arXiv subjects

L. Ciotti

Publications and source records attributed to L. Ciotti.

At least 73 records · Page 4Linked to original sources

arXiv2006

Dissipationless collapse, weak homology and central cores of elliptical galaxies

By means of high-resolution N-body simulations we revisited the dissipationless collapse scenario for galaxy formation. We considered both single-component collapses and collapses of a cold stellar distribution in a live dark matter halo. Single-component collapses lead to stellar systems whose projected profiles are fitted very well by the Sersic R^(1/m) law with 3.6 < m < 8. The stellar end-products of collapses in a dark matter halo are still well described by the R^(1/m) law, but with 1.9 < m < 12, where the lowest m values are obtained when the halo is dominant. In all the explored cases the profiles at small radii deviate from their global best-fit R^(1/m) model, being significantly flatter. The break-radius values are comparable with those measured in `core' elliptical galaxies, and are directly related to the coldness of the initial conditions. The dissipationless collapse of initially cold stellar distributions in pre-existing dark matter haloes may thus have a role in determining the observed weak homology of elliptical galaxies.

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arXiv2005

Axisymmetric and triaxial MOND density-potential pairs

We present a simple method, based on the deformation of spherically symmetric potentials, to construct explicit axisymmetric and triaxial MOND density-potential pairs. General guidelines to the choice of suitable deformations, so that the resulting density distribution is nowhere negative, are presented. This flexible method offers for the first time the possibility to study the MOND gravitational field for sufficiently general and realistic density distributions without resorting to sophisticated numerical codes. The technique is illustrated by constructing the MOND density-potential pair for a triaxial galaxy model that, in the absence of deformation, reduces to the Hernquist sphere. Such analytical solutions are also relevant to test and validate numerical codes. Here we present a new numerical potential solver designed to solve the MOND field equation for arbitrary density distributions: the code is tested with excellent results against the analytic MOND triaxial Hernquist model and the MOND razor-thin Kuzmin disk, and a simple application is finally presented.

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arXiv2005

Hydrostatic models for the rotation of extra-planar gas in disk galaxies

We show that fluid stationary models are able to reproduce the observed, negative vertical gradient of the rotation velocity of the extra-planar gas in spiral galaxies. We have constructed models based on the simple condition that the pressure of the medium does not depend on density alone (baroclinic instead of barotropic solutions: isodensity and isothermal surfaces do not coincide). As an illustration, we have successfully applied our method to reproduce the observed velocity gradient of the lagging gaseous halo of NGC 891. The fluid stationary models discussed here can describe a hot homogeneous medium as well as a "gas" made of discrete, cold HI clouds with an isotropic velocity dispersion distribution. Although the method presented here generates a density and velocity field consistent with observational constraints, the stability of these configurations remains an open question.

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arXiv2005

The contribution of rotational velocity to the FP of elliptical galaxies

The contribution of ordered rotation to the observed tilt and thickness of the Fundamental Plane of elliptical galaxies is studied by means of oblate, two-integrals cuspy galaxy models with adjustable flattening, variable amount of ordered rotational support, and possible presence of a dark matter halo and of a central super-massive black hole. We find that, when restricting the measure of the velocity dispersion to the central galactic regions, rotation has a negligible effect, and so cannot be responsible of the observed tilt. However, streaming velocity effect can be significant when observing small and rotationally supported galaxies through large (relative) aperture (as for example in Fundamental Plane studies at high redshift), and can lead to unrealistically low mass-to-light ratios. The effect of a central supermassive black hole on the kinematical fields, and the models position in the v/sigma-ellipticity plane are also discussed.

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arXiv2005

The evolution of early-type galaxies at z~1 from the K20 survey

We have performed VLT spectroscopy of an almost complete sample of 18 early-type galaxies with 0.88 < z < 1.3 plus two at z=0.67, selected from the K20 survey, and derived the velocity dispersion for 15+2 of them. By combining these data with HST and VLT images, we study the Fundamental Plane (FP), the Faber-Jackson and the Kormendy relations at z~1, and compare them with the local ones. The FP at z~1 keeps a remarkably small scatter, and shows both an offset and a rotation, which we interpret in terms of evolution of the mass-to-light ratio, and possibly of the size. We show evidence that the evolution rate depends on galaxy mass, being faster for less massive galaxies. We discuss the possible factors driving the evolution of spheroids and compare our results with the predictions of the hierachical models of galaxy formation.

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arXiv2005

A simple method to construct exact density-potential pairs from a homeoidal expansion

We start from a study of the density-potential relation for classical homeoids in terms of an asymptotic expansion for small deviations from spherical symmetry. We then show that such expansion is a useful device that allows us to construct a variety of exact density-potential pairs with spheroidal, toroidal, or triaxial shapes for which the deviation from spherical symmetry is finite. As concrete analytical applications, we describe: (1) The construction of a family of toroidal axisymmetric density-potential pairs one of which is associated with a perfectly flat rotation curve (for a member of this family, the supporting two-integral phase-space distribution function is obtained in closed form); (2) The determination of the aperture velocity dispersion in a wide class of two-integral axisymmetric models not stratified on homeoids with central black hole, which may be useful for the discussion of the dynamical contributions to the characteristics of the Fundamental Plane of early-type galaxies; and (3) For such class of models, the construction of the v/sigma-ellipticity relation, often considered to assess the role of rotation in the structure of elliptical galaxies.

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arXiv2004

Radiative feedback from quasars and the growth of massive black holes in stellar spheroids

We discuss the importance of feedback via photoionization and Compton heating on the co-evolution of massive black holes (MBHs) at the center of spheroidal galaxies and their stellar and gaseous components. We first assess the energetics of the radiative feedback from a typical quasar on the ambient interstellar medium. We then demonstrate that the observed Mbh-sigma relation could be established following the conversion of most of the gas of an elliptical progenitor into stars, specifically when the gas-to-stars mass ratio in the central regions has dropped to a low level ~0.01 or less, so that gas cooling is no longer able to keep up with the radiative heating by the growing central MBH. A considerable amount of the remaining gas will be expelled and both MBH accretion and star formation will proceed at significantly reduced rates thereafter, in agreement with observations of present day ellipticals. We find further support for this scenario by evolving over an equivalent Hubble time a simple, physically based toy model that additionally takes into account the mass and energy return for the spheroid evolving stellar population, a physical ingredient often neglected in similar approaches.

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arXiv2004

Radiative Feedback from Quasars and the Growth of Supermassive Black Holes

We discuss the role of feedback via photoionization and Compton heating in the co-evolution of massive black holes at the center of spheroidal galaxies and their stellar and gaseous components. We first assess the energetics of the radiative feedback from a typical quasar on the ambient interstellar gas. We then demonstrate that the observed Mbh-sigma relation could be established at a relatively early epoch in galactic evolution when the formation of the stellar bulge was almost completed and the gas-to-stars mass ratio was reduced to a low level ~0.01 such that cooling could not keep up with radiative heating. A considerable amount of gas was expelled at that time and black hole accretion proceeded at a much lower rate thereafter.

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arXiv2004

Active Galaxies and Radiative Heating

There is abundant evidence that heating processes in the central regions of elliptical galaxies has both prevented large-scale cooling flows and assisted in the expulsion of metal rich gas. We now know that each such spheroidal system harbors in its core a massive black hole weighing approximately 0.13% of the mass in stars and also know that energy was emitted by each of these black holes with an efficiency exceeding 10% of its rest mass. Since, if only 0.5% of that radiant energy were intercepted by the ambient gas, its thermal state would be drastically altered, it is worth examining in detail the interaction between the out-flowing radiation and the equilibrium or inflowing gas. On the basis of detailed hydrodynamic computations we find that relaxation oscillations are to be expected with the radiative feedback quite capable of regulating both the growth of the central black hole and also the density and thermal state of the gas in the galaxy. Mechanical input of energy by jets may assist or dominate over these radiative effects. We propose specific observational tests to identify systems which have experienced strong bursts of radiative heating from their central black holes.

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arXiv2004

Stationary fluid models for the extra-planar gas in spiral galaxies

We show how to construct families of stationary hydrodynamical configurations that reproduce the observed vertical gradient of the rotation velocity of the extra-planar gas in spiral galaxies. We then present a simple model for the lagging halo of the spiral galaxy NGC 891, which is in agreement with the HI observations. Our method is based on well known properties of baroclinic solutions, and it is an elementary application of a much more general and flexible method.

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arXiv2004

A physically motivated toy model for the BH-spheroid coevolution

We present a summary of the results obtained with a time-dependent, one-zone toy model aimed at exploring the importance of radiative feedback on the co-evolution of massive black holes (MBHs) at the center of stellar spheroids and their stellar and gaseous components. We consider cosmological infall of gas as well as the mass and energy return for the evolving stellar population. The AGN radiative heating and cooling are described by assuming photoionization equilibrium of a plasma interacting with the average quasar SED. Our results nicely support a new scenario in which the AGN accretion phase characterized by a very short duty-cycle (and now common in the Universe) is due to radiative feedback. The establishment of this phase is recorded as a fossil in the Magorrian and Mbh-sigma relations.

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arXiv2004

Collisionless evaporation from cluster elliptical galaxies: a contributor to the intracluster stellar population

By means of simple numerical models we discuss whether "collisionless stellar evaporation" from cluster elliptical galaxies could be an effective mechanism for the production of intracluster stellar populations. The effectiveness of this mechanism is due to the fact that, for realistic galaxy and cluster models, the galaxy oscillation periods near equilibrium configurations in the cluster tidal field are of the same order of stellar orbital times in the external parts of the galaxies themselves. With the aid of Monte-Carlo simulations we explore the evolution of stellar orbits in oscillating galaxies placed near different equilibrium positions. We found that, over an Hubble time, the main effect is a substantial expansion of the galactic outskirts, particularly affecting the galaxy at the cluster center and those orbiting near the cluster core radius: overall, approximately the 10% of the galaxy mass is affected. Thus, the proposed mechanism could be of some importance in the shaping of the halo of cD galaxies and in making easier ``galaxy harassment'' in the formation of the intracluster stellar population.

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arXiv2004

Two-body relaxation in modified Newtonian dynamics

A naive extension to MOND of the standard computation of the two-body relaxation time Tb implies that Tb is comparable to the crossing time regardless of the number N of stars in the system. This computation is questionable in view of the non-linearity of MOND's field equation. A non-standard approach to the calculation of Tb is developed that can be extended to MOND whenever discreteness noise generates force fluctuations that are small compared to the mean-field force. It is shown that this approach yields standard Newtonian results for systems in which the mean density profile is either plane-parallel or spherical. In the plane-parallel case we find that in the deep-MOND regime Tbb scales with N as in the Newtonian case, but is shorter by the square of the factor by which MOND enhances the gravitational force over its Newtonian value for the same system. Application of these results to dwarf galaxies and groups and clusters of galaxies reveals that in MOND luminosity segregation should be far advanced in groups and clusters of galaxies, two body relaxation should have substantially modified the density profiles of galaxy groups, while objects with masses in excess of ~10M_sun should have spiralled to the centres of dwarf galaxies.(Abridged)

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arXiv2004

On the use of X-rays to determine dynamical properties of elliptical galaxies

The X-ray emitting interstellar medium of early type galaxies is often used as a tool to determine their total mass M and stellar orbital anisotropy beta profiles, based on the hypothesis of hydrostatic equilibrium for the hot gas. Here we investigate the effects that deviations from equilibrium have on M and beta estimates, by using simple analytical calculations and hydrodynamical simulations representative of gas rich galaxies. We show that the deviations of the X-ray determined beta^{est} and M^{est} from the true values are linked by a remarkably simple relation; in particular, M is underestimated if beta is overestimated, and more radially anisotropic orbital distributions than true are deduced in presence of gas infall velocities of the order of the local stellar velocity dispersion (as are likely in the central regions of galactic cooling flows). The results of this analysis are applied to the most thoroughly investigated bright elliptical, NGC4472. We show that beta^{est} recently derived from X-rays corresponds to a galaxy that is unstable by radial orbit instability. Then, assuming as true beta and M the optically derived values, we show that the differences beta^{est}-beta and M^{est}-M agree with the predictions found here in the case of lack of hydrostatic equilibrium, which points to the latter as a possible explanation for the discrepancies (abridged).

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arXiv2003

Free-free absorption effects on Eddington luminosity

In standard treatments the Eddington luminosity is calculated by assuming that the electron-photon cross section is well described by the Thomson cross section which is gray (frequency independent). Here we discuss some consequence of the introduction of free-free opacity in the Eddington luminosity computation: in particular, due to the dependence of free-free emission on the square of the gas density, it follows that the associated absorption cross section increases linearly with the gas density, so that in high density environments Eddington luminosity is correspondingly reduced. We present a summary of an ongoing exploration of the parameter space of the problem, and we conclude that Eddington luminosity in high density environments can be lowered by a factor of ten or more, making it considerably easier for black holes to accelerate and eject ambient gas.

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arXiv2003

Self-consistent stellar dynamical tori

We present preliminary results on a new family of distribution functions that are able to generate axisymmetric, truncated (i.e., finite size) stellar dynamical models characterized by toroidal shapes. The relevant distribution functions generalize those that are known to describe polytropic spheres, for which all the dynamical and structural properties of the system can be expressed in explicit form as elementary functions of the system gravitational potential. The model construction is then completed by a numerical study of the associated Poisson equation. We note that our axisymmetric models can also include the presence of an external gravitational field, such as that produced by a massive disk or by a central mass concentration (e.g., a supermassive black hole).

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arXiv2003

The role of Compton heating in cluster cooling flows

Recent observations by Chandra and XMM-Newton demonstrate that the central gas in "cooling flow" galaxy clusters has a mass cooling rate that decreases rapidly with decreasing temperature. This contrasts the predictions of a steady state cooling flow model. On the basis of these observational results, the gas can be in a steady state only if a steady temperature dependent heating mechanism is present; alternatively the gas could be in an unsteady state, i.e., heated intermittently. Intermittent heating can be produced by accretion on the supermassive black hole residing in the central cluster galaxy, via Compton heating. This mechanism can be effective provided that the radiation temperature of the emitted spectrum is higher than the gas temperature. Here we explore whether this heating mechanism can be at the origin of the enigmatic behavior of the hot gas in the central regions of ``cooling flow'' clusters. Although several characteristics of Compton heating appear attractive in this respect, we find that the fraction of absorbed heating for realistic gas and radiation temperatures falls short by two orders of magnitude of the required heating.

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arXiv2003

Clues on galaxy and cluster formation from their scaling relations

By means of high-resolution N-body simulations in a LambdaCDM cosmology, we verify that scaling relations similar to those observed for nearby galaxy clusters are also defined by their dark matter hosts; the slopes, however, are not the same. We then show that the scaling relations of galaxy clusters can be explained as the result of the cosmological collapse of density fluctuations at the appropriate scales, plus a systematic trend of the M/L ratio with cluster mass. The empirical fact that the exponent of the Faber-Jackson relation of elliptical galaxies is significantly different (higher) than that of clusters, force us to conclude that the galaxy scaling laws might derive from the cosmological collapse of density fluctuations at the epoch when galactic scales became non-linear, plus modifications afterward due to early-time dissipative merging.

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