Weak Homology of Bright Elliptical Galaxies

Studies of the Fundamental Plane of early-type galaxies, from small to intermediate redshifts, are often carried out under the guiding principle that the Fundamental Plane reflects the existence of an underlying mass-luminosity relation for such galaxies, in a scenario where elliptical galaxies are homologous systems in dynamical equilibrium. Here I will re-examine the issue of whether empirical evidence supports the view that significant systematic deviations from strict homology occur in the structure and dynamics of bright elliptical galaxies. In addition, I will discuss possible mechanisms of dynamical evolution for these systems, in the light of some classical thermodynamical arguments and of recent N-body simulations for stellar systems under the influence of weak collisionality.Comment: 13 pages, 7 figures, to appear in "Galaxies and Chaos", Contopoulos, G. and Voglis, N. (eds), Lecture Notes in Physics, Springer-Verlag, Heidelber

Improving the accuracy of mass reconstructions from weak lensing: from the shear map to the mass distribution

In this paper we provide a statistical analysis of the parameter-free method often used in weak lensing mass reconstructions. It is found that a proper assessment of the errors involved in such a non-local analysis requires the study of the relevant two-point correlation functions. After calculating the two-point correlation function for the reduced shear, we determine the expected error on the inferred mass distribution and on other related quantities, such as the total mass, and derive the error power spectrum. This allows us to optimize the reconstruction method, with respect to the kernel used in the inversion procedure. In particular, we find that curl-free kernels are bound to lead to more accurate mass reconstructions. Our analytical results clarify the arguments and the numerical simulations by Seitz & Schneider (1996).Comment: 11 pages and 2 Postscript figures, uses A&A TeX macros. Submitted to A&A. Changed conten

Improving the accuracy of mass reconstructions from weak lensing: local shear measurements

Different options can be used in order to measure the shear from observations in the context of weak lensing. Here we introduce new methods where the isotropy assumption for the distribution of the source galaxies is implemented directly on the observed quadrupole moments. A quantitative analysis of the error associated with the finite number of source galaxies and with their ellipticity distribution is provided, applicable even when the shear is not weak. Monte Carlo simulations based on a realistic sample of source galaxies show that our procedure generally leads to errors ~30% smaller than those associated with the standard method of Kaiser and Squires (1993).Comment: 9 pages and 3 Postscript figures, uses A&A TeX macros. To be published in A&

COOLING FLOW MODELS OF THE X--RAY EMISSION AND TEMPERATURE PROFILES FOR A SAMPLE OF ELLIPTICAL GALAXIES

A simple spherically-symmetric, steady-state, cooling-flow description with gas loss (following Sarazin \& Ashe 1989), within galaxy models constrained by radially extended stellar dynamical data, is shown to provide generally reasonable fits to the existing data on X-ray emission profiles and temperatures for a set of bright elliptical galaxies in Virgo and Fornax. Three free parameters are needed to specify the model: the external mass flux, the external pressure, and a dimensionless factor, which regulates the mass deposition rate along the flow. Three different assumptions on the supernova rate have been considered. A moderate value for the supernova rate in elliptical galaxies is found to be preferred. Confining pressures of p_{ext}\sim4\div15\times10^3\kelvin cm$^{-3}$ and significant accretion rates of external material, up to 4\msolar/\yr, are suggested by our models. A possible correlation between $L_X/L_B$ and the iron abundance in the gas inside ellipticals is pointed out.Comment: 20 pages + 8 figures, uuencoded gzipped tar file containing latex text file and 7 .ps files; only change is that figures have been provided with labels. To appear in Ap

A fast direct method of mass reconstruction for gravitational lenses

Statistical analyses of observed galaxy distortions are often used to reconstruct the mass distribution of an intervening cluster responsible for gravitational lensing. In current projects, distortions of thousands of source galaxies have to be handled efficiently; much larger data bases and more massive investigations are envisaged for new major observational initiatives. In this article we present an efficient mass reconstruction procedure, a direct method that solves a variational principle noted in an earlier paper, which, for rectangular fields, turns out to reduce the relevant execution time by a factor from 100 to 1000 with respect to the fastest methods currently used, so that for grid numbers N = 400 the required CPU time on a good workstation can be kept within the order of 1 second. The acquired speed also opens the way to some long-term projects based on simulated observations (addressing statistical or cosmological questions) that would be, at present, practically not viable for intrinsically slow reconstruction methods.Comment: 6 pages, 2 figures. Uses A&A macros. Accepted for pubblication on A&

Looking at the Fundamental Plane through a gravitational lens

We consider the Fundamental Plane of elliptical galaxies lensed by the gravitational field of a massive deflector (typically, a cluster of galaxies). We show that the Fundamental Plane relation provides a straightforward measurement of the projected mass distribution of the lens with a typical accuracy of ~0.15 in the dimensionless column density kappa. The proposed technique breaks the mass-sheet degeneracy completely and is thus expected to serve as an important complement to other lensing-based analyses. Moreover, its ability to measure directly the mass distribution on the small pencil beams that characterize the size of background galaxies may lead to crucial tests for current scenarios of structure formation.Comment: ApJL, in pres

Weak lensing and cosmology

Recently, it has been shown that it is possible to reconstruct the projected mass distribution of a cluster from weak lensing provided that both the geometry of the universe and the probability distribution of galaxy redshifts are known; actually, when additional photometric data are taken to be available, the galaxy redshift distribution could be determined jointly with the cluster mass from the weak lensing analysis. In this paper we develop, in the spirit of a ``thought experiment,'' a method to constrain the geometry of the universe from weak lensing, provided that the redshifts of the source galaxies are measured. The quantitative limits and merits of the method are discussed analytically and with a set of simulations, in relation to point estimation, interval estimation, and test of hypotheses for homogeneous Friedmann-Lemaitre models. The constraints turn out to be significant when a few thousand source galaxies are used.Comment: 17 pages, 8 figures. Uses A&A LaTeX style. Accepted for pubblication by A&A. Several changes made: new model for the lens; Sect. 7 and App. A. adde

Probing the rotation curve of the outer accretion disk in FU Orionis objects with long-wavelength spectroscopy

Studies of the Spectral Energy Distribution of Young Stellar Objects suggest that the outer disk of FU Orionis objects might be self-gravitating. In this paper we propose a method to test directly whether, in these objects, significant deviations from Keplerian rotation occur. In a first approach, we have used a simplified model of the disk vertical structure that allows us to quickly bring out effects related to the disk self-gravity. We find that the often studied optical and near-infrared line profiles are produced too close to the central object to provide significant evidence for non-Keplerian rotation. Based on parameters relevant for the case of FU Ori, we show that high-resolution long-wavelength spectroscopy, of the far-infrared H$_2$ pure rotational lines (sometimes observed in ``passive'' protostellar disks) and sub-mm CO lines, should be well suited to probe the rotation curve in the outer disk, thus measuring to what extent it is affected by the disk self-gravity. The results of the present exploratory paper should be extended soon to a more realistic treatment of the disk vertical structure.Comment: 14 pages, A&A in pres

The evolution of field early-type galaxies to z~0.7

We have measured the Fundamental Plane (FP) parameters for a sample of 30 field early-type galaxies (E/S0) in the redshift range 0.1<z<0.66. We find that: i) the FP is defined and tight out to the highest redshift bin; ii) the intercept \gamma evolves as d\gamma/dz=0.58+0.09-0.13 (for \Omega=0.3, \Omega_{\Lambda}=0.7), or, in terms of average effective mass to light ratio, as d\log(M/L_B)/dz=-0.72+0.11-0.16, i.e. faster than is observed for cluster E/S0 -0.49+-0.05. In addition, we detect [OII] emission >5\AA in 22% of an enlarged sample of 42 massive E/S0 in the range 0.1<z<0.73, in contrast with the quiescent population observed in clusters at similar z. We interpret these findings as evidence that a significant fraction of massive field E/S0 experiences secondary episodes of star-formation at z<1.Comment: ApJ Letters, in pres

Characterising the Gravitational Instability in Cooling Accretion Discs

We perform numerical analyses of the structure induced by gravitational instabilities in cooling gaseous accretion discs. For low enough cooling rates a quasi-steady configuration is reached, with the instability saturating at a finite amplitude in a marginally stable disc. We find that the saturation amplitude scales with the inverse square root of the cooling parameter beta = t_cool / t_dyn, which indicates that the heating rate induced by the instability is proportional to the energy density of the induced density waves. We find that at saturation the energy dissipated per dynamical time by weak shocks due is of the order of 20 per cent of the wave energy. From Fourier analysis of the disc structure we find that while the azimuthal wavenumber is roughly constant with radius, the mean radial wavenumber increases with radius, with the dominant mode corresponding to the locally most unstable wavelength. We demonstrate that the density waves excited in relatively low mass discs are always close to co-rotation, deviating from it by approximately 10 per cent. This can be understood in terms of the flow Doppler-shifted phase Mach number -- the pattern speed self-adjusts so that the flow into spiral arms is always sonic. This has profound effects on the degree to which transport through self-gravity can be modelled as a viscous process. Our results thus provide (a) a detailed description of how the self-regulation mechanism is established for low cooling rates, (b) a clarification of the conditions required for describing the transport induced by self-gravity through an effective viscosity, (c) an estimate of the maximum amplitude of the density perturbation before fragmentation occurs, and (d) a simple recipe to estimate the density perturbation in different thermal regimes.Comment: 16 pages, 22 figures. Accepted for publication in MNRAS 11 November 200