Ph.D. Thesis

Numerical Studies of Gas Flow in Spiral Galaxies

by B. Kevin Edgar

Under the supervision of Professors Paul Woodward and Robert Kennicutt

ABSTRACT

A numerical method is described which is specifically designed to treat the dynamics of an infinitesimally this, differentially rotating, gaseous disk. The method is based on the Piecewise Parabolic Method (PPM), a higher-order extension of Godunov's method. Gravitational forces representing a linear spiral density wave in the stellar component of a galaxy are included. The calculation is Eulerian and is performed in a uniformly rotating frame of reference using plane polar coordinates. The equations are formulated in an exact perturbation form to explicitly eliminate all large, opposing terms representing force balance in the unperturbed, axisymmetric state, allowing the accurate computation of small perturbations. The method is ideally suited to the study of the gaseous response to a spiral density wave in a disk galaxy.
 
A series two-dimensional hydrodynamical models is computed to test the gravitational response of a uniform, isothermal, massless gaseous disk to an imposed spiral gravitational perturbation. The parameters describing the mass distribution, rotation properties, and the spiral wave are based on the galaxy NGC 628. The solutions have shocks inside and outside corotation, depleting the region around corotation. The rate at which this region is depleted depends strongly on the strength of the imposed spiral perturbation. Potential perturbations of 10% of greater produce large radial inflows. The time needed for the gas to fall to the inner Linblad resonance in such models is only a small fraction of the Hubble time. The implied rapid evolution suggests that if galaxies exist with such large perturbations, either gas must be replenished from outside the galaxy or the perturbations must be transitory. Inside corotation with the spiral pattern, the loss of angular momentum by the gas increases the angular momentum of the stars, reducing the wave amplitude.
 

Master's Thesis

Surface Photometry of Spiral Arms

by B. Kevin Edgar

Under the supervision of Professor Robert Kennicutt

ABSTRACT

Multi-color surface photometry of the spiral patterns of the 4 grad design spiral galaxies NGC 210, 628, 1300, and 7479 is presented. Radial profiles combined with synthesized stellar population models are used to separate the light into disk and arm components. The underlying disks are uniform in color, with a (B-V) color of ~0.75. The integrated spectrum of the arms is a composite of two populations confirming the presence of stellar density waves in the underlying disks. Three of the galaxies contain a moderate amplitude density wave (enhancement, K, of ~1.35-1.45); the fourth, NGC 1300, has a much stronger density enhancement. The accuracy of the CCD data suggests that precise photometry (differential colors accurate to < 0.m1) is required to detect moderate amplitude density waves.