arXiv subjects

J A Sellwood

Publications and source records attributed to J A Sellwood.

16 recordsLinked to original sources

arXiv2025

A Milky Way mass model for isolated simulations

This paper presents an equilibrium model of a Milky Way-like spiral galaxy that supports open, mostly 2- and 3-arm spiral patterns but does not form a bar. It is suggested as a more realistic alternative model to that employed by the Agora collaboration; their model has a much lower disk mass and therefore forms only multi-arm spiral patterns. This improved model should enable simulations that test star-formation and feedback models in a more realistic isolated galaxy. Three versions of the same model having $2.1\times 10^5$, $2.1\times 10^6$, and $2.1\times 10^7$ particles are available for download.

astro-ph.GA↗

arXiv2025

A comment on "Why is the Galactic disk so cool?", by Hamilton et al

Frankel et al (2020) reported that the rate of diffusion in angular momentum by stars in the disk of the Milky Way was about ten times faster than the rate of heating, which places a stringent requirement on the nature of disk star scattering. In a recent posting, Hamilton et al (2024) integrated orbits of test particles in a Galactic model that included transient spirals, and found that the ratio of the rates heating to radial migration in their calculations was generally larger than reported by Frankel et al. They concluded that the observed slow heating rate poses a significant challenge to dynamical models of the Milky Way and seemed to require revision of our current theories of spiral structure. Here we show that that our earlier models of radial migration we published in 2002, after correction for 3D motion, account naturally for the finding of Frankel et al, but leave little room for additional heating by mechanisms other than transient spirals.

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arXiv2025

The quest for a stable disk

The majority of disk galaxies manifest spirals and/or bars that are believed to result from dynamical instabilities. However, some galaxies have featureless disks, which are therefore inferred to be dynamically stable. Yet despite many years of effort, theorists have been unable to construct realistic models of galaxy disks that possess no instabilities and therefore could remain featureless. This conclusion has been reached through simulations for the most part, some of which have been confirmed by linear stability analyses. Toomre claimed that the Mestel disk, embedded in an equal mass halo, to be a notable counter-example, but his prediction of stability could not be reproduced in simulations due to complicated non-linear effects that caused secular growth of Poisson noise-driven disturbances until strong features emerged. Here we revisit this issue and show that simply eliminating the most nearly circular orbits from Toomre's disk model can inhibit troublesome secular growth. We also present both 2D and 3D simulations of particle disks that remain featureless for over 50 orbit periods. We report that spiral evolution naturally depletes circular orbits and that the radial velocity distribution in the featureless disks of S0 galaxies should have negative kurtosis.

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arXiv2024

Particle selection from an equilibrium DF

When starting an N-body simulation of an isolated galaxy, it is desirable to select particles from a distribution function to ensure that the model is in equilibrium. Random sampling from a DF is widely used, but results in a set of particles that differs by shot noise from that intended. This paper presents a method to reduce sampling noise that has been developed by the author in a many collaborations over a number of years. The technique has been partly described in past papers, though the ideas have not previously been gathered together, nor have its advantages been clearly demonstrated in past work. Of course, sampling errors can also be reduced by a brute force increase in the number of particles, but methods to achieve the same effect with fewer particles have obvious advantages. Here we not only describe the method, but also present three sets of simulations to illustrate the practical advantages of reducing sampling error. The improvements are not dramatic, but are clearly worth having.

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arXiv2023

The stability of some galaxy disks is still perplexing

The problem of how some disk galaxies avoid forming bars remains unsolved. Many galaxy models having reasonable properties continue to manifest vigorous instabilities that rapidly form strong bars and no widely-accepted idea has yet been advanced to account for how some disk galaxies manage to avoid this instability. It is encouraging that not all galaxies formed in recent cosmological simulations possess bars, but the dynamical explanation for this result is unclear. The unstable mode that creates a bar is understood as a standing wave in a cavity that reflects off the disk center and the corotation radius, with amplification at corotation. Here we use simulations to address one further idea that may perhaps inhibit the feedback loop and therefore contribute to stability, which is to make the disk center dynamically hot and/or to taper away mass from the inner disk, which could be masked by a bulge. Unfortunately, we find that neither strategy makes much difference to the global stability of the disk in the models we have tried. While deep density cutouts do indeed prevent feedback through the center, they still reflect incoming waves and thereby provoke a slightly different instability that again leads to a strong bar.

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arXiv2022

Spiral instabilities: Mode saturation and decay

This paper continues a series reporting different aspects of the behaviour of disc galaxy simulations that support spiral instabilities. The focus in this paper is to demonstrate how linear spiral instabilities saturate and decay, and how the properties of the disc affect the limiting amplitude of the spirals. Once again, we employ idealized models that each possess a single instability that we follow until it has run its course. Remarkably, we find a tight correlation between the growth rate of the mode and its limiting amplitude, albeit from only six simulations. We show that non-linear orbit deflections near corotation cause the mode to saturate, and that the more time available in a slowly-growing mode creates the critical deflections at lower amplitude. We also find that scattering at the inner Lindblad resonance is insignificant until after the mode has saturated. Our objective in this series of papers, which we believe we have now achieved, has been to develop a convincing and well-documented account of the physical behaviour of the spiral patterns that have been observed in simulations by others, and by ourselves, for many decades. Understanding the simulations is an important step towards the greater objective, which is to find observational evidence from galaxies that could confront the identified mechanism.

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arXiv2021

Uncertainties in Galaxy Rotation Curves

Assessing the likelihood that the rotation curve of a galaxy matches predictions from galaxy formation simulations requires that the uncertainties in the circular speed as a function of radius derived from the observational data be statistically robust. Few uncertainties presented in the literature meet this requirement. In this paper we present a new standalone tool, makemap, that estimates the fitted velocity at each pixel from Gauss-Hermite fits to a 3D spectral data cube, together with its uncertainty obtained from a modified bootstrap procedure. We apply this new tool to neutral hydrogen spectra for 18 galaxies from the THINGS sample, and present new velocity maps with uncertainties. We propagate the estimated uncertainties in the velocity map into our previously-described model fitting tool DiskFit to derive new rotation curves. The uncertainties we obtain from these fits take into account not only the observational errors, but also uncertainties in the fitted systemic velocity, position of the rotation centre, inclination of the galaxy to the line of sight, and forced non-circular motion. They are therefore much better-defined than values that have previously been available. Our estimated uncertainties on the circular speeds differ from previous estimates by factors ranging up to of five, being smaller in some cases and larger in others. We conclude that kinematic models of well-resolved HI datasets vary widely in their precision and reliability, and therefore potentially in their value for comparisons with predictions from cosmological galaxy formation simulations.

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arXiv2017

Rapid formation of black holes in galaxies: a self-limiting growth mechanism

We present high-quality fluid dynamical simulations of isothermal gas flows in a rotating barred potential. We show that a large quantity of gas is driven right into the nucleus of a model galaxy when the potential lacks a central mass concentration, but the inflow stalls at a nuclear ring in comparison simulations that include a central massive object. The radius of the nuclear gas ring increases linearly with the mass of the central object. We argue that bars drive gas right into the nucleus in the early stages of disk galaxy formation, where a nuclear star cluster and perhaps a massive black hole could be created. The process is self-limiting, however, because inflow stalls at a nuclear ring once the mass of gas and stars in the nucleus exceeds ~1% of the disk mass, which shuts off rapid growth of the black hole. We briefly discuss the relevance of these results to the seeding of massive black holes in galaxies, the merger model for quasar evolution, and the existence of massive black holes in disk galaxies that lack a significant classical bulge.

astro-ph.GA↗

arXiv2010

Quantifying Non-circular Streaming Motions in Disc Galaxies

High-quality velocity maps of galaxies frequently exhibit signatures of non-circular streaming motions. We here apply the software tool, "velfit" recently proposed by Spekkens & Sellwood, to five representative galaxies from the THINGS sample. We describe the strengths and weaknesses of the tool, and show that it is both more powerful and yields results that are more easily interpreted than the commonly used procedure. We demonstrate that it can estimate the magnitudes of forced non-circular motions over a broad range of bar strengths from a strongly barred galaxy, through cases of mild bar-like distortions to placing bounds on the shapes of halos in galaxies having extended rotation curves. We identify mild oval distortions in the inner parts of two dwarf galaxies, NGC 2976 and NGC 7793, and show that the true strength of the non-axisymmetric gas flow in the strongly barred galaxy NGC 2903 is revealed more clearly in our fit to an optical Halpha map than to the neutral hydrogen data. The method can also yield a direct estimate of the ellipticity of a slowly-rotating potential distortion in the flat part of a rotation curve, and we use our results to place tight bounds on the possible ellipticity of the outer halos of NGC 3198 and NGC 2403.

astro-ph.CO↗

arXiv2004

What is the Evidence for Dark Matter?

Newtonian mechanics indicates that galaxies and galaxy clusters are much more massive than we would have guessed from their luminosities, with the discrepancy being generally attributed to dark matter halos. An alternative hypothesis is that accelerations in very weak gravitational fields are larger than predicted by Newton's laws, and there is no need for dark matter. Even though we do not currently have a satisfactory theory associated with this rival hypothesis, we can ask whether any observational tests could rule it out or prefer it over the dark matter hypothesis. Current evidence suggests that neither hypothesis enjoys a decisive advantage over the other. If dark matter turns out to be the correct interpretation however, then theories of galaxy formation face some quite severe fine-tuning problems.

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arXiv2002

Radial Mixing in Galactic Discs

We show that spiral waves in galaxy discs churn the stars and gas in a manner that largely preserves the overall angular momentum distribution and leads to little increase in random motion. Changes in the angular momenta of individual stars are typically as large as ~50% over the lifetime of the disk. The changes are concentrated around the corotation radius for an individual spiral wave, but since transient waves with a wide range of pattern speeds develop in rapid succession, the entire disk is affected. This behaviour has profound consequences for the metallicity gradients with radius in both stars and gas. The ISM is also stirred by the same mechanism. We find observational support for stirring, propose a simple model for the distribution of stars over metallicity and age, and discuss other possible consequences.

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arXiv2000

The Stability of Disks in Cusped Potentials

We confirm that a high rate of shear at the center is able to stabilize an entire stellar disk against bar-forming (m=2) modes, irrespective of the dark halo density. Our simulations of unstable power-law disks also yield the growth rate and pattern speed of the dominant lop-sided (m=1) mode in close agreement with that predicted by linear theory in our cleanest case. The one-armed modes, which dominate models with extensive disks, can be controlled by tapering the outer disk. Two-armed instabilities of hard-centered disks are more difficult to identify because they are easily overwhelmed by particle noise. Nevertheless, we have detected the predicted m=2 modes in simulations with very large numbers of particles. Such bisymmetric instabilities in these disks are provoked only by sharp edges and are therefore easily eliminated. We have constructed a cool disk model with an almost flat rotation curve and a quasi-exponential density profile that is unambiguously stable. The halo in this stable model has a large core radius, with the disk and bulge providing almost all the rotational support in the inner parts.

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arXiv1997

Disk Mass from Large-scale Dynamics

The radial distribution of mass in a disk galaxy is strongly constrained by its rotation curve. The separate contributions from the individual stellar populations and dark matter (DM) are not easily disentangled, however, especially since there is generally no feature to indicate where the component dominating the central attraction switches from luminous to dark matter. Here I summarize three recent thesis projects at Rutgers University which all suggest that DM has a low density in the inner parts of bright galaxies, and that most of the mass therefore resides in the disk. In addition, I present some preliminary work on the Milky Way.

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arXiv1997

Galaxy dynamics by N-body simulation

I compare various popular and unpopular techniques for simulating large collisionless stellar systems. I give a quantitative comparison of the raw cpu times required for five separate codes, including tree codes and basis function expansions, which demonstrate that grid codes are most efficient for large numbers of particles. Since efficiency is only one consideration when choosing a code, I discuss other strengths and weaknesses of the various methods. While some applications may require the maximum possible number of particles, I argue that quiet start techniques can often permit reliable results to be obtained with moderate particle numbers. I suggest a quiet start procedure for spherical stellar systems and show that it leads to a significant reduction in the relaxation rate. The combination of efficient codes and quiet starts allows many galactic dynamical problems to be tackled without the need for supercomputers.

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arXiv1996

Axisymmetric bending oscillations of stellar disks

Self-gravitating stellar disks with random motion support both exponentially growing and, in some cases, purely oscillatory axisymmetric bending modes, unlike their cold disk counterparts. A razor-thin disk with even a very small degree of random motion in the plane is both unstable and possesses a discrete spectrum of neutral modes, irrespective of the sharpness of the edge. Random motion normal to the disk plane has a stabilizing effect but at the same time allows bending waves to couple to the internal vibrations of the particles, which causes the formerly neutral modes to decay through Landau damping. Focusing first on instabilities, I here determine the degree of random motion normal to the plane needed to suppress global, axisymmetric, bending instabilities in a family of self-gravitating disks. As found previously, bending instabilities are suppressed only when the thickness exceeds that expected from a na\"ıve local criterion when the degree of pressure support within the disk plane is comparable to, or exceeds, the support from rotation. A modest disk thickness is adequate for the bending stability of most disk galaxies, except perhaps near their centers. The discretization of the neutral spectrum in a zero-thickness disk is due to the existence of a turning point for bending waves in a warm disk, which is absent when the disk is cold. When the disk is given a finite thickness, the discrete neutral modes generally become strongly damped through wave-particle interactions. It is surprising therefore that I find some simulations of warm, stable disks can support (quasi-)neutral, large-scale, bending modes that decay very slowly, if at all.

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arXiv1995

Bar Formation, Evolution and Destruction

I review the various mechanisms for creating bars in rotating stellar disks, and conclude that the swing-amplified feed-back loop, which produces rapidly tumbling bars, remains the most probable. The bar continues to evolve after its formation in a number of ways; here I discuss what appears to be inevitable thickening normal to the plane, continued spiral activity in the outer disk and also underscore the increasingly important problem presented by angular momentum loss to the halo. Finally, I examine possible means, excluding interaction, by which bars in galaxies could be destroyed.

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