I compute the estimated distribution function f(q) for the apparent axis ratio q of various types of stellar systems, using a nonparametric kernel method. I then invert f(q) to find the distribution of intrinsic axis ratios, using two different hypotheses: first, that the stellar systems are all oblate, and second, that they are all prolate. The shapes of globular clusters in our own galaxy are consistent, at the 99% confidence level, with both the oblate and prolate hypothesis. The shapes of dwarf galaxies in the Virgo cluster are consistent, at the 99% confidence level, with the prolate hypothesis, but inconsistent with the oblate hypothesis. The shapes of star clusters in the Large Magellanic Cloud, of ordinary elliptical galaxies, of brightest cluster ellipticals, and of galaxy clusters are all inconsistent, at the 99% confidence level, with both the oblate and prolate hypotheses. The globular clusters in our galaxy are older than their half-mass relaxation time, and are most likely rotationally flattened oblate spheroids. The other stellar systems considered are generally younger than their half-mass relaxation time, and thus are triaxial bodies flattened by anisotropy of their velocity dispersion.

The Intrinsic Shapes of Stellar Systems

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.

Three mechanisms for bar thickening

We report the discovery of the minor planet 2003 VB12 (popularly named Sedna), the most distant object ever seen in the solar system. Pre-discovery images from 2001, 2002, and 2003 have allowed us to refine the orbit sufficiently to conclude that 2003 VB12 is on a highly eccentric orbit which permanently resides well beyond the Kuiper belt with a semimajor axis of 480$\pm$40 AU and a perihelion of 76$\pm$4AU. Such an orbit is unexpected in our current understanding of the solar system, but could be the result of scattering by a yet-to-be-discovered planet, perturbation by an anomalously close stellar encounter, or formation of the solar system within a cluster of stars. In all of these cases a significant additional population is likely present, and in the two most likely cases 2003 VB12 is best considered a member of the inner Oort cloud, which then extends to much smaller semimajor axes than previously expected. Continued discovery and orbital characterization of objects in this inner Oort cloud will verify the genesis of this unexpected population.

/pdf/discovery-of-a-candidate-inner-oort-cloud-planetoid-3dp7riyvuk.pdf

Discovery of a candidate inner Oort cloud planetoid

We test whether the population of "extreme" trans-Neptunian objects (eTNOs) detected in the Y4 Dark Energy Survey (DES) data exhibit azimuthal asymmetries which might be evidence of gravitational perturbations from an unseen super-Earth in a distant orbit. By rotating the orbits of the detected eTNOs, we construct a synthetic population which, when subject to the DES selection function, reproduces the detected distribution of eTNOs in the orbital elements $a,e,$ and $i$ as well as absolute magnitude $H$, but has uniform distributions in mean anomaly $M$, longitude of ascending node $\Omega,$ and argument of perihelion $\omega.$ We then compare the detected distributions in each of $\Omega, \omega,$ and $\varpi\equiv\Omega+\omega$ to those expected from the isotropic population, using Kuiper's variant of the Kolmogorov-Smirnov test. The three angles are tested for each of 4 definitions of the eTNO population, choosing among $a>(150,250)$ AU and perihelion $q>(30,37)$ AU. These choices yield 3--7 eTNOs in the DES Y4 sample. Among the twelve total tests, two have the likelihood of drawing the observed angles from the isotropic population at $p 250, q>37$ AU, and the 4 detections at $a>250, q>30$ AU, have $\Omega$ distribution with $p=0.03$ of coming from the isotropic construction, but this is not strong evidence of anisotropy given the 12 different tests. The DES data taken on their own are thus consistent with azimuthal isotropy and do not require a "Planet 9" hypothesis. The limited sky coverage and object count mean, however, that the DES data by no means falsify this hypothesis.

/pdf/testing-the-isotropy-of-the-dark-energy-survey-s-extreme-f5v9rf4e4t.pdf

Testing the Isotropy of the Dark Energy Survey’s Extreme Trans-Neptunian Objects

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. 

Spirals in Galaxies

Galactic bars are unstable to a vertical buckling instability which heats the disk and in some cases forms a boxy/peanut shaped bulge. We analyze the buckling instability as an application of classical Euler buckling followed by nonlinear gravitational Landau damping in the collisionless system. We find that the buckling instability is dictated by the kinematic properties and geometry of the bar. The analytical result is compared to simulations of isolated galaxies containing the disk and dark matter components. Our results demonstrate that violent buckling does not destroy bars while a less energetic buckling can dissolve the bar. The disks that undergo gentle buckling remain stable to bar formation which may explain the observed bar fraction in the local universe. Our results align with the results from recent surveys.

Violent buckling benefits galactic bars

Disks of low-mass bodies on high-eccentricity orbits in near-Keplerian potentials can be dynamically unstable to buckling out of the plane. In this Letter, we present N-body simulations of the long-term behavior of such a system, finding apsidal clustering of the orbits in the disk plane. The timescale over which the clustering is maintained increases with number of particles, suggesting that lopsided configurations are stable at large N. This discovery may explain the observed apsidal () clustering of extreme trans-Neptunian Objects in the outer solar system.

Apsidal Clustering following the Inclination Instability

We investigate the evolutionary nexus between the morphology and internal kinematics of the central regions of collisional, rotating, multi-mass stellar systems, with special attention to the spatial characterisation of the process of mass segregation. We report results from idealized, purely $N$-body simulations that show multi-mass, rotating, and spherical systems rapidly form an oblate, spheroidal massive core, unlike single-mass rotating or multi-mass non-rotating configurations with otherwise identical initial properties, indicating that this evolution is a result of the interplay between the presence of a mass spectrum and angular momentum. This feature appears to be long-lasting, preserving itself for several relaxation times. The degree of flattening experienced by the systems is directly proportional to the initial degree of internal rotation. In addition, this morphological effect has a clear characterisation in terms of orbital architecture, as it lowers the inclination of the orbits of massive stars. We offer an idealised dynamical interpretation that could explain the mechanism underpinning this effect and we highlight possible useful implications, from kinematic hysteresis to spatial distribution of dark remnants in dense stellar systems.

/pdf/central-dynamics-of-multimass-rotating-star-clusters-555c08yyic.pdf

Central dynamics of multimass rotating star clusters

Disks of low-mass bodies on high-eccentricity orbits in near-Keplerian potentials can be dynamically unstable to buckling out of the plane. In this letter, we present $N$-body simulations of the long-term behavior of such a system, finding apsidal clustering of the orbits in the disk plane. The timescale over which the clustering is maintained increases with number of particles, suggesting that lopsided configurations are stable at large $N$. This discovery may explain the observed apsidal ($\varpi$) clustering of extreme trans-Neptunian Objects in the outer solar system.

An overwhelming majority of galactic spiral arms trail with respect to the rotation of the galaxy, though a small sample of leading spiral arms has been observed. The formation of these leading spirals is not well understood. Here we show, using collisionless $N$-body simulations, that a barred disc galaxy in a retrograde dark matter halo can produce long-lived ($\sim3$ Gyr) leading spiral arms. Due to the strong resonant coupling of the disc to the halo, the bar slows rapidly and spiral perturbations are forced ahead of the bar. We predict that such a system, if observed, will also host a dark matter wake oriented perpendicular to the stellar bar. More generally, we propose that any mechanism that rapidly decelerates the stellar bar will allow leading spiral arms to flourish.

Angela Collier

Papers

Violent buckling benefits galactic bars

Apsidal Clustering following the Inclination Instability

Central dynamics of multimass rotating star clusters

Apsidal Clustering following the Inclination Instability

Bar-driven leading spiral arms in a counter-rotating dark matter halo