arXiv subjects

R. E. de Souza

Publications and source records attributed to R. E. de Souza.

12 recordsLinked to original sources

arXiv2016

Chandrasekhar's Dynamical Friction and non-extensive statistics

The motion of a point like object of mass $M$ passing through the background potential of massive collisionless particles ($m << M$) suffers a steady deceleration named dynamical friction. In his classical work, Chandrasekhar assumed a Maxwellian velocity distribution in the halo and neglected the self gravity of the wake induced by the gravitational focusing of the mass $M$. In this paper, by relaxing the validity of the Maxwellian distribution due to the presence of long range forces, we derive an analytical formula for the dynamical friction in the context of the $q$-nonextensive kinetic theory. In the extensive limiting case ($q = 1$), the classical Gaussian Chandrasekhar result is recovered. As an application, the dynamical friction timescale for Globular Clusters spiraling to the galactic center is explicitly obtained. Our results suggest that the problem concerning the large timescale as derived by numerical $N$-body simulations or semi-analytical models can be understood as a departure from the standard extensive Maxwellian regime as measured by the Tsallis nonextensive $q$-parameter.

astro-ph.GA↗

arXiv2013

Pushing the limits of the Gaia space mission by analyzing galaxy morphology

The ESA Gaia mission, to be launched during 2013, will observe billions of objects, among which many galaxies, during its scanning of the sky. This will provide a large space-based dataset with unprecedented spatial resolution. Because of its natural Galactic and Astrometric priority, Gaia's observational strategy was optimized for point sources. Nonetheless, it is expected that 10^6 sources will be extragalactic, and a large portion of them will be angularly small galaxies. Although the mission was designed for point sources, a dedicated analysis of the raw data will allow the recovery of morphology of those objects at a 0.2" level. This may constitute a unique all-sky survey of such galaxies. We describe the conceptual design of the method we created for performing the morphological analysis of these objects as well as first results obtained from data simulations of low-resolution, highly binned, satellite data. Based on the obtained results we conclude that it is possible to push the limits of the Gaia space mission by analyzing galaxy morphology. (Abridged)

astro-ph.IM↗

arXiv2012

Nonextensive Effects on Chandrasekhar's Dynamical Friction

The motion of a point like object of mass M passing through the background potential of massive collisionless particles (m << M) suffers a steady deceleration named dynamic friction. In his classical work, Chandrasekhar assumed a Maxwellian velocity distribution in the halo and neglected the self gravity of the wake induced by the gravitational focusing of the mass M. In this paper, by relaxing the validity of the Maxwellian distribution due to the presence of long range forces, we derive an analytical formula for the dynamic friction in the context of the q-nonextensive kinetic theory. In the extensive limiting case (q = 1), the classical Gaussian Chandrasekhar result is recovered. As an application, the dynamic friction timescale for Globular Clusters spiraling to the galactic center is explicitly obtained. Our results suggest that the problem concerning the large timescale as derived by numerical N-body simulations or semi-analytical models can be understood as a departure from the standard extensive Maxwellian regime as measured by the Tsallis nonextensive q-parameter.

astro-ph.SR↗

arXiv2009

Comparing photometric results of real and N-body bars

We compare the results of the photometrical analysis of barred galaxies with those of a similar analysis from N-body simulations. The photometry is for a sample of nine barred galaxies observed in the J and Ks bands with the CANICA near infrared (NIR) camera at the 2.1-m telescope of the Observatorio Astrofisico Guillermo Haro (OAGH) in Cananea, Sonora, Mexico. The comparison includes radial ellipticity profiles and surface brightness (density for the N-body galaxies) profiles along the bar major and minor axes. We find very good agreement, arguing that the exchange of angular momentum within the galaxy plays a determinant role in the evolution of barred galaxies.

astro-ph.CO↗

arXiv2009

Lowered Nonextensive Stellar Distribution

The structure of globular clusters and elliptical galaxies are described in an unified way through a new class of lowered models inspired on the nonextensive kinetic theory. These power law models are specified by a single parameter q which quantifies to what extent they depart from the class of lowered stellar distributions discussed by Michie and King. For q equal to unity, the Michie-King profiles are recovered. However, for q smaller than unity there is a gradual modification in the shape of the density profiles which depends on the degree of tidal damage imposed on the model, thereby also providing a good fit for globular clusters. It is also shown that a subclass of these models, those with a deeper potential and $q$ slightly less than unity, present a distribution resembling the de Vaucoulers $r^{1/4}$ profile which yields a good description of the structure of elliptical galaxies. This subset of models follows this trend, with a slight departure over nearly 10 orders of magnitudes.

astro-ph.GA↗

arXiv2007

Kinematic segregation of nearby disk stars from the Hipparcos database

To better understand our Galaxy, we investigate the pertinency of describing the sys tem of nearby disk stars in terms of a two-components Schwarzschild velocity distributio n.Using the proper motion and parallax information of Hipparcos database, we determine t he parameters characterizing the local stellar velocity field of a sample of 22000 disk stars. The sample we use is essentially the same as the one described by the criteria ad opted to study the LSR and the stream motion of the nearby stellar population

astro-ph↗

arXiv2004

Power Law Stellar Distributions

The density profiles and other quantities of physical interest for spherically symmetric systems are computed by assuming that a collisionless stellar gas may relax to the non-Gaussian power law distribution suggested by the nonextensive kinetic theory. There are two different classes of solutions. The first class behaves like a subset of the polytropic Lane-Emden spheres, whereas the second one corresponds to a transition between two different polytropic indices. Unlike the isothermal Maxwellian sphere, the total mass and sizes of both classes are finite for a large range of the nonextensive q-parameter.

astro-ph↗

arXiv2004

BUDDA: A New Two-Dimensional Bulge/Disk Decomposition Code for Detailed Structural Analysis of Galaxies

We present BUDDA (BUlge/Disk Decomposition Analysis), a new code devoted to perform a two-dimensional bulge/disk decomposition directly from the images of galaxies. The bulge component is fitted with a generalized Sersic profile whereas disks have an exponential profile. No other components are included. Bars and other sub-structures, like lenses, rings, inner bars and inner disks, are studied with the residual images obtained through the subtraction of bulges and disks from the original images. This means that a detailed structural analysis of galaxies may be performed with a small number of parameters, and sub-structures may be directly studied with no a priori assumptions. To illustrate its usefulness, we present the results obtained with a sample of 51 mostly early-type galaxies (but covering the whole Hubble sequence). These results show some of the applications in which the code may be used: the determination of parameters for Fundamental Plane and structural studies, quantitative morphological classification of galaxies and identification and study of hidden sub-structures. We have determined the structural parameters of the galaxies in our sample and found many examples of hidden inner disks in ellipticals, secondary bars, nuclear rings and dust lanes in lenticulars and spirals, and also wrong morphological classification cases.

astro-ph↗

arXiv2003

NGC 4608 and NGC 5701: Barred Galaxies without Disks?

We have performed for the first time in the literature a 2D structural analysis on the barred lenticular and face--on galaxies NGC 4608 and NGC 5701. The results indicate that these galaxies either have never had large disks or their disks were almost completely destroyed by their strong bars, due to secular evolution processes. We discuss these surprising conclusions checking for signs of secular evolution, considering bar forming instabilities, and suggesting, based on N-body simulations, a new mechanism to form bars in spheroids, which includes non-spherical halos. Quantitative predictions from our new mechanism are compared with those from other recent models for bar formation and evolution.

astro-ph↗

arXiv2001

The Distance to SN 1999em from the Expanding Photosphere Method

We present optical and IR spectroscopy of the first two months of evolution of the Type II SN 1999em. We combine these data with high-quality optical/IR photometry beginning only three days after shock breakout, in order to study the performance of the ``Expanding Photosphere Method'' (EPM) in the determination of distances. With this purpose we develop a technique to measure accurate photospheric velocities by cross-correlating observed and model spectra. The application of this technique to SN 1999em shows that we can reach an average uncertainty of 11% in velocity from an individual spectrum. Our analysis shows that EPM is quite robust to the effects of dust. In particular, the distances derived from the VI filters change by only 7% when the adopted visual extinction in the host galaxy is varied by 0.45 mag. The superb time sampling of the BVIZJHK light-curves of SN 1999em permits us to study the internal consistency of EPM and test the dilution factors computed from atmosphere models for Type II plateau supernovae. We find that, in the first week since explosion, the EPM distances are up to 50% lower than the average, possibly due the presence of circumstellar material. Over the following 65 days, on the other hand, our tests lend strong credence to the atmosphere models, and confirm previous claims that EPM can produce consistent distances without having to craft specific models to each supernova. This is particularly true for the VI filters which yield distances with an internal consistency of 4%. From the whole set of BVIZJHK photometry, we obtain an average distance of 7.5+/-0.5 Mpc, where the quoted uncertainty (7%) is a conservative estimate of the internal precision of the method obtained from the analysis of the first 70 days of the supernova evolution.

astro-ph↗

arXiv1998

Shape of the Galactic Orbits in Clusters

A kinematical analysis applied to a sample of galaxy clusters indicates that the differences between the velocity distribution of elliptical and spiral galaxies are associated with the shape of their orbit families. The orbital anisotropies present on each morphological population could be measured with the use of a parameter which is the ratio of the radial and tangential velocity dispersions, and can be recovered through the observed line-of-sight velocity distribution. When a Gaussian velocity distribution is assumed, having different dispersions along the radial and tangential directions, we conclude that the orbits of elliptical galaxies in clusters are close to radial, while spirals have more circular shaped or isotropic orbits. Lenticulars galaxies shares an intermediate orbital parameter, between spirals and ellipticals.

astro-ph↗

arXiv1994

Southern Binary Galaxies. I. a Sample of Isolated Pairs

A catalogue of binary galaxies with 621 pairs has been determined by applying a surface density enhancement procedure to {\it The Surface Photometry Catalogue of the ESO-Uppsala Galaxies}. The method does not require any redshift information. An additional restriction, based on objective criteria that take into account the completeness of the source catalogue, led to a sample of 189 isolated pairs which are listed. We have obtained the optical luminosity function of binary galaxies in the catalogue, from which we estimate that the luminosity density of galaxies in binaries is $\sim$4% of that found for field galaxies. The general properties of our sample are similar to those from CPG and CMG.

astro-ph↗