Local stability of a gravitating filament: a dispersion relation

Filamentary structures are ubiquitous in astrophysics and are observed at various scales. On a cosmological scale, matter is usually distributed along filaments, and filaments are also typical features of the interstellar medium. Within a cosmic filament, matter can contract and form galaxies, whereas an interstellar gas filament can clump into a series of bead-like structures which can then turn into stars. To investigate the growth of such instabilities, we derive a local dispersion relation for an idealized self-gravitating filament, and study some of its properties. Our idealized picture consists of an infinite self-gravitating and rotating cylinder with pressure and density related by a polytropic equation of state. We assume no specific density distribution, treat matter as a fluid, and use hydrodynamics to derive the linearized equations that govern the local perturbations. We obtain a dispersion relation for axisymmetric perturbations and study its properties in the (k_R, k_z) phase space, where k_R and k_z are respectively the radial and longitudinal wavenumbers. While the boundary between the stable and unstable regimes is symmetrical in k_R and k_z and analogous to the Jeans criterion, the most unstable mode displays an asymmetry that could constrain the shape of the structures that form within the filament. Here the results are applied to a fiducial interstellar filament, but could be extended for more astrophysical systems such as cosmological filaments and tidal tails.Comment: 8 pages, 1 figure, published in A&

Near-IR photometry of disk galaxies: search for nuclear isophotal twist and double bars

We present a near-IR, mainly H band, photometry of 72 nearby disk galaxies. The main goal of the survey was to search for isophotal twist inside their nuclear regions. As the twist can be due in some cases to projection effects, rather than resulting from a dynamical phenomenon, we deproject - under the simplifying assumption of a 2D geometry - all galaxies whose disk position angle and inclination are known, the latter not exceeding 75 degrees. We show the ellipticity, position angle and surface brightness radial profiles, and discuss how a projection of 2D and 3D bars can distort the isophotes, give an illusion of a non-existing double bar or mask a real one. We report 15 new double-barred galaxies and confirm 2 detected previously. We identify 14 additional twists not known before and we also find nuclear triaxial structures in three SA galaxies. The frequency of Seyferts among galaxies with nuclear bars or twists is high. As a secondary product, we publish structural parameters (length and axis ratio) of large-scale bars in order to extend still scarce data on bars in the near-IR.Comment: 11 pages of text (Astron. & Astroph. LaTeX l-aa macro) with 3 postscript figures, 7 additional pages of non-main-body postscript figures containing contour and ellipse fitting plots of 72 galaxies; accepted by Astronomy & Astrophysics Suppl. Se

Does Land Tenure Insecurity Drive Deforestation in the Brazilian Amazon?

The purpose of this paper is to highlight the detrimental impact of land tenure insecurity on deforestation in the Brazilian Amazon. It is related to recent controversies about the detrimental impact of land laws on deforestation, which seem to legitimize land encroachments. The latter is mainly the result of land tenure insecurity which is a key characteristic of this region and results from a long history of interactions between rural social unrest and land reforms or land laws. A simple model is developed where strategic interactions between farmers lead to excessive deforestation. One of the empirical implications of the model is a positive relationship between land tenure insecurity and the extent of deforestation. The latter is tested on data from a panel of Brazilian Amazon municipalities. The negative effect of land tenure insecurity proxied by the number of squatters on deforestation is not rejected when estimations are controlled for the possible endogeneity of squatters. One of the main policy implications is that ex post legalizations of settlements must be accompanied by the enforcement of environmental obligations.deforestation;land tenure insecurity;squatters;Panel Data Analysis;Brazil

Property rights and deforestation in the Brazilian Amazon

This paper focuses on the impact of property rights insecurity on deforestation in the Brazilian Legal Amazon. Deforestation is considered as a risk management strategy: property rights insecurity reduces the present value of forests and fosters forest conversion into agricultural and pasture lands. Moreover, deforestation is the consequence of strategic interactions between landowners and squatters. Landowners clear the forest preventively in order to assert the productive use of land and to reduce the expropriation risk. Squatters invade land plots, clear the forest and may afterwards gain official recognition with formal property titles. A particular attention is paid to the measure of land property rights insecurity in the Brazilian context. It is assumed that property rights insecurity has a multidimensional character taken into account by the number of homicides related to land conflicts and expropriation procedures. Principal component analysis allows synthesising such information. An econometric model of deforestation is estimated on a panel dataset on the 1988-2000 period and the nine states of the Brazilian Legal Amazon. The hypothesis that insecure land property rights contribute to higher rates of deforestation is not rejected when the simultaneity bias between insecure property rights and deforestation is addressed. This result questions the modality of the Brazilian land reform that considers forested areas as unproductive and thus open for expropriation procedures.deforestation;insecure property rights;Brazilian Legal Amazon

Lopsided Spiral Galaxies

The light distribution in the disks of many galaxies is non-axisymmetric or `lopsided' with a spatial extent much larger along one half of a galaxy than the other, as in M101. Recent near-IR observations show that lopsidedness is common. The stellar disks in nearly 30 % of galaxies have significant lopsidedness, greater than 10 % measured as the Fourier amplitude of the m=1 component normalized to the average value. This asymmetry is traced particularly well by the atomic hydrogen gas distribution lying in the outer parts. The lopsidedness also occurs in the nuclear regions, where the nucleus is offset with respect to the outer isophotes. The galaxies in a group environment show higher lopsidedness. The origin of lopsidedness could be due to the disk response to a tidally distorted halo, or via gas accretion. The lopsidedness has a large impact on the dynamics of the galaxy, its evolution, the star formation in it, and on the growth of the central black hole and on the nuclear fueling, merging of binary black holes etc. The disk lopsidedness can be used as a diagnostic to study the halo asymmetry. This is an emerging area in galactic structure and dynamics. In this review, the observations to measure the lopsided distribution, as well as the theoretical progress made so far to understand its origin and properties, and the related open problems will be discussed. (abridged).Comment: 75 pages, 28 figures, a review article, to be published by Physics Report

On Eigenvalue spacings for the 1-D Anderson model with singular site distribution

We study eigenvalue spacings and local eigenvalue statistics for 1D lattice Schrodinger operators with Holder regular potential, obtaining a version of Minami's inequality and Poisson statistics for the local eigenvalue spacings. The main additional new input are regular properties of the Furstenberg measures and the density of states obtained in some of the author's earlier work.Comment: 13 page

First Detection of Molecular Gas in the Shells of CenA

Shells are faint arc-like stellar structures, which have been observed around early type galaxies and are thought to be the result of an interaction. HI gas has recently been detected in shells, a surprising result in view of the theoretical predictions that most of the gas should decouple from stars and fall into the nucleus in such interactions. Here we report the first detection of molecular gas (CO) in shells, found 15kpc away from the center of NGC5128 (CenA), a giant elliptical galaxy that harbors an active nucleus (AGN). The ratio between CO and HI emission in the shells is the same as that found in the central regions, which is unexpected given the metallicity gradient usually observed in galaxies. We propose that the dynamics of the gas can be understood within the standard picture of shell formation if one takes into account that the interstellar medium is clumpy and hence not highly dissipative. The observed metal enrichment could be due to star formation induced by the AGN jet in the shells. Furthermore our observations provide evidence that molecular gas in mergers may be spread out far from the nuclear regions.Comment: Accepted for publication in Astronomy & Astrophysics Letters, (Vol. 356), 4 pages + 1 color figur