arXiv subjects

J. A. Sellwood

Publications and source records attributed to J. A. Sellwood.

At least 19 recordsLinked to original sources

arXiv2025

The odd primordial halo of the Milky Way implied by Gaia. A shallow core, but a steep decline

Primordial dark matter halos are well understood from cold dark matter-only simulations. Since they can contract significantly as baryons settle into their centers, direct comparisons with observed galaxies are complicated. We present an approach to reversing the halo contraction by numerically calculating the halo response to baryonic infall and iterating the initial condition. This allowed us to derive spherically averaged primordial dark matter halos for observed galaxies. We applied this approach to the Milky Way and found that the latest Gaia measurements for the rotation velocities imply an odd primordial Galactic halo: Its concentration and total mass differ by more than 3$σ$ from the predictions, and the density profile presents an inner core that is too shallow and an outer decline that is too steep to be compatible with the cold dark matter paradigm.

astro-ph.GA↗

arXiv2025

After 54 years of bar instability studies: a fresh surprise

The well-known bar instability of rotationally-supported disk galaxy models has been studied extensively since its first discovery over half a century ago. We were therefore very surprised to find cases of disks embedded in rigid halos, which on the basis of widely-cited criteria should be unstable, that appeared to be robustly stable. Here we show that the unstable bar mode in such simulations was being suppressed by changes to the disk caused by other instabilities having higher angular symmetry that were the first to saturate. Although this may seem like a promising solution to the long-standing puzzle presented by the apparent stability of real disk galaxies, we also show that instability is restored in the same models when the rigid halo is replaced by a live population of particles, where the usual stability conditions apply. Our study has been confined to a narrow range of models, and we cannot therefore exclude the possibility that mode interference may be able to prevent bar formation in other models having live halos.

astro-ph.GA↗

arXiv2025

Star Formation and Magnetic Field Amplification due to Galactic Spirals

We use global MHD galaxy simulations to investigate the effects of spiral arms on the evolution of magnetic fields and star formation within a self-regulated interstellar medium (ISM). The same galaxy is simulated twice: once with self-consistent stellar spiral arms and once more with the stellar spirals suppressed via a novel numerical approach, using the Ramses AMR code. Spiral arms continually promote star formation, with 2.6 times higher rates in the spiral galaxy. The higher rate is due to high gas columns gathered along the spiral arms, rather than increasing the star formation efficiency at a given gas column. In both cases, the magnetic field is initially amplified via a small-scale dynamo driven by turbulence due to supernova feedback. Only the spiral galaxy exhibits late-time, consistent field growth due to a large-scale dynamo (e-folding time $\sim600$ Myr). This results in volume-averaged field strengths of $\sim 1$ $μ$G after 1 Gyr of evolution. The mean-fields tend to align themselves with the spiral arms and are coherent up to 10 kpc scales. We demonstrate a novel large-scale dynamo mechanism, whereby spiral-driven radial flows enable the mean-field amplification.

astro-ph.GA↗

arXiv2022

The ultra-diffuse galaxy AGC 114905 needs dark matter

Recent 21 cm line observations of the ultra-diffuse galaxy AGC~114905 indicate a rotating disc largely supported against gravity by orbital motion, as usual. Remarkably, this study has revealed that the form and amplitude of the HI rotation curve is completely accounted for by the observed distribution of baryonic matter, stars and neutral gas, implying that no dark halo is required. It is surprising to find a DM-free galaxy for a number of reasons, one being that a bare Newtonian disk having low velocity dispersion would be expected to be unstable to both axi- and non-axisymmetric perturbations that would change the structure of the disc on a dynamical timescale, as has been known for decades. We present $N$-body simulations of the DM-free model, and one having a low-density DM halo, that confirm this expectation: the disc is chronically unstable to just such instabilities. Since it is unlikely that a galaxy that is observed to have a near-regular velocity pattern would be unstable, our finding calls into question the suggestion that the galaxy may lack, or have little, dark matter. We also show that if the inclination of this near face-on system has been substantially overestimated, the consequent increased amplitude of the rotation curve would accommodate a halo massive enough for the galaxy to be stable.

astro-ph.GA↗

arXiv2022

Spirals in galaxies

Spirals in galaxies have long been thought to be caused by gravitational instability in the stellar component of the disk, but discerning the precise mechanism had proved elusive. Tidal interactions, and perhaps bars, may provoke some spiral responses, but spirals in many galaxies must be self-excited. We survey the relevant observational data and aspects of disk dynamical theory. The origin of the recurring spiral patterns in simulations of isolated disk galaxies has recently become clear and it is likely that the mechanism is the same in real galaxies, although evidence to confirm this supposition is hard to obtain. As transient spiral activity increases random motion, the patterns must fade over time unless the disk also contains a dissipative gas component. Continuing spiral activity alters the structure of the disks in other ways: reducing metallicity gradients and flattening rotation curves are two of the most significant. The overwhelming majority of spirals in galaxies have two- or three-fold rotational symmetry, indicating that the cool, thin disk component is massive. Spirals in simulations of halo-dominated disks instead manifest many arms, and consequently do not capture the expected full spiral-driven evolution. We conclude by identifying areas where further work is needed.

astro-ph.GA↗

arXiv2021

Internally-driven warps in disc galaxies

Any perturbation to a disc galaxy that creates a misalignment between the planes of the inner and outer disc, will excite a slowly evolving bending wave in the outer disc. The torque from the stiff inner disc drives a retrograde, leading-spiral bending wave that grows in amplitude as it propagates outward over a period of several Gyr. This behaviour creates warps that obey the rules established from observations, and operates no matter what the original cause of the misalignment between the inner and outer disc. The part of the disc left behind by the outwardly propagating wave is brought into alignment with the inner disc. Here we confirm that mild warps in simulations of disc galaxies can be excited by shot noise in the halo, as was recently reported. We show that the quadrupole component of the noise creates disc distortions most effectively. Bending waves caused by shot noise in carefully constructed equilibrium simulations of isolated galaxies are far too mild to be observable, but perturbations from halo substructure and galaxy assembly must excite larger amplitude bending waves in real galaxies.

astro-ph.GA↗

arXiv2021

Spiral instabilities: Linear and nonlinear effects

We present a study of the spiral responses in a stable disc galaxy model to co-orbiting perturbing masses that are evenly spaced around rings. The amplitudes of the responses, or wakes, are proportional to the masses of the perturbations, and we find that the response to a low-mass ring disperses when it is removed -- behaviour that is predicted by linear theory. Higher mass rings cause nonlinear changes through scattering at the major resonances, provoking instabilities that were absent before the scattering took place. The separate wake patterns from two rings orbiting at differing frequencies, produce a net response that is an apparently shearing spiral. When the rings have low mass, the evolution of the simulation is both qualitatively and quantitatively reproduced by linear superposition of the two separate responses. We argue that apparently shearing transient spirals in simulations result from the superposition of two or more steadily rotating patterns, each of which is best accounted for as a normal mode of the non-smooth disc.

astro-ph.GA↗

arXiv2021

Spiral instabilities: little interaction with a live halo

In order to address the question of whether spiral disturbances in galaxy discs are gravitationally coupled to the halo, we conduct simulations of idealized models of disc galaxies. We compare growth rates of spiral instabilities in identical mass models in which the halo is held rigid or is represented by particles drawn from an equilibrium distribution function. We examine cases of radial and azimuthal bias in the halo velocity ellipsoid in one of our models, and an isotropic velocity distribution in both. We find at most marginal evidence for an enhanced growth rate of spiral modes caused by a halo supporting response. We also find evidence for very mild dynamical friction between the spiral disturbance and the halo. We offer an explanation to account for the different behaviour between spiral modes and bar modes, since earlier work had found that bar instabilities became significantly more vigorous when a responsive halo was substituted for an equivalent rigid mass distribution. The barely significant differences found here justify the usual simplifying approximation of a rigid halo made in studies of spiral instabilities in galaxies.

astro-ph.GA↗

arXiv2020

Three Mechanisms for Bar Thickening

We present simulations of bar-unstable stellar discs in which the bars thicken into box/peanut shapes. Detailed analysis of the evolution of each model revealed three different mechanisms for thickening the bars. The first mechanism is the well-known buckling instability, the second is the vertical excitation of bar orbits by passage through the 2:1 vertical resonance, and the third is a gradually increasing fraction of bar orbits trapped into this resonance. Since bars in many galaxies may have formed and thickened long ago, we have examined the models for fossil evidence in the velocity distribution of stars in the bar, finding a diagnostic to discriminate between a bar that had buckled from the other two mechanisms.

astro-ph.GA↗

arXiv2020

Spiral Instabilities in N-body Simulations: Emergence from noise II

An earlier paper presented the potentially significant discovery that disturbances in simplified simulations of a stellar disc model that was predicted to be stable in linear theory grew to large amplitude over a long period of time. The ultimate appearance of true instabilities was attributed to non-linear scattering by a succession of collective waves excited by shot noise from the finite number of particles. The paper concluded that no finite number of particles, however large, could mimic a smooth disc. As this surprising finding has been challenged as an artifact of the numerical scheme employed, we here present a new calculation of the same model using a different grid geometry that confirms the original behaviour.

astro-ph.GA↗

arXiv2019

Spiral instabilities: Mechanism for recurrence

We argue that self-excited instabilities are the cause of spiral patterns in simulations of unperturbed stellar discs. In previous papers, we have found that spiral patterns were caused by a few concurrent waves, which we claimed were modes. The superposition of a few steadily rotating waves inevitably causes the appearance of the disc to change continuously, and creates the kind of shearing spiral patterns that have been widely reported. Although we have found that individual modes last for relatively few rotations, spiral activity persists because fresh instabilities appear, which we suspected were excited by the changes to the disc caused by previous disturbances. Here we confirm our suspicion by demonstrating that scattering at either of the Lindblad resonances seeds a new groove-type instability. With this logical gap closed, our understanding of the behaviour in the simulations is almost complete. We believe that our robust mechanism is a major cause of spiral patterns in the old stellar discs of galaxies, including the Milky Way where we have previously reported evidence for resonance scattering in the recently released Gaia data.

astro-ph.GA↗

arXiv2019

The global stability of M33: still a puzzle

The inner disc of the local group galaxy M33 appears to be in settled rotational balance, and near IR images reveal a mild, large-scale, two-arm spiral pattern with no strong bar. We have constructed N-body models that match all the extensive observational data on the kinematics and surface density of stars and gas in the inner part of M33. We find that currently favoured models are unstable to the formation of a strong bar of semi-major axis 2 < a_B < 3 kpc within 1 Gyr, which changes the dynamical properties of the models to become inconsistent with the current, apparently well-settled, state. The formation of a bar is unaffected by how the gas component is modelled, by increasing the mass of the nuclear star cluster, or by making the dark matter halo counter-rotate, but it can be prevented by either reducing the mass-to-light ratio of the stars to Upsilon_V ~ 0.6 or Upsilon_K ~ 0.23 in solar units or by increasing the random motions of the stars. Also a shorter and weaker bar results when the halo is rigid and unresponsive. However, all three near-stable models support multi-arm spirals, and not the observed large-scale bi-symmetric spiral. A two-arm spiral pattern could perhaps be tidally induced, but such a model would require an implausibly low mass disc to avoid a bar and there is no visible culprit. Thus the survival of the current state of this exceptionally well-studied galaxy is not yet understood. We also suspect that many other unbarred galaxies present a similar puzzle.

astro-ph.GA↗

arXiv2019

Discriminating among theories of spiral structure using Gaia DR2

We compare the distribution in position and velocity of nearby stars from the Gaia DR2 radial velocity sample with predictions of current theories for spirals in disc galaxies. Although the rich substructure in velocity space contains the same information, we find it more revealing to reproject the data into action-angle variables, and we describe why resonant scattering would be more readily identifiable in these variables. We compute the predicted changes to the phase space density, in multiple different projections, that would be caused by a simplified isolated spiral pattern, finding widely differing predictions from each theory. We conclude that the phase space structure present in the Gaia data shares many of the qualitative features expected in the transient spiral mode model. We argue that the popular picture of apparently swing-amplified spirals results from the superposition of a few underlying spiral modes.

astro-ph.GA↗

arXiv2018

The RINGS Survey III: Medium-Resolution Halpha Fabry-Perot Kinematic Dataset

The distributions of stars, gas, and dark matter in disk galaxies provide important constraints on galaxy formation models, particularly on small spatial scales (<1kpc). We have designed the RSS Imaging spectroscopy Nearby Galaxy Survey (RINGS) to target a sample of 19 nearby spiral galaxies. For each of these galaxies, we are obtaining and modeling Halpha and H1~21~cm spectroscopic data as well as multi-band photometric data. We intend to use these models to explore the underlying structure and evolution of these galaxies in a cosmological context, as well as whether the predictions of LCDM are consistent with the mass distributions of these galaxies. In this paper, we present spectroscopic imaging data for 14 of the RINGS galaxies observed with the medium spectral resolution Fabby-Perot etalon on the Southern African Large Telescope. From these observations, we derive high spatial resolution line of sight velocity fields of the Halpha line of excited hydrogen, as well as maps and azimuthally averaged profiles of the integrated Halpha and [NII] emission and oxygen abundances. We then model these kinematic maps with axisymmetric models, from which we extract rotation curves and projection geometries for these galaxies. We show that our derived rotation curves agree well with other determinations and the similarity of the projection angles with those derived from our photometric images argues against these galaxies having intrinsically oval disks.

astro-ph.GA↗

arXiv2017

An Overview of the MHONGOOSE Survey: Observing Nearby Galaxies with MeerKAT

MHONGOOSE is a deep survey of the neutral hydrogen distribution in a representative sample of 30 nearby disk and dwarf galaxies with HI masses from 10^6 to ~10^{11} M_sun, and luminosities from M_R ~ -12 to M_R ~ -22. The sample is selected to uniformly cover the available range in log(M_HI). Our extremely deep observations, down to HI column density limits of well below 10^{18} cm^{-2} - or a few hundred times fainter than the typical HI disks in galaxies - will directly detect the effects of cold accretion from the intergalactic medium and the links with the cosmic web. These observations will be the first ever to probe the very low-column density neutral gas in galaxies at these high resolutions. Combination with data at other wavelengths, most of it already available, will enable accurate modelling of the properties and evolution of the mass components in these galaxies and link these with the effects of environment, dark matter distribution, and other fundamental properties such as halo mass and angular momentum. MHONGOOSE can already start addressing some of the SKA-1 science goals and will provide a comprehensive inventory of the processes driving the transformation and evolution of galaxies in the nearby universe at high resolution and over 5 orders of magnitude in column density. It will be a Nearby Galaxies Legacy Survey that will be unsurpassed until the advent of the SKA, and can serve as a highly visible, lasting statement of MeerKAT's capabilities.

astro-ph.GA↗

arXiv2016

Mass Distribution and Bar Formation in Growing Disk Galaxy Models

We report idealized simulations that mimic the growth of galaxy disks embedded in responsive halos and bulges. The disks manifested an almost overwhelming tendency to form strong bars that we found very difficult to prevent. We found that fresh bars formed in growing disks after we had destroyed the original, indicating that bar formation also afflicts continued galaxy evolution, and not just the early stages of disk formation. This behavior raises still more insistently the previously unsolved question of how some galaxies avoid bars. Since our simulations included only collisionless star and halo particles, our findings may apply to gas-poor galaxies only; however the conundrum persists for the substantial unbarred fraction of those galaxies. Our original objective was to study how internal dynamics rearranged the distribution of mass in the disk as a generalization of our earlier study with rigid spherical components. With difficulty, we were able to construct some models that were not strongly influenced by bars, and found that halo compression, and angular momentum exchange with the disk did not alter our earlier conclusion that spiral activity is largely responsible for creating smooth density profiles and rotation curves.

astro-ph.GA↗

arXiv2016

Bar instability in disk-halo systems

We show that the exponential growth rate of a bar in a stellar disk is substantially greater when the disk is embedded in a live halo than in a rigid one having the same mass distribution. We also find that the vigor of the instability in disk-halo systems varies with the shape of the halo velocity ellipsoid. Disks in rigid halos that are massive enough to be stable by the usual criteria, quickly form bars in isotropic halos and much greater halo mass is needed to avoid a strong bar; thus stability criteria derived for disks in rigid halos do not apply when the halo is responsive. The study presented here is of an idealized family of models with near uniform central rotation and that lack an extended halo; we present more realistic models with extended halos in a companion paper. The puzzle presented by the absence of strong bars in some galaxies having gently rising inner rotation curves is compounded by the results presented here.

astro-ph.GA↗

arXiv2015

The RINGS Survey: High-Resolution H-alpha Velocity Fields of Nearby Spiral Galaxies with the SALT Fabry-Perot

We have obtained high-spatial-resolution spectrophotometric data on several nearby spiral galaxies with the Southern African Large Telescope (SALT) Fabry-Pérot interferometer on the Robert Stobie Spectrograph (RSS) as a part of the RSS Imaging spectroscopy Nearby Galaxy Survey (RINGS). We have successfully reduced two tracks of Fabry-Pérot data for the galaxy NGC 2280 to produce a velocity field of the H-alpha line of excited hydrogen. We have modeled these data with the DiskFit modeling software and found these models to be in excellent agreement both with previous measurements in the literature and with our lower-resolution HI velocity field of the same galaxy. Despite this good agreement, small regions exist where the difference between the H-alpha and HI velocities is larger than would be expected from typical dispersions. We investigate these regions of high velocity difference and offer possible explanations for their existence.

astro-ph.GA↗