring model to explain the existence of virtually all known dSphs across a wide range of distances from their hosts. A subset of rotationally-supported dwarfs with gamma <1 are eventually disrupted by the primary; those that survive as dSphs are generally on orbits that are biased towards lower eccentricities and/or larger pericenters relative to those of typical CDM satellites. The latter could explain the rather peculiar orbits of several classic LG dSphs such as Fornax, Leo I, Tucana, and Cetus.

astro-ph.CO↗

arXiv2012

Stellar populations in tidally stirred dwarf galaxies

Using N-body simulations we study the evolution of separate stellar populations in dwarf galaxies in the context of the tidal stirring scenario for the formation of dwarf spheroidal (dSph) galaxies in the Local Group. The dwarf galaxies, initially composed of a stellar disk and a dark matter halo, are placed on seven different orbits around the Milky Way. The stars are divided into two populations, within and outside the half-light radius, and their positions are followed for 10 Gyr. We find that the populations retain different density distributions even over such long timescales. Some of the stars of the outer population migrate to the central part of the dwarf forming an extended core while the stars of the inner population develop a tail in the outer parts. In addition, the outer population is more heavily stripped by tidal forces from the Milky Way and may become subdominant at all radii on tight enough orbits. We conclude that the tidal stirring model is fully compatible with the presence of multiple stellar populations in dSph galaxies.

astro-ph.GA↗

arXiv2012

A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: VII. Kinematics of the Main Body of the Sagittarius dSph

We have assembled a large-area spectroscopic survey of giant stars in the Sagittarius (Sgr) dwarf galaxy core. Using medium resolution (R ~15,000), multifiber spectroscopy we have measured velocities of these stars, which extend up to 12 degrees from the galaxy's center (3.7 core radii or 0.4 times the King limiting radius). From these high quality spectra we identify 1310 Sgr members out of 2296 stars surveyed distributed across 24 different fields across the Sgr core. Additional slit spectra were obtained of stars bridging from the Sgr core to its trailing tail. Our systematic, large area sample shows no evidence for significant rotation, a result at odds with the ~20 km/s rotation required as an explanation for the bifurcation seen in the Sgr tidal stream; the observed small (<= 4 km/s) velocity trend along primarily the major axis is consistent with models of the projected motion of an extended body on the sky with no need for intrinsic rotation. The Sgr core is found to have a flat velocity dispersion (except for a kinematically colder center point) across its surveyed extent and into its tidal tails, a property that matches the velocity dispersion profiles measured for other Milky Way dwarf spheroidal (dSph) galaxies. We comment on the possible significance of this observed kinematical similarity for the dynamical state of the other classical Milky Way dSphs in light of the fact that Sgr is clearly a strongly tidally disrupted system.

astro-ph.GA↗

arXiv2012

How to make an ultra-faint dwarf spheroidal galaxy: tidal stirring of disky dwarfs with shallow dark matter density profiles

In recent years the Sloan Digital Sky Survey has unraveled a new population of ultra-faint dwarf galaxies (UFDs) in the vicinity of the Milky Way (MW) whose origin remains a puzzle. Using a suite of collisionless N-body simulations, we investigate the formation of UFDs in the context of the tidal stirring model for the formation of dwarf spheroidal galaxies in the Local Group (LG). Our simulations are designed to reproduce the tidal interactions between MW-sized host galaxies and rotationally supported dwarfs embedded in 10^9 M_sun dark matter (DM) halos. We explore a variety of inner density slopes ρ\propto r^{-α} for the dwarf DM halos, ranging from core-like (α= 0.2) to cuspy (α= 1), and different dwarf orbital configurations. Our experiments demonstrate that UFDs can be produced via tidal stirring of disky dwarfs on relatively tight orbits, consistent with a redshift of accretion by the host galaxy of z \sim 1, and with intermediate values for the halo inner density slopes (ρ\propto r^{-0.6}). The inferred slopes are in excellent agreement with those resulting from both the modeling of the rotation curves of dwarf galaxies and recent cosmological simulations of dwarf galaxy formation. Comparing the properties of observed UFDs with those of their simulated counterparts, we find remarkable similarities in terms of basic observational parameters. We conclude that tidal stirring of rotationally supported dwarfs represents a viable mechanism for the formation of UFDs in the LG environment.

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