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Radio Galaxies and Quasars
Published in Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr, Fundamentals of Radio Astronomy, 2019
Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr
Active galactic nuclei (AGN)!modelActive galactic nuclei (AGN)!supermassive black holeBlack hole!supermassive The model of an active galactic nucleus involves the infall of gas towards a supermassive black hole, generally with M≈106to109M⊙. The heating of gas as it spirals in toward a black hole is an efficient mechanism for converting gravitational potential energy into radiation. Numerous studies of the kinematics of the central regions of galaxies provide strong evidence that supermassive black holes commonly exist in galactic centers. As the interstellar gas falls inward, its angular momentum causes it to form a rotating disk of gas around the black hole; this is termed an accretion disk. Accretion diskActive galactic nuclei (AGN)!accretion disk Because of conservation of energy, as the gas falls inward its gravitational potential energy is converted to kinetic energy which is converted by collisions of the gas in the disk to thermal energy. The accretion disk, therefore, is heated to high temperatures with higher temperatures at smaller radii, closer to the black hole. By the time the gas gets close to the black hole it is sufficiently hot to emit X-rays.
Experiments and long-term high-performance computations on amplitude modulations of strato-rotational flows
Published in Geophysical & Astrophysical Fluid Dynamics, 2021
G. Meletti, S. Abide, S. Viazzo, A. Krebs, U. Harlander
The gas-dust region of the accretion disks can be approximated as a simple differentially rotating shear flow – known as Taylor–Couette (TC) flows – with near-Keplerian velocity profiles (Dubrulle et al.2004). A classic TC system consists of two concentric cylinders that rotate with angular velocities and , and has a mean azimuthal velocity profile given by where is the rotation ratio between inner and outer cylinders, and is the aspect ratio between inner and outer cylinder radius (r). Equation (1) is the analytical solution of the Navier–Stokes equations in cylindrical coordinates (φ, r, z) for incompressible Newtonian fluids in infinite long cylinders. When the first term of right-hand side of (1) is zero, the velocity is a potential field, therefore curl free. This defines the Rayleigh limit, , that separates stable from unstable flows.