M. Gambera

Migration of giant planets in planetesimal discs

Planets orbiting a planetesimal circumstellar disc can migrate inward from their initial positions because of dynamical friction between planets and planetesimals. The migration rate depends on the disc mass and on its time evolution. Planets that are embedded in long-lived planetesimal discs, having total mass of 10 −4 0.01M⊙, can migrate inward a large distance and can survive only if the inner disc is truncated or because of tidal interaction with the star. In this case the semi-major axis, a, of the planetary orbit is less than 0.1AU. Orbits with larger a are obtained for smaller value of the disc mass or for a rapid evolution (depletion) of the disc. This model may explain several of the orbital features of the giant planets that were discovered in last years orbiting nearby stars as well as the metallicity enhancement found in several stars associated with short-period planets.

A Three-dimensional Wavelet Analysis of Substructure in the Coma Cluster: Statistics and Morphology

Evidence of clustering within the Coma Cluster is found by means of a multiscale analysis of the combined angular-redshift distribution. We have compiled a catalog of 798 galaxy redshifts from published surveys of the region of the Coma Cluster. We examine the presence of substructure and of voids at different scales ranging from ~1 to ~16 h-1 Mpc, using subsamples of the catalog ranging from cz = 3000 km s-1 to cz = 28,000 km s-1. Our substructure detection method is based on the wavelet transform and on segmentation analysis. The wavelet transform allows us to find structures at different scales, and the segmentation method allows a quantitative statistical and morphological analysis of the sample. From the whole catalog, we select a subset of 320 galaxies, with redshifts between cz = 5858 km s-1 and cz = 8168 km s-1, which we identify as belonging to the central region of Coma and upon which we have performed a deeper analysis, on scales ranging from 180 h-1 kpc to 1.44 h-1 Mpc. Our results are expressed in terms of the number of structures or voids and their sphericity for different values of the threshold detection and at all the scales investigated. According to our analysis, there is strong evidence of multiple hierarchical substructure, on scales ranging from a few hundreds of kiloparsecs to about 4 h-1 Mpc. The morphology of these substructures is rather spherical. On a scale of 720 h-1 kpc we find two main subclusters that have been found before, but our wavelet analysis shows even more substructures, whose redshift position is approximately marked by the following bright galaxies: NGC 4934 and 4840, NGC 4889, NGC 4898 and 4864, NGC 4874 and 4839, NGC 4927, and NGC 4875.

Substructure recovery by three‐dimensional discrete wavelet transforms

We present and discuss a method to identify substructures in combined angular-redshift samples of galaxies within Clusters. The method relies on the use of Discrete Wavelet Transform (hereafter DWT) and has already been applied to the analysis of the Coma cluster (Gambera et al. 1997). The main new ingredient of our method with respect to previous studies lies in the fact that we make use of a 3D data set rather than a 2D. We test the method on mock cluster catalogs with spatially localized substructures and on a N-body simulation. Our main conclusion is that our method is able to identify the existing substructures provided that: a) the subclumps are detached in part or all of the phase space, b) one has a statistically significant number of redshifts, increasing as the distance decreases due to redshift distortions; c) one knows {\it a priori} the scale on which substructures are to be expected. We have found that to allow an accurate recovery we must have both a significant number of galaxies (

DYNAMICAL EVOLUTION OF CLUSTERS OF GALAXIES: THE EFFECT OF HIGH-VELOCITY SUBSTRUCTURE CLUMPS

\approx 200

The Effects of Shear and Rotation Anisotropy Upon the Process of Gravitational Instability

for clusters at z

Tidal Torques Dynamical Friction and the Structure of Clusters of Galaxies

\geq 0.4

The collapse of a spherical density perturbation in the presence of dynamical friction

or about 800 at z

Non radial motions and the shapes and the abundance of clusters of galaxies

\leq

Substructure effects on the collapse of density perturbations

0.4) and a limiting magnitude for completeness

PEAK MASS IN LARGE-SCALE STRUCTURE AND DYNAMICAL FRICTION

m_B=16