J. H. Hunter

High-resolution observations, kinematics, and dynamics of the barred spiral NGC 1073

High spatial resolution neutral hydrogen observations of the barred spiral galaxy NGC 1073 are used to map the kinematics and gas density with a velocity resolution of about 12 km/sec. The kinematics indicate that the galaxy is seen nearly face-on, i.e., at an inclination of about 18 deg. The rotation curve and mass distribution models indicate that the features of a truncation signature may be present in NGC 1073. Other notable features in the H I are the lack of spiral features, the presence of a gas bar, and the very good circular symmetry. Dynamical models that reproduce the H I observations are constrained by the large-scale structure and kinematics of gas, and by the near-infrared morphology of the bar. Different aspects of observations, data reduction and analysis, and dynamical modeling of NGC 1073 are discussed. The dynamical components considered are (1) an asymmetric n = 0 Toomre disk, (2) a triaxial stellar bar, and (3) an oval distortion of the axisymmetric disk. 28 refs.

Comparison of stellar and gasdynamics of a barred galaxy

The stellar and gas dynamics of several models of barred galaxies were studied, and results for some representative cases are reported for galaxies in which the stars and gas respond to the same potentials. Inside corotation there are two main families of periodic orbits, designated x1 and 4/1. Close to the center, the x1 orbits are like elongated ellipses. As the 4/1 resonance is approached, these orbits become like lozenges, with apices along the bar and perpendicular to it. The family 4/1 consists of orbits like parallelograms which produce the boxy component of the bar. The orbits in spirals outside corotation enhance the spiral between the outer -4/1 resonance and the outer Lindblad resonance. Between corotation and the -4/1 resonance in strong spirals, the orbits are mostly stochastic and fill almost circular rings. A spiral field must be added to gasdynamical models to obtain gaseous arms extending from the end of a bar. 38 refs.

On the Effects of Including Counterrotating Angular Momentum in Simulations of Galactic Disk Systems

We describe the evolution of two-dimensional disk systems constructed with significant portions of their angular momentum counterrotating. We confirm that counterrotating angular momentum stabilizes an otherwise bar unstable disk. Further, we show that is possible to realize novel bar/disk and bar/bar configurations by varying our initial conditions. We find that it is possible to construct models that evolve to the following configurations: two counterrotating bars, a cospatial bar and bulgelike component, and a slow counterrotating bar. We examine the possible implications of such systems and whether they might exist in nature.

KELVIN-HELMHOLTZ AND THERMAL-DYNAMIC INSTABILITIES WITH SELF-GRAVITY: A NEW GRAVITATIONAL INTERFACE INSTABILITY

In the vortex sheet limit, we generalize our previous work on compressible, anisentropic Kelvin-Helmholtz, and related instabilities by including self-gravity in the calculations. In addition to significantly modifying the Kelvin-Helmholtz modes, if the background media are of unequal density, self-gravity gives rise to a new instability that persists in the static limit. If the media have significant density contrast (ρ1/ρ2=½, say), the growth rate of this new gravitational interface instability is of the order of the free-fall time in the denser medium, and, unlike a Jeans instability, it depends only weakly on the perturbation wavelength. Such instabilities may initiate star formation near the boundaries of molecular clouds in the ISM on timescales of ~ 106-107 yr.

On the mass of M31

Recent work by several groups has established the properties of the dwarf satellites to M31. We reexamine the reported kinematics of this group employing a fresh technique we have developed previously. By calculating the distribution of a χ statistic (which we define in the paper) for the M31 system, we conclude that the total mass (disc plus halo) of the primary is unlikely to be as great as that of our own Milky Way. In fact the χ distribution for M31 indicates that, like NGC 3992, it does not have a massive halo. In contrast, the analysis of the satellites of NGC 1961 and NGC 5084 provides strong evidence for massive haloes surrounding both spiral galaxies.

Self‐gravity driven instabilities of interfaces in the interstellar medium

In order to understand star formation it is important to understand the dynamics of atomic and molecular clouds in the interstellar medium (ISM). Nonlinear hydrodynamic flows are a key component to the ISM. One route by which nonlinear flows arise is the onset and evolution of interfacial instabilities. Interfacial instabilities act to modify the interface between gas components at different densities and temperatures. Such an interface may be subject to a host of instabilities, including the Rayleigh-Taylor, Kelvin-Helmholtz, and Richtmyer-Meshkov instabilities. Recently, a new density interface instability was identified. This self-gravity interfacial instability (SGI) causes any displacement of the interface to gr ow on roughly a free-fall time scale, even when the perturbation wavelength is much less than the Jeans length. In previous work, we used numerical simulations to confirm the expectations of linear theory and examine the nonlinear evolution of the SGI. We now continue our study by generalizing our initial conditions to allow the acceleration due to self-gravity to be non-zero across the interface. We also consider the behaviour of the SGI for perturbation wavelengths near the Jeans wavelength. We conclude that the action of self-gravity across a density interface may play a significant role in the ISM either by fueling the growth of new instabilities or modifying the evolution of existing instabilities.

Mass Ditribution in Spiral Galaxies II

The VLA telescope was used to observe the atomic hydrogen emission from the Magellanic Cloudlike satellite surrounding several large spiral galaxies. The disks of these systems were resolved and their rotation masses were determined. The satellite kinematics were analyzed to determine an orbital mass in a volume of an average about 3.5 times that of the disk. Simulations were carried out on models on two kinds of models: those in which the satellite orbits were constrained to remain within the halo and those for which the constraint was not imposed. The most successful models have halo radii three to four times the disk radii of their primaries and their satellites have orbital eccentricities less than 0.3. The ratio of halo mass to disk mass is less than five with an average mass of about three disk masses. The observation that most of the satellites are undamaged militates against model galaxies with large halos and satellites with moderately eccentric orbits. 15 refs.

Small Satellite Probes of Spiral Galaxies

Small, bound satellites of larger spiral galaxies are employed in the determination of the statistical presence of mass beyond the rotation disk of the largest (primary) galaxy. Optical and neutral hydrogen data are combined to develop mass estimates using the small bound groups. The resulting distribution of the internal/external mass ratios provides limits on halo mass models, and a glimpse at this mass estimation technique.

Interface Instabilities in the Interstellar Medium

In the present communication, we reexamine two limiting cases of star-forming mechanisms involving self-gravity, thermodynamics, and velocity fields, that we believe must be ubiquitous in the ISM -- the generally oblique collision of supersonic gas streams or turbulent eddies. The general case of oblique collisions has not yet been examined. However, two limiting cases have been studied in detail: (1) The head-on collision of two identical gas streams that form dense, cool accretion shocks that become unstable and may form Jeans mass clouds, which subsequently undergo collapse. (2) Linearly unstable tangential velocity discontinuities, which result in Kelvin-Helmholtz (K-H) instabilities and related phenomena. The compressible K-H instabilities exhibit rich and unexpected behaviors. Moreover a new thermal-dynamic (T-D) mode was discovered that arises from the coupling of the perturbed thermal behavior and the unperturbed flow. The T-D mode has the curious characteristic that it may be strongly unstable to interface modes when the global modes in either medium are absolutely thermally stable. In the present communication additional models of case 1 are described and discussed, and self-gravity is added in the linear theory of tangential discontinuities, case 2. We prove that self-gravity fundamentally changes the behavior of interfacial modes -- density discontinuities (or steps) are inherently unstable on roughly the free-fall timescale of the denser medium to perturbations of all wavelengths.

Numerical Studies of Disksa

In the present communication, two distinct problems will be considered, one in more detail than the other. The bulk of the paper will be devoted to summarizing an efficient method, developed by two of the authors (J. H. H. and E. M.), for constructing self-consistant models of flat, truncated disks with specified rotation curves. In the final section of the paper, two of the authors (J. H. H. and M. N. E.) outline a theory, developed in collaboration with G. Contopoulos, that may account for large-scale vortices in the gas that often are found in the neighborhood of the L4,5 points of the bar in models of barred galaxies.