arXiv subjects

John Dubinski

Publications and source records attributed to John Dubinski.

40 records · Page 3Linked to original sources

arXiv1994

The Settling of Warped Disks in Oblate Dark Halos

When a galaxy forms, the disk may initially be tilted with respect to a flattened dark halo. The misalignment between the disk and the halo is a common explanation for galactic disk warps, since in this state, disks have precessing bending modes which resemble real warps. A dubious assumption in this theory is that the gravitational response of the halo is negligible. We therefore calculate the response of an oblate halo to a precessing inclined exponential disk using a variety of techniques. We construct models with a rigid exponential disk precessing in a particle halo, a particle disk precessing inside a static bulge/halo potential, and a self-consistent model with a particle disk, bulge and halo. When the disk:halo mass ratio is small ($\sim$10\%) within 5 exponential scale radii, the disk settles to the equatorial plane of the halo within 5 orbital times. When the disk and halo mass are comparable, the halo rapidly aligns with the disk within a few orbital times, while the disk inclination drops. The rapid response of the halo to a inclined disk suggests that the warps seen in galactic disks are not likely the bending modes of a precessing disk inside of triaxial halos. If a galaxy forms inclined to the principal plane of a dark halo, either the disk will settle to a principal plane or the inner halo will twist to align with the disk. The outer halo will remain misaligned for a much longer time and therefore may still exert a torque. Warped bending modes may still exist if the misalignment of the outer halo persists for a Hubble time.

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arXiv1993

Self-Gravity and Dissipation in Polar Rings

We construct models of highly-inclined (``polar'') rings in an external potential including both self--gravity and dissipation due to a drag force. We adopt the oblate spheroidal scale--free logarithmic potential with axis ratio $q=0.85$ and an initial inclination of 80$^\circ$ for the self--gravitating rings. Stellar (dissipationless) rings suffer from mass loss during their evolution which drives a secular change of the mean inclination toward the poles of the potential. As much as half of the ring mass escapes in the process and forms an inner and an outer shell of precessing orbits. If the remaining mass is more than $\sim$ 0.02 of the enclosed galaxy mass, rings remain bound and and are not destroyed by differential precession. The rings precess at a constant rate for more than a precession period $τ_p$ finding the configuration predicted by Sparke in 1986 which warps at larger radii toward the poles of the potential. We model shear viscosity with a velocity-dependent drag force and find that nuclear inflow dominates over self--gravity if the characteristic viscous inflow time scale $τ_{vi}$ is shorter than $\sim 25 τ_p$. Rings with $τ_{vi}/τ_p \ \sles \ 25$ collapse toward the nucleus of the potential within one precession period independent of the amount of self--gravity. Our results imply that stars and gas in real polar rings exhibit markedly different dynamical evolutions.

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arXiv1993

Lowered Evans Models: Analytic Distribution Functions of Oblate Halo Potentials

We describe an analytic distribution function of a finite, oblate stellar system that is useful for the practical modelling of dark halos. The function is determined by lowering Evans's (1993) distribution function of a flattened, cored isothermal system in analogy to the lowering of the singular, isothermal sphere in the definition of the King (1966) models. We derive analytic expressions of the density, maximal streaming velocity and velocity dispersion profiles as a function of the potential. As for the King models, the potential must be calculated numerically. We also present a recipe for generating N-body realizations of this distribution function and examine the stability in three models with dimensionless spins $λ=$0.0, 0.05 and 0.18 using N-body simulations with 50,000 particles. The $λ=0.18$ model is unstable to the formation of a triaxial bar within $\sim 5$ King radii while the other models appear stable. We conclude that the slowly rotating systems are useful for modelling flattened dark halos.

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arXiv1993

The Effect of Dissipation on the Shapes of Dark halos

The dissipative infall of gas during the formation of a galaxy modifies the density profile and shape of the dark halo. Gas dissipates energy radiatively and sinks to the center of the dark halo forming the luminous part of a galaxy. The resulting central density enhancement can alter the halo's orbital distribution. We simulate dissipative infall inside of an initially triaxial N-body dark halo by slowly growing a potential in the center of the particle distribution. The dark halo transforms from a prolate-triaxial halo ($T \sim 0.8$) to an oblate-triaxial halo ($T \sim 0.5$) while approximately preserving the flattening ($c/a \sim 0.5$). The main implication is that dark halos are rounder and more oblate than previous predictions of purely collisionless simulations with the new constraint that $b/a \gapp 0.7$. At the same time, the distribution of intrinsic flattenings of dark halos ($\langle c/a \rangle = 0.5$, $\langle (c/a)^2 \rangle ^{1/2} = 0.15$) is preserved during the period of baryonic infall. The oval distortions of disk galaxies should therefore be slightly less than original predictions from collisionless dark halos. The predicted distribution of shapes for dark halos from cosmological N-body simulations is in better agreement with distribution of ellipticities of elliptical galaxies if we assume all halos and galaxies become oblate-triaxial in response to baryonic dissipation. The observed distribution of kinematic misalignment angles is also consistent with this shape distribution if

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