1,721,105 research outputs found
Modeling elliptical galaxies: Phase-space constraints on two-component (γ1, γ2) models
No full textIn the context of the study of the properties of the mutual mass distribution of the bright and dark matter in elliptical galaxies, I present a family of two-component, spherical, self-consistent galaxy models, in which one density distribution follows a γ1 profile and the other a γ2 profile [hereafter (γ1, γ2) models], with different total masses and "core" radii. A variable amount of (radial) orbital anisotropy is allowed in both components, following the Osipkov-Merritt parameterization. For these models, I derive analytically the necessary and sufficient conditions that the model parameters must satisfy in order to correspond to a physical system (the so-called model consistency). Moreover, the possibility of adding a black hole at the center of radially anisotropic γ models is discussed, determining analytically a lower limit of the anisotropy radius as a function of γ. The analytical phase-space distribution function for (1, 0) models is presented, together with the solution of the Jeans equations and the quantities entering the scalar virial theorem. It is proved that a globally isotropic γ = 1 component is consistent for any mass and core radius of the superimposed γ = 1 model ; on the other hand, only a maximum value of the core radius is allowed for the γ = 0 model when a γ = 1 density distribution is added. The combined effects of mass concentration and orbital anisotropy are investigated, and an interesting behavior of the distribution function of the anisotropic γ = 0 component is found: there exists a region in the parameter space where a sufficient amount of anisotropy results in a consistent model, while the structurally identical but isotropic model would be inconsistent
Tomography of collisionless stellar systems
No full textIn this paper the concept of tomography of a collisionless stellar system of general shape is introduced, and a generalization of the Projected Virial Theorem is obtained. Applying the tomographic procedure we then derive a new family of virial equations which coincides with the already known ones for spherically symmetric systems. This result is obtained without any use of explicit expressions for the line-of-sight velocity dispersion, or spherical coordinate system. © 1994 Kluwer Academic Publishers
On the Rotation Curve of Disk Galaxies in General Relativity
Full text linkRecently, it has been suggested that the phenomenology of flat rotation curves observed at large radii in the equatorial plane of disk galaxies can be explained as a manifestation of general relativity (GR) instead of the effect of dark matter (DM) halos. In this paper, by using the well-known weak-field, low-velocity gravitomagnetic formulation of GR, the expected rotation curves in GR are rigorously obtained for purely baryonic disk models with realistic density profiles and compared with the predictions of Newtonian gravity for the same disks in absence of DM. As expected, the resulting rotation curves are indistinguishable, with GR corrections at all radii of the order v 2/c 2 ≍ 10-6. Next, the gravitomagnetic Jeans equations for two-integral stellar systems are derived, and then solved for the Miyamoto-Nagai disk model, showing that finite-thickness effects do not change the previous conclusions. Therefore, the observed phenomenology of galactic rotation curves at large radii requires DM in GR exactly as in Newtonian gravity, unless the cases here explored are reconsidered in the full GR framework with substantially different results (with the surprising consequence that the weak-field approximation of GR cannot be applied to the study of rotating systems in the weak-field regime). In this article, the mathematical framework is described in detail, so that the present study can be extended to other disk models, or to elliptical galaxies (where DM is also required in Newtonian gravity, but their rotational support can be much less than in disk galaxies)
Stellar systems following the R 1/m luminosity law: IV. The total energy and the central concentration of galaxies
No full textWe expand our previous analytical and numerical studies of the family of Sérsic models, which are routinely used to describe early-type galaxies and the bulges of spiral galaxies. In particular, we focus on the total energy budget, an important dynamical property that has not been discussed in detail in previous works. We use two different methods to calculate the total energy for the Sérsic model family that result in two independent expressions that can be used along the entire sequence of Sérsic models. We use these expressions to investigate whether the Spitzer concentration index is a reliable measure for the intrinsic 3D concentration of galaxies, and we conclude that it is not a very useful measure for the central concentration. The popular Third Galaxy Concentration index, on the other hand, is shown to be a reliable measure for the intrinsic 3D concentration, even though it is based on the surface brightness distribution and not on the intrinsic 3D density
Motion of a rigid body in a tidal field - An application to elliptical galaxies in clusters
No full textWe investigate the motion, near the equilibrium configurations, of an initially spinless rigid body subject to an external tidal field. Two cases are considered: when the center of mass of the body is at rest at the equilibrium point of the field generated by a generic mass distribution, and when it is placed on a circular orbit subject to a spherically symmetric potential. A complete analysis of the equilibrium configurations is carried out for both cases. First, we derive the conditions for the stable equilibria, and then we analyze the frequencies of oscillations around the equilibrium positions. In view of these results, we consider the problem of alignment of galaxies in clusters. After estimating the period of the oscillations induced on the galaxies by the tidal field of the cluster, we discuss the possible effect of resonances between stellar orbits inside the galaxy and the oscillations of the galaxy as a whole; this may be a mechanism responsible for producing an intracluster stellar population
Stellar systems following the R1/m luminosity law. II. Anisotropy, velocity profiles, and the fundamental plane of elliptical galaxies
No full textFollowing a first paper on this subject (Ciotti 1991, hereafter Paper I), we study the dynamical properties of spherical galaxies with surface luminosity profile described by the R1/m-law, in which a variable degree of orbital anisotropy is allowed. The parameter m for the present models covers the range [1, 10]. For these models we study the self-consistently generated phase-space distribution function (DF), and we derive - as a function of m - the minimum value of the anisotropy radius for the model consistency (i.e., in order to have a nowhere negative DF). Then we study the region in the parameter space where the R1/m models are likely to be stable against radial-orbit instability, and we compare its size with that of the larger region corresponding to the consistency requirement. For stable anisotropic models the spatial and projected velocity dispersion profiles are obtained solving the Jeans equation, and compared to those of the globally isotropic case, already discussed in Paper I. The relevance of the results in connection with the fundamental plane (FP) of elliptical galaxies is pointed out: the effect on the projected velocity dispersion due to the maximum orbital anisotropy allowed by the stability requirement is well within the FP thickness, and so no fine-tuning for anisotropy is required. Finally, the velocity profiles are constructed as a function of the projected radius and for various degrees of anisotropy, and their deviations from a gaussian discussed
Decoupled hot gas flows in elliptical galaxies
No full textWe present the results of a new set of hydrodynamical simulations of hot gas flows in model elliptical galaxies with the following characteristics: the spatial luminosity distribution approaches a power law form at small radii, in accordance with the results of recent ground based observations, and with the Hubble Space Telescope; the dark matter has a peaked profile too, as indicated by high resolution numerical simulations of dissipationless collapse; the dark to luminous mass ratio spans a large range of values, including low values found by optical studies confined to within two effective radii; finally, the type la supernova rate is that given by the latest estimates of optical surveys, or zero, as suggested by the iron abundances recently measured in the hot gas. We find that the resulting gas flows are strongly decoupled: an inflow develops in the central region of the galaxies, while the external parts are still degassing, i.e., the flows are mostly partial winds. This behavior can be explained in terms of the local energy balance of the hot gas. A large spread in the X-ray luminosity LX at fixed optical luminosity LB can be produced as in previous simulations that used King models plus massive quasi-isothermal dark halos, and higher supernova rates; the key factor causing large LX variations is now the size of the central inflow region. The highest LX observed correspond to global inflows. Finally, non negligible amounts of cold gas can be produced by the partial winds; this could be an explanation for the possible discovery of cold matter at the center of elliptical galaxies, an alternative to the presence of a steady state cooling flow
Stellar systems following the R 1/m luminosity law: III. Photometric, intrinsic, and dynamical properties for all Sérsic indices
No full textThe Sérsic or R1/m model has become the de facto standard model to describe the surface brightness profiles of early-Type galaxies and the bulges of spiral galaxies. The photometric, intrinsic, and dynamical properties of this model have been investigated, but mainly for fairly large Sérsic indices m. For small values of m, appropriate for low-mass and dwarf ellipticals, a detailed investigation of these properties is still lacking. In this study, we used a combination of numerical and analytical techniques to investigate the Sérsic model over the entire range of Sérsic parameters, focussing on the small m regime, where a number of interesting and surprising properties are found. For all values mâ < â 1, the model is characterised by a finite central luminosity density, and for m < 1/2, even a central depression in the luminosity density profile. This behaviour translates to the dynamical properties: we show that all Sérsic models with m ≥ 1/2 can be supported by an isotropic velocity dispersion tensor, and that these isotropic models are stable to both radial and non-radial perturbations. The models with m < 1/2, on the other hand, cannot be supported by an isotropic velocity dispersion tensor
Decoupled and inhomogeneous gas flows in S0 galaxies
No full textA recent analysis of the Einstein sample of early-type galaxies has revealed that at any fixed optical luminosity LB, S0 galaxies have lower mean X-ray luminosity LX per unit LB than elliptical galaxies. Following a previous analytical investigation of this problem (Ciotti & Pellegrini), we have performed two-dimensional numerical simulations of the gas flows inside S0 galaxies in order to ascertain the effectiveness of rotation and/or galaxy flattening in reducing the LX/LB ratio. The flow in models without supernova (SNIa) heating is considerably ordered, and essentially all the gas lost by the stars is cooled and accumulated in the galaxy center. If rotation is present, the cold material settles in a disk on the galactic equatorial plane. Models with a time-decreasing SNIa heating host gas flows that can be much more complex. After an initial wind phase, gas flows in energetically strongly bound galaxies tend to reverse to inflows. This occurs in the polar regions, while the disk is still in the outflow phase. In this phase of strong decoupling, cold filaments are created at the interface between inflowing and outflowing gas. Models with more realistic values of the dynamical quantities are preferentially found in the wind phase with respect to their spherical counterparts of equal LB. The resulting LX of this class of models is lower than in spherical models with the same LB and SNIa heating. At variance with cooling flow models, rotation is shown to have only a marginal effect in this reduction, while the flattening is one of the driving parameters for such underluminosity, in accordance with the analytical investigation. © 1998. The American Astronomical Society. All rights reserved
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