Ewa L. Lokas

The Ohio State University, Columbus, Ohio, United States

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Publications (46)124.37 Total impact

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    ABSTRACT: We study the origin and properties of unbound stars in the kinematic samples of dwarf spheroidal galaxies. For this purpose we have run a high resolution N-body simulation of a two-component dwarf galaxy orbiting in a Milky Way potential. We create mock kinematic data sets by observing the dwarf in different directions. When the dwarf is observed along the tidal tails the kinematic samples are strongly contaminated by unbound stars from the tails. However, most of the unbound stars can be removed by the method of interloper rejection proposed by den Hartog & Katgert. We model the velocity dispersion profiles of the cleaned-up kinematic samples using solutions of the Jeans equation. We show that even for such a strongly stripped dwarf the Jeans analysis, when applied to cleaned samples, allows us to reproduce the mass and mass-to-light ratio of the dwarf with accuracy typically better than 25%.
    Proceedings of the International Astronomical Union 01/2008; 244:321-325.
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    Ewa L. Lokas
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    ABSTRACT: NGC 5128 is a well-studied elliptical galaxy with an excellent kinematic data set for planetary nebulae which has been modelled up till now only by solving the Jeans equation for spherical systems. As a first approximation beyond spherical symmetry we model the galaxy as an axisymmetric system flattened by rotation with isotropic velocity distribution. We propose a new version of such an isotropic rotator having cuspy density profile with NFW-like behavior. The solutions of the Jeans equations for a single component of such form do not reproduce the data well: the rotation curve rises too slowly with radius and the velocity dispersion profile drops too fast. The data are well fitted however by a system built with two components: a more compact, flattened, less massive, fast-rotating and cold `bulge' and an extended, almost spherical, more massive, slow-rotating and hot `halo'. This picture agrees well with the results of recent N-body simulations of galaxy mergers which tend to produce oblate rotating spheroids. The total mass of the system is estimated to be (9.1 +/- 3.5) x 10^11 solar masses and the mass-to-light ratio is only 26 +/- 17 solar units. Comment: 4 pages, 3 figures, revised version accepted for publication in ApJ Letters
    The Astrophysical Journal 11/2007; · 6.73 Impact Factor
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    ABSTRACT: A new revision of the gamma flux that we expect to detect in Imaging Atmospheric Cherenkov Telescopes (IACTs) from neutralino annihilation in the Draco dSph is presented in the context of the minimal supersymmetric standard models (MSSM) compatible with the present phenomenological and cosmological constraints, and using the dark matter (DM) density profiles compatible with the latest observations. This revision takes also into account the important effect of the Point Spread Function (PSF) of the telescope, and is valid not only for Draco but also for any other DM target. We show that this effect is crucial in the way we will observe and interpret a possible signal detection. Finally, we discuss the prospects to detect a possible gamma signal from Draco for current or planned gamma-ray experiments, i.e. MAGIC, GLAST and GAW. Even with the large astrophysical and particle physics uncertainties we find that the chances to detect a neutralino annihilation signal in Draco seem to be very scarce for current experiments. However, the prospects for future IACTs with upgraded performances (especially lower threshold energies and higher sensitivities) such as those offered by the CTA project, might be substantially better.
    Physical review D: Particles and fields 02/2007;
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    ABSTRACT: A new revision of the gamma flux that we expect to detect in Imaging Atmospheric Cherenkov Telescopes (IACTs) from SUSY dark matter annihilation in the Draco dSph is presented using the dark matter density profiles compatible with the latest observations. This revision takes also into account the important effect of the Point Spread Function (PSF) of the Cherenkov telescope. We show that this effect is crucial in the way we will observe and interpret a possible signal profile in the telescope. Given these new considerations, some light can be shed on the recent signal excess reported by the CACTUS experiment. Comment: 7 pages, 5 figures, to appear in the Proceedings of the workshop "The dark side of the Universe", Madrid, June 20-24, 2006
    09/2006;
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    ABSTRACT: The aim of this paper is to study the efficiency of different approaches to interloper treatment in dynamical modelling of galaxy clusters. Using cosmological N-body simulation of standard LCDM model, we select 10 massive dark matter haloes and use their particles to emulate mock kinematic data in terms of projected galaxy positions and velocities as they would be measured by a distant observer. Taking advantage of the full 3D information available from the simulation, we select samples of interlopers defined with different criteria. The interlopers thus selected provide means to assess the efficiency of different interloper removal schemes found in the literature. We study direct methods of interloper removal based on dynamical or statistical restrictions imposed on ranges of positions and velocities available to cluster members. In determining these ranges, we use either the velocity dispersion criterion or a maximum velocity profile. We also generalize the common approaches taking into account both the position and velocity information. Another criterion is based on the dependence of the commonly used virial mass and projected mass estimators on the presence of interlopers. We find that the direct methods exclude on average 60-70 percent of unbound particles producing a sample with contamination as low as 2-4 percent. Next, we consider indirect methods of interloper treatment which are applied to the data stacked from many objects. In these approaches, interlopers are treated in a statistical way as a uniform background which modifies the distribution of cluster members. Using a Bayesian approach, we reproduce the properties of composite clusters and estimate the probability of finding an interloper as a function of distance from the object centre.
    Astronomy and Astrophysics 07/2006; · 5.08 Impact Factor
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    ABSTRACT: Elements of kinematical and dynamical modeling of elliptical galaxies (Es) are presented. In projection, NFW models resemble Sersic models, but with a very narrow range of shapes (m=3+/-1). The total density profile of Es cannot be NFW-like because the predicted local M/L and aperture velocity dispersion within an effective radius (Re) are much lower than observed. Stars must then dominate Es out to a few Re. Fitting an NFW model to the total density profile of Sersic+NFW (stars+dark matter [DM]) Es results in very high concentration parameters, as found by X-ray observers. Kinematical modeling of Es assuming an isotropic NFW DM model underestimates M/L at the virial radius by a factor of 1.6 to 2.4, because dissipationless LCDM halos have slightly different density profiles and slightly radial velocity anisotropy. In N-body+gas simulations of Es as merger remnants of spirals embedded in DM halos, the slope of the DM density profile is steeper when the initial spiral galaxies are gas-rich. The Hansen & Moore (2006) relation between anisotropy and the slope of the density profile breaks down for gas and DM, but the stars follow an analogous relation with slightly less radial anisotropies for a given density slope. Using kurtosis (h_4) to infer anisotropy in Es is dangerous, as h_4 is also sensitive to small levels of rotation. The stationary Jeans equation provides accurate masses out to 8 Re. The discrepancy between the modeling of Romanowsky et al. (2003), indicating a dearth of DM in Es, and the simulations analyzed by Dekel et al. (2005), which match the spectroscopic observations of Es, is partly due to radial anisotropy and to observing oblate Es face-on. However, one of the 15 solutions to the orbit modeling of Romanowsky et al. is found to have an amount and concentration of DM consistent with LCDM predictions. Comment: Invited talk, 21st IAP meeting, Mass Profiles and Shapes of Cosmological Structures, ed. G. A. Mamon, F. Combes, C. Deffayet & B. Fort (Paris: EDP). 10 pages, 7 figures (8 plots)
    EAS Publications Series 01/2006;
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    Gary A. Mamon, Ewa L. Lokas
    Monthly Notices of the Royal Astronomical Society 01/2006; 370:1582-1582. · 5.52 Impact Factor
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    ABSTRACT: Using cosmological N-body simulations we study the line-of-sight velocity distribution of dark matter haloes focusing on the lowest-order even moments, dispersion and kurtosis, and their application to estimate the mass profiles of cosmological structures. For each of the ten massive haloes selected from the simulation box we determine the virial mass, concentration and the anisotropy parameter. In order to emulate observations from each halo we choose randomly 300 particles and project their velocities and positions along the line of sight and on the surface of the sky, respectively. After removing interlopers we calculate the profiles of the line-of-sight velocity moments and fit them with the solutions of the Jeans equations. The estimates of virial mass, concentration parameter and velocity anisotropy obtained in this way are in good agreement with the values found from the full 3D analysis. Comment: 2 pages, 1 figure, poster contribution to the proceedings of the XXIst IAP Colloquium "Mass Profiles and Shapes of Cosmological Structures", Paris 4-9 July 2005, Editors: G. Mamon, F. Combes, C. Deffayet, B. Fort, EDP Sciences, in press
    EAS Publications Series 08/2005;
  • Gary A. Mamon, Ewa L. Lokas
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    ABSTRACT: Density profiles (NFW and similar) of structures found in cosmological ΛCDM N-body simulations cannot represent the total matter distribution in elliptical galaxies, or else their local mass-to-light ratio would be smaller than the value expected from the stellar component, and also their central velocity dispersions averaged within a circular aperture or thin slit would be lower than expected from what is observed through the Faber-Jackson relation. This implies that the stellar component dominates in the observed regions of ellipticals and it is therefore more difficult to reject the hypothesis that elliptical galaxies have a dark matter component similar to what is observed in the cosmological simulations or to infer the inner slope of the dark matter density profile. One must measure line-of-sight velocity dispersions out to 5 effective radii and with 15 km/s accuracy to infer the dark matter content within a factor of 3 inside the virial radius. The new density profile and slight radial orbit anisotropy found in recent cosmological simulations imply that, for a given M/L within the virial radius, the line of sight velocity dispersions at 5 effective radii are smaller than inferred with NFW dark matter and isotropic orbits, and conversely, the low velocity dispersions recently measured at 5 effective radii for the planetary nebulae around ellipticals are explained by global mass-to-light ratios within the virial radius that are twice as large as inferred when the dark matter is supposed NFW and the orbits isotropic.
    11/2004; -1:691.
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    Gary A. Mamon, Ewa L. Lokas, Teresa Sanchis
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    ABSTRACT: We provide the first direct lifting of the mass/anisotropy degeneracy for a cluster of galaxies, by jointly fitting the line of sight velocity dispersion and kurtosis profiles of the Coma cluster, assuming an NFW tracer density profile, a generalized-NFW dark matter profile and a constant anisotropy profile. We find that the orbits in Coma must be quasi-isotropic, and find a mass consistent with previous analyses, but a concentration parameter 50% higher than expected in cosmological N-body simulations. We then test the accuracy of our method on realistic non-spherical systems with substructure and streaming motions, by applying it to the ten most massive structures in a cosmological N-body simulation. We find that our method yields fairly accurate results on average (within 20%), although with a wide variation (factor 1.7 at 1 sigma) for the concentration parameter, with decreased accuracy and efficiency when the projected mean velocity is not constant with radius.
    Proceedings of the International Astronomical Union 05/2004;
  • Gary A. Mamon, Ewa L. Lokas
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    ABSTRACT: Density profiles (NFW and similar) of structures found in cosmological LambdaCDM N-body simulations cannot represent the total matter distribution in elliptical galaxies, or else their local mass-to-light ratio would be smaller than the value expected from the stellar component, and also their central velocity dispersions averaged within a circular aperture or thin slit would be lower than expected from what is observed through the Faber-Jackson relation. This implies that the stellar component dominates in the observed regions of ellipticals and it is therefore more difficult to reject the hypothesis that elliptical galaxies have a dark matter component similar to what is observed in the cosmological simulations or to infer the inner slope of the dark matter density profile. One must measure line-of-sight velocity dispersions out to 5 effective radii and with 15 km/s accuracy to infer the dark matter content within a factor of 3 inside the virial radius. The new density profile and slight radial orbit anisotropy found in recent cosmological simulations imply that, for a given M/L within the virial radius, the line of sight velocity dispersions at 5 effective radii are smaller than inferred with NFW dark matter and isotropic orbits, and conversely, the low velocity dispersions recently measured at 5 effective radii for the planetary nebulae around ellipticals are explained by global mass-to-light ratios within the virial radius that are twice as large as inferred when the dark matter is supposed NFW and the orbits isotropic.
    01/2004;
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    ABSTRACT: Using numerical simulations and the Sheth-Tormen approximation we study the mass function of dark matter halos in voids. We find that the void mass function is significantly lower and its shape is different than that of the field halos. We predict that in the standard LCDM model a void with radius 10 h^-1 Mpc should have 50 halos with circular velocity v_c > 50 km/s and 600 halos with v_c > 20 km/s.
    08/2002;
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    Ewa L. Lokas
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    ABSTRACT: I review the main characteristics of structure formation in the quintessential Universe. Assuming equation of state w=p/\varrho=$const I provide a brief description of the background cosmology and discuss the linear growth of density perturbations, the strongly nonlinear evolution, the power spectra and rms fluctuations as well as mass functions focusing on the three values w=-1, -2/3 and -1/3. Finally I describe the presently available and future constraints on w.
    Acta Physica Polonica Series B 01/2002; · 1.01 Impact Factor
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    Ewa L. Lokas, Yehuda Hoffman
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    ABSTRACT: We generalize the spherical collapse model for the formation of bound objects to apply in a Universe with arbitrary positive cosmological constant. We calculate the critical condition for collapse of an overdense region and give exact values of the characteristic densities and redshifts of its evolution.
    09/2001;
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    Ewa L. Lokas, Y. Hoffman
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    ABSTRACT: We generalize the spherical collapse model for the formation of dark matter halos to apply in a universe with arbitrary positive cosmological constant. We calculate the critical condition for collapse of an overdense region and give exact values of the characteristic densities and redshifts of its evolution.
    12/2000;
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    Ewa L. Lokas, Yehuda Hoffman
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    ABSTRACT: Using the formalism of the spherical infall model the structure of collapsed and virialized dark halos is calculated for a variety of scale-free initial conditions. In spite of the scale-free cosmological nature of the problem, the collapse of individual objects is not self-similar. Unlike most of previous calculations the dynamics used here relies only on adiabatic invariants and not on self-similarity. The paper focuses on the structure of the innermost part of the collapsed halos and addresses the problem of central density cusps. The slopes of density profiles at 1% of virial radius are calculated for a variety of cosmological models and are found to vary with the mass of the halos and power spectrum of the initial conditions. The inner slopes range between r^-2.3 and r^-2 with the limiting case of r^-2 reached for the largest masses. The steep cusps found here correspond to the limiting case where all particles move on radial orbits. The introduction of angular momentum will make the density profile shallower. We expect this to resolve the discrepancy found between the calculated profiles and the ones found in high resolution N-body simulations, where the exponent ranges from -0.5 to -1.5. The robust prediction here is that collisionless gravitational collapse in an expanding universe is expected to form density cups and not halos with a core structure. Comment: 4 pages, 4 figures, uses emulateapj5.sty, revised version accepted by ApJL (one new figure added, model described in more detail)
    The Astrophysical Journal 05/2000; · 6.73 Impact Factor
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    Ewa L. Lokas
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    ABSTRACT: I apply the modified spherical infall model to test the dependence of the shape of density profiles of dark matter halos on redshift. The model predicts that the density profile of a halo of a given mass steepens with time and its concentration parameter changes like c~1/(1+z_0)^2.
    11/1999;
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    Ewa L. Lokas
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    ABSTRACT: I propose a modification of the spherical infall model for the evolution of density fluctuations with initially Gaussian probability distribution and scale-free power spectra. I introduce a generalized form of the initial density distribution around an overdense region and cut it off at half the inter-peak separation accounting in this way for the presence of neighbouring fluctuations. Contrary to the original predictions of the model, resulting density profiles within virial radii no longer have power-law dependence on the distance, but are well fitted by the universal formula of changing slope obtained as a result of N-body simulations. The profiles of halos of a given mass are in general flatter than the corresponding ones from the simulations, but the trend of steeper profiles for smaller masses is reproduced. The profiles of galaxy size objects are in general in better agreement with N-body simulations than those of larger ones.
    10/1999;
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    ABSTRACT: Using third-order perturbation theory, we derive a relation between the divergence of the peculiar velocity and the density. Specifically, we compute the expectation value of the divergence given density. Our calculations assume Gaussian initial conditions and are valid for Gaussian filtering of the evolved density and velocity fields. The mean velocity divergence turns out to be a third-order polynomial in the density contrast. We test the power-spectrum dependence of the coefficients of the polynomial for scale-free and standard CDM spectra and find it rather weak. Over scales larger than about 5 h−1 Mpc, the scatter in the relation is small compared with that introduced by random errors in the observed density and velocity fields. The relation can be useful for recovering the peculiar velocity from the associated density field, and also for non-linear analyses of the anisotropies of structure in redshift surveys.
    Monthly Notices of the Royal Astronomical Society 10/1998; 300(4):1027 - 1034. · 5.52 Impact Factor
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    ABSTRACT: Using third-order perturbation theory, we derive a relation between the mean divergence of the peculiar velocity given density and the density itself. Our calculations assume Gaussian initial conditions and are valid for Gaussian filtering of the evolved density and velocity fields. The mean velocity divergence turns out to be a third-order polynomial in the density contrast. We test the power spectrum dependence of the coefficients of the polynomial for scale-free and standard CDM spectra and find it rather weak. Over scales larger than about 5 megaparsecs, the scatter in the relation is small compared to that introduced by random errors in the observed density and velocity fields. The relation can be useful for recovering the peculiar velocity from the associated density field, and also for non-linear analyses of the anisotropies of structure in redshift surveys.
    03/1998;