arXiv subjects

A. Mangalam

Publications and source records attributed to A. Mangalam.

39 records · Page 3Linked to original sources

arXiv2003

Physics of Supermassive Disks: Formation and Collapse

Supermassive disks are thought to be precursors of supermassive black holes that are believed to power quasars and exist at centers of galaxies. Formation scenarios of such disks are reviewed and it is argued that gas dynamical schemes are favourable compared to stellar dynamical schemes which could however be important feeding mechanisms for the growth of the black hole. A new self-similar model of a collapse of a self-gravitating disk due to radiation induced stresses applicable to two different situations of radiative viscosity and Compton drag is presented. The collapse timescale purely due to radiative viscosity is found to be a fraction of Hubble time, $τ_γ\sim{σ_T c/ (m_p G)} ({L_{edd}/ L}) \simeq 6 \times 10^9$yrs is slow and probably magnetic fields play an important role before general relativistic effects take over. A model of self-gravitating disk collapsing due to Compton drag by the Cosmic Microwave Background is also presented which is found to be effective at redshifts $1400>z\gsim 300$. It is proposed that the small $\lsim 10^5 \Msun$ objects that form by this mechanism by $z \sim 20$ can merge and coalesce by dynamical friction to form the high redshift quasars seen. Supermassive stars which are systems (and could be end products of a supermassive disk phase) en route to the final collapse are also briefly reviewed.

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arXiv2001

Formation of a Proto-Quasar from Accretion Flows in a Halo

We present a detailed model for the formation of massive objects at the centers of galaxies. The effects of supernovae heating and the conditions of gas loss are revisited. The escape time of the gas is compared with the cooling time, which provides an additional condition not previously considered. Its consequences for the allowed mass range of the halo is calculated. It is shown that sufficient gas is retained to form massive dark objects and quasars even for moderately massive halos but a decline is expected at low redshifts. Subsequently, a gaseous disk forms with a radial extent of a ~kpc, spun up by tidal torques and magnetized by supernovae fields with fields strengths of $10-100 μG$. In a model of a self-similar accretion flow in an initially dominant halo, it is shown that for typical halo parameters, about $10^8 M_\odot$ accretes via small magnetic stresses (or alternatively by self-gravity induced instability or by alpha viscosity) in $10^8$ years into a compact region. A model of a self-gravitating evolution of a compact magnetized disk, which is relevant when a significant fraction of the disk mass falls in, is presented, and it has a rapid collapse time scale of a million years. The two disk solutions, one for accretion in an imposed halo potential and the other for self-gravitating disk, obtained here, have general utility and can be adapted to other contexts like protostellar disks as well.

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arXiv1999

Constrained Violent Relaxation to a Spherical Halo

Violent relaxation during the collapse of a galaxy halo is known to be incomplete in realistic cases such as cosmological infall or mergers. We adopt a physical picture of strong but short lived interactions between potential fluctuations and particle orbits, using the broad framework outlined by Tremaine (1987) for incorporating incompleteness of the relaxation. We are guided by results from plasma physics, viz. the quasilinear theory of Landau damping, but allow for significant differences in our case. Crucially, wave particle scattering does not drive the system to an equilibrium distribution function of the exponential type, even in regions of phase space allowed by the constraints. The physical process is mixing without friction in ``action'' space, for which the simplest possible model is a constant phase space density modulated by the constraints. Our distribution function does not use the exponential functions of the energy prevalent in other work, which we regard as inappropriate to collisionless systems. The halo of the self-consistent, parameter-free solutions show an r^(-4) behavior in density at large r, an r^(1/4) surface brightness profile in the region 0.1-8 r_e, and a radially anisotropic velocity dispersion profile outside an isotropic core. The energy distribution seen in simulations N(E) singles out the pericenter cutoff model as the most realistic among the variants we have explored.

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