Instabilities in disc galaxies: from noise to grooves to spirals

Using the linearized Boltzmann equation, we investigate how grooves carved in the phase space of a half-mass Mestel disc can trigger the vigorous growth of two-armed spiral eigenmodes. Such grooves result from the collisional dynamics of a disc subject to finite-N shot noise, as swing-amplified noise patterns push stars towards lower-angular momentum orbits at their inner Lindblad radius. Supplementing the linear theory with analytical arguments, we show that the dominant spiral mode is a cavity mode with reflections off the forbidden region around corotation and off the deepest groove. Other subdominant modes are identified as groove modes. We provide evidence that the depletion of near-circular orbits, and not the addition of radial orbits, is the crucial physical ingredient that causes these new eigenmodes. Thus, it is possible for an isolated, linearly stable stellar disc to spontaneously become linearly unstable via the self-induced formation of phase-space grooves through finite-N dynamics. These results may help explain the growth and maintenance of spiral patterns in real disc galaxies.Comment: 11 pages, 7 figures, submitted to MNRA

NICMOS Imaging of Infrared-Luminous Galaxies

We present near-infrared images obtained with the HST NICMOS camera for a sample of 9 luminous (LIGs: L_IR (8-1000 microns) >~ 10^11 L_sun) and 15 ultra-luminous (ULIGS: L_IR >~ 10^12 L_sun) infrared galaxies. The sample includes representative systems classified as warm (f_25/f_60 > 0.2) and cold (f_25/f_60 <~ 0.2) based on the mid-infrared colors and systems with nuclear emission lines classified as HII (i.e. starburst), QSO, Seyfert and LINER. The morphologies of the sample galaxies are diverse and provide further support for the idea that they are created by the collision or interactions of spiral galaxies. Although no new nuclei are seen in the NICMOS images, the NICMOS images do reveal new spiral structures, bridges, and circumnuclear star clusters...Comment: LaTex, 27 pages with 14 gif and 4 jpg figures, AJ, in press, contour figures of the NICMOS images can be viewed at http://nedwww.ipac.caltech.edu/level5/Scoville/frames.htm

Atomic X-ray Spectroscopy of Accreting Black Holes

Current astrophysical research suggests that the most persistently luminous objects in the Universe are powered by the flow of matter through accretion disks onto black holes. Accretion disk systems are observed to emit copious radiation across the electromagnetic spectrum, each energy band providing access to rather distinct regimes of physical conditions and geometric scale. X-ray emission probes the innermost regions of the accretion disk, where relativistic effects prevail. While this has been known for decades, it also has been acknowledged that inferring physical conditions in the relativistic regime from the behavior of the X-ray continuum is problematic and not satisfactorily constraining. With the discovery in the 1990s of iron X-ray lines bearing signatures of relativistic distortion came the hope that such emission would more firmly constrain models of disk accretion near black holes, as well as provide observational criteria by which to test general relativity in the strong field limit. Here we provide an introduction to this phenomenon. While the presentation is intended to be primarily tutorial in nature, we aim also to acquaint the reader with trends in current research. To achieve these ends, we present the basic applications of general relativity that pertain to X-ray spectroscopic observations of black hole accretion disk systems, focusing on the Schwarzschild and Kerr solutions to the Einstein field equations. To this we add treatments of the fundamental concepts associated with the theoretical and modeling aspects of accretion disks, as well as relevant topics from observational and theoretical X-ray spectroscopy.Comment: 63 pages, 21 figures, Einstein Centennial Review Article, Canadian Journal of Physics, in pres

Satellite Communications

This study is motivated by the need to give the reader a broad view of the developments, key concepts, and technologies related to information society evolution, with a focus on the wireless communications and geoinformation technologies and their role in the environment. Giving perspective, it aims at assisting people active in the industry, the public sector, and Earth science fields as well, by providing a base for their continued work and thinking

High-precision polarimetric, broad-band spectroscopic and temporal studies of black hole X-ray binaries

Black holes are among the most unusual objects in the Universe. Powered through accretion, they are strong sources of radiation in a broad energy range, from radio to hard X-rays. Stellar-mass black holes manifest themselves in binary systems, when their companion star – main sequence or giant – starts to lose matter, which is then captured and accreted by the black hole. Accreting black hole binaries are complex systems. We observe emission from multiple components, such as accretion disc, hot accretion flow (corona), jet, and even donor star. A large number of accreting black holes are transient sources – they undergo periods of violent activity, increasing luminosity by several orders of magnitude. Throughout the course of an active phase – an outburst – the spectral energy distribution and relative contribution of each component can change dramatically, resulting in a gradual evolution of the observed spectra of the black hole binary transient. In the first part of the thesis I describe the nature of the black hole binaries, focusing on the emission mechanisms. Using the archival photometric data of GX 339−4 I demonstrate how observed optical and infrared spectral properties of the source can be explained with a three-component jet, hot flow and accretion disc model. In the second part I discuss polarization mechanisms. Polarization is a fundamental property of light and it carries information about the geometrical structure of the source and scattering or polarizing media. I review processes that can produce polarized (or polarize unpolarized) radiation in the accreting black hole binaries and interstellar medium. In the third part I introduce the novel optical polarimeter (DIPol-UF), which was built in Tuorla Observatory as part of an international collaboration. I outline the challenges of remotely operating high-precision polarimeter and describe the control software that I developed specifically for this instrument. Finally, I discuss the properties of intrinsic polarization of low-mass X-ray binaries based on polarimetric data of V404 Cyg and MAXI J1820+070. Both objects showed small and variable intrinsic polarization during an outburst, with polarization angle coinciding with jet position angle. I demonstrate how high-precision polarimetry can augment photometric and timing studies of X-ray binaries, shedding more light onto the nature of optical emission in these objects

Cinématique haute résolution des galaxies de l'échantillon SINGS et observations du H[alpha] profond de la galaxie NGC 7793

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal