1,721,067 research outputs found
Local gravitational instability of two-component thick discs in three dimensions
No full textAims. The local gravitational instability of rotating discs is believed to be an important mechanism in different astrophysical processes, including the formation of gas and stellar clumps in galaxies. We aim to study the local gravitational instability of two-component thick discs in three dimensions. Methods. We use as a starting point a recently proposed analytic three-dimensional (3D) instability criterion for discs with non-negligible thickness that takes the form Q3D < 1, where Q3D is a 3D version of the classical 2D Toomre Q parameter for razor-thin discs. Here, we extend the 3D stability analysis to two-component discs, considering first the influence on Q3D of a second unresponsive component, and then the case in which both components are responsive. We present the application to two-component discs with isothermal vertical distributions, which can represent, for instance, galactic discs with both stellar and gaseous components. Finally, we relax the assumption of vertical isothermal distribution, by studying one-component self-gravitating discs with polytropic vertical distributions for a range of values of the polytropic index corresponding to convectively stable configurations. Results. We find that Q3D < 1, where Q3D can be computed from observationally inferred quantities, is a robust indicator of local gravitational instability, depending only weakly on the presence of a second component and on the vertical gradient of temperature or velocity dispersion. We derive a sufficient condition for local gravitational instability in the midplane of two-component discs, which can be employed when both components have Q3D > 1
Galaxy merging, the fundamental plane of elliptical galaxies and the M BHσ0 relation
No full textWe explore the effects of dissipationless merging on the fundamental plane of elliptical galaxies using an N-body code based on a new, high-performance numerical scheme. We investigate the two extreme cases of galaxy growth by equal-mass merging and accretion of small stellar systems; in a subset of simulations we also consider the presence of dark matter haloes around the merging galaxies. Curiously, we found that the fundamental plane is preserved by major merging, while in the accretion scenario its edge-on thickness is only marginally reproduced, with substantial thickening in the case of merging with low angular momentum. We also found that both the Faber-Jackson and Kormendy relations are not reproduced by the simulations, in accordance with the results of a preliminary analysis based on a simple application of the virial theorem. Finally, we discuss the implications of our results for the origin of the M BH-σ0 and Magorrian relations. We found that dissipationless merging is unable to reproduce the MBH-σ 0 relation, if the black hole masses add linearly (while the Magorrian relation is nicely reproduced); in contrast, a black hole merging with substantial emission of gravitational waves reproduces the M BH-σ0 relation but fails to reproduce the Magorrian relation. We argue that our results strongly point towards a major role of dissipation in the formation of early-type galaxies and in the growth of their central supermassive black holes, thus supporting the idea of a link between galaxy formation and quasi-stellar object activity
Brownian motion of supermassive black holes in galaxy cores
No full textWe investigate the dynamics of supermassive black holes (SMBHs) in galactic cores by means of a semi-Analytic model based on the Langevin equation, including dynamical friction and stochastic noise accounting for the gravitational interactions with stars. The model is validated against direct N-body simulations of intermediate-mass black holes in stellar clusters where a realistic number of particles is accessible. For the galactic case, we find that the SMBH experiences a Brownian-like motion with a typical displacement from the geometric center of the Galaxy of a few parsecs, for system parameters compatible with M87
The cosmic evolution of the stellar mass-velocity dispersion relation of early-type galaxies
Full text linkWe study the evolution of the observed correlation between central stellar velocity dispersion σe and stellar mass M∗ of massive (M∗ ≿ 3 × 1010 M☉) early-type galaxies (ETGs) out to redshift z ≈ 2.5, taking advantage of a Bayesian hierarchical inference formalism. Collecting ETGs from state-of-the-art literature samples, we build a fiducial sample (0 ≾ z ≾ 1), which is obtained with homogeneous selection criteria, but also a less homogeneous extended sample (0 ≾ z ≾ 2.5). Based on the fiducial sample, we find that at z ≾ 1 the M∗-σe relation is well represented by σe ∝ M∗β(1 + z)ζ, with β ≃ 0.18 independent of redshift and ζ ≃ 0.4 (at a given M∗, σe decreases for decreasing z, for instance by a factor of ≈1.3 from z = 1 to z = 0). When the slope β is allowed to evolve, we find it increasing with redshift: β(z) ≃ 0.16 + 0.26log (1 + z) describes the data as well as constant β ≃ 0.18. The intrinsic scatter of the M∗-σe relation is ≃0.08 dex in σe at given M∗, independent of redshift. Our results suggest that, on average, the velocity dispersion of individual massive (M∗ ≿ 3 × 1011M☉) ETGs decreases with time while they evolve from z ≈ 1 to z ≈ 0. The analysis of the extended sample, over the wider redshift range 0 ≾ z ≾ 2.5, leads to results similar to that of the fiducial sample, with slightly stronger redshift dependence of the normalization (ζ ≃ 0.5) and weaker redshift dependence of the slope (dβ/dlog (1 + z) ≃ 0.18) when β varies with time. At z = 2 ETGs with M∗ ≈ 1011 M☉ have, on average, ≈1.7 higher σe than ETGs of similar stellar mass at z = 0
Radial orbital anisotropy and the Fundamental Plane of elliptical galaxies
No full textThe existence of the Fundamental Plane imposes strong constraints on the structure and dynamics of elliptical galaxies, and thus contains important information on the processes of their formation and evolution. Here we focus on the relations between the Fundamental Plane thinness and tilt and the amount of radial orbital anisotropy: in fact, the problem of the compatibility between the observed thinness of the Fundamental Plane and the wide spread of orbital anisotropy admitted by galaxy models has often been raised. By using N-body simulations of galaxy models characterized by observationally motivated density profiles, and also allowing for the presence of live, massive dark matter haloes, we explore the impact of radial orbital anisotropy and instability on the Fundamental Plane properties. The numerical results confirm a previous semi-analytical finding (based on a different class of one-component galaxy models): the requirement of stability matches almost exactly the thinness of the Fundamental Plane. In other words, galaxy models that are radially anisotropic enough to be found outside the observed Fundamental Plane (with their isotropic parent models lying on the Fundamental Plane) are unstable, and their end-products fall back on the Fundamental Plane itself. We also find that a systematic increase of radial orbit anisotropy with galaxy luminosity cannot explain by itself the whole tilt of the Fundamental Plane, the galaxy models becoming unstable at moderately high luminosities: at variance with the previous case, their end-products are found well outside the Fundamental Plane itself. Some physical implications of these findings are discussed in detail
Gas rotation and dark matter halo shape in cool-core clusters of galaxies
No full textAims. We study the possibility that the gas in cool-core clusters of galaxies has non-negligible rotation support, the impact of gas rotation on mass estimates from current X-ray observations, and the ability of forthcoming X-ray observatories to detect such rotation. Methods. We present three representative models of massive cool-core clusters with a rotating intracluster medium (ICM) in equilibrium in cosmologically motivated spherical, oblate, or prolate dark matter halos, represented by physical density- potential pairs. In the models, the gas follows a composite-polytropic distribution, and has rotation velocity profiles consistent with current observational constraints and similar to those found in clusters formed in cosmological simulations. We show that the models are consistent with available measurements of the ICM properties of the massive cluster population: the thermodynamic profiles, the shape of the surface brightness distribution, the hydrostatic mass bias, and the broadening of X-ray emitting lines. Using the configuration for the microcalorimeter onboard the XRISM satellite, we generated a set of mock X-ray spectra for our cluster models, which we then analyzed to make predictions about the rotation speed that will be obtained with such an instrument. We then assessed what fraction of the hydrostatic mass bias of our models could be accounted for by detecting the rotation speed with XRISM spectroscopy over the range (0.1 - 1)r500, sampled with three nonoverlapping pointings. Results. Current data leave room for rotating ICM in cool-core clusters, with peaks in the rotation speed as high as 600kms-1. We show that such rotation, if present, will be detected with upcoming X-ray facilities such as XRISM and that 60 - 70% of the hydrostatic mass bias due to rotation can be accounted for using the line-of-sight velocity measured from X-ray spectroscopy with XRISM, with a residual bias smaller than 3% at an overdensity of 500. In this way, XRISM will allow us to pin down any mass bias of a different origin from the rotation
Time variability of AGN and heating of cooling flows
No full textThere is increasing evidence that AGN mechanical feedback is important in the energetics of cooling flows in galaxies and galaxy clusters. We investigate the implications of the variability of AGN mechanical luminosity L_m on observations of cooling flows and radio galaxies in general. It is natural to assume that l=ln(L_m/L_x) is a Gaussian process. Then L_m will be log-normally distributed at fixed cooling luminosity L_x, and the variance in a measure of L_m will increase with the time-resolution of the measure. We test the consistency of these predictions with existing data. These tests hinge on the power spectrum of l(t). Monitoring of Seyfert galaxies combined with estimates of the duty cycle of quasars imply flicker noise spectra, similar to those of microquasars. We combine a sample of sources in cooling flows that have cavities with the assumption that the average mechanical luminosity of the AGN equals L_x. Given that the mechanical luminosities are characterized by flicker noise, we find that their spectral amplitudes lie between the estimated amplitudes of quasars and the measured values for the radio luminosities of microquasars. The model together with the observation that powerful radio galaxies lie within a narrow range in optical luminosity, predicts the luminosity function of radio galaxies, in agreement with observations. Forthcoming radio surveys will test the prediction that the luminosity function turns over at about the smallest luminosities so far probed. [Abridged
Galactic cannibalism in the galaxy cluster C0337-2522 at z = 0.59
No full textAccording to the galactic cannibalism model, cD galaxies are formed in the centre of galaxy clusters by merging of massive galaxies and accretion of smaller stellar systems; however, observational examples of the initial phases of this process are lacking. We have identified a strong candidate for this early stage of cD galaxy formation: a group of five elliptical galaxies in the core of the X-ray cluster C0337-2522 at redshift z = 0.59. With the aid of numerical simulations, in which the galaxies are represented by N-body systems, we study their dynamical evolution up to z = 0; the cluster dark matter distribution is also described as an N-body system. We explore the hypothesis that some of the five galaxies will have merged before z = 0, making reasonable assumptions on the structural and dynamical characteristics of the cluster. We then compare the properties of the merger remnant with those of real ellipticals (such as its accordance with the fundamental plane, the Faber-Jackson and the MBH-σ0 relations) and, in particular, we check whether the remnant has the surface brightness profile typical of cD galaxies. We find that a multiple merging event in the considered group of galaxies will take place before z = 0 and that the merger remnant preserves the fundamental plane and the Faber-Jackson relations, while its behaviour with respect to the MBH-σ 0 relation is quite sensitive to the details of black hole merging. However, the end-products of our simulations are more similar to a 'normal' giant elliptical than to a cD galaxy with its characteristic diffuse luminous halo, thus confirming previous indications that the formation of cD galaxies is not a necessary consequence of galaxy merging at the cluster centre
The effect of tides on the Sculptor dwarf spheroidal galaxy
No full textDwarf spheroidal galaxies (dSphs) appear to be some of the most dark matter (DM)-dominated objects in the Universe. Their dynamical masses are commonly derived using the kinematics of stars under the assumption of equilibrium. However, these objects are satellites of massive galaxies (e.g. the Milky Way) and thus can be influenced by their tidal fields. We investigate the implication of the assumption of equilibrium focusing on the Sculptor dSph by means of ad hoc N-body simulations tuned to reproduce the observed properties of Sculptor following the evolution along some observationally motivated orbits in the Milky Way gravitational field. For this purpose, we used state-of-the-art spectroscopic and photometric samples of Sculptor's stars. We found that the stellar component of the simulated object is not directly influenced by the tidal field, while ≈ 30-60 of the mass of the more diffuse DM halo is stripped. We conclude that, considering the most recent estimate of the Sculptor proper motion, the system is not affected by the tides and the stellar kinematics represents a robust tracer of the internal dynamics. In the simulations that match the observed properties of Sculptor, the present-day dark-to-luminous mass ratio is ≈6 within the stellar half-light radius (≈0.3 kpc) and >50 within the maximum radius of the analysed data set (≈1.5°, ≈2 kpc)
Testing quasi-linear modified Newtonian dynamics theory with Galactic globular clusters in a weak external field
Full text linkWe developed self-consistent dynamical models of stellar systems in the framework of quasi-linear modified Newtonian dynamics (QUMOND). The models are constructed from the anisotropic distribution function of Gunn and Griffin, combined with the modified Poisson equation defining this gravitation theory and take into account the external field effect. We have used these models, and their Newtonian analogues, to fit the projected density and the velocity dispersion profiles of a sample of 18 Galactic globular clusters, using the most updated data sets of radial velocities and Gaia proper motions. We have thus obtained, for each cluster, estimates of the dynamical mass-to-light ratio (M/L) for each theory of gravity. The selected clusters have accurate proper motions and a well-sampled mass function down to the very low-mass regime. This allows us to constrain the degree of anisotropy and to provide, from comparison with stellar evolution isochrones, a dynamics-independent estimate of the minimum mass-to-light ratio (M/L)min. Comparing the best-fitting dynamical M/L with (M/L)min, we find that for none of the analysed clusters the two gravity theories are significantly incompatible with the observational data, although for one of them (NGC 5024) the dynamical M/L predicted by QUMOND lies at 2.8sigma below (M/L)min. Though the proposed approach suffers from some limitations (in particular the lack of a treatment of mass segregation), the obtained results suggest that the kinematics of globular clusters in a relatively weak external field can be a powerful tool to prove alternative theories of gravitation
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