Engineering topics

Weil, Melinda L.

Publications and source records attributed to Weil, Melinda L..

6 records

NASA NTRS1994

Kinematic misalignments in remnants of multiple mergers

Observations indicate that the rotation and minor axes in elliptical galaxies are usually well aligned. However, computer models of dissipationless mergers of pairs of disk galaxies have failed to establish such a correlation, implying that most ellipticals probably did not originate in this manner. Here we consider remnants produced from repeated merging in small groups of a half-dozen or so disk galaxies and find that the internal kinematics of such objects are more representative of actual elliptical galaxies. Although our coverage of parameter space is not complete, the remnants produced in our simulations invariably possess small kinematic misalignments. These results suggest that the majority of elliptical galaxies have had evolutionary histories more complex than those envisaged by simple merger hypotheses.

Weil, Melinda L.↗

NASA NTRS1993

Segregation of gas and stars in shell galaxies

Using a code which is capable of evolving composite systems of collisionless matter and gas, we explore mergers like those thought responsible for the shells seen around many elliptical galaxies. If a small companion containing both gas and stars is accreted by a more massive primary, the stellar and gaseous debris are rapidly segregated: while the stars are free to oscillate back and forth in the primary's potential, thereby forming shells, the oppositely directed flows near the center of the primary effectively dissipate the orbital kinetic energy of the gas. Consequently, the gas settles into compact disks or rings in the nucleus of the primary, depending on orbital parameters. We note implications of these findings for the production of polar rings and the onset of nuclear activity in galaxies.

Weil, Melinda L.↗

NASA NTRS1993

Gas distribution and starbursts in shell galaxies

Detailed maps of most elliptical galaxies reveal that, whereas the greatest part of their luminous mass originates from a smooth distribution with a surface brightness approximated by a de Vaucouleurs law, a small percentage of their light is contributed by low surface brightness distortions termed 'fine structures'. The sharp-edged features called 'shells' are successfully reproduced by merger and infall models involving accretion from less massive companions. In this context, dwarf spheroidal and compact disk galaxies are likely progenitors of these stellar phenomena. However, it is probable that the sources of shell-forming material also contain significant amounts of gas. This component may play an important role in constraining the formation and evolution of shell galaxies. To investigate the effects of the gaseous component, numerical simulations were performed to study the tidal disruption of dwarf galaxies containing both gas and stars by more massive primaries, and the evolution of the ensuing debris. The calculations were performed with a hybrid N-body/hydrodynamics code. Collisionless matter is evolved using a conventional N-body technique and gas is treated using smoothed particle hydrodynamics in which self-gravitating fluid elements are represented as particles evolving according to Lagrangian hydrodynamic equations. An isothermal equation of state is employed so the gas remains at a temperature 104 K. Owing to the large mass ratio between the primary and companion, the primary is modeled as a rigid potential and the self-gravity of both galaxies is neglected.

Weil, Melinda L.↗

NASA NTRS1993

Spokes in ring galaxies

We examine the response of self-gravitating primary galaxies consisting of dark matter halos and disks containing both stars and gas to collisions with less massive companions. The primaries were constructed using a technique which makes it possible to realize multi-component systems that are stable and virtually in precise equilibrium. A total of 65,536 particles were employed to represent the primary and 4096 to represent the companion. Half the particles in the primary comprise its halo and the other half its disk. Gas makes up 10 percent of the disk mass and is represented by 8192 of the disk particles. A system of units is used where the gravitational constant, total disk mass, and disk exponential scale length are unity. The primary motivation of the present study is to determine whether effects associated with dissipation and self-gravity can account for the unusual morphology of the Cartwheel galaxy.

Hernquist, Lars↗

NASA NTRS1992

Starbursts in the nuclei of shell galaxies

Simulations of shell-forming galaxy mergers are reported in which gas and stars are tracked separately to study the implications of observations that indicate fine structures in elliptical galaxies. The hybrid N-body/hydrodynamics code by Hernquist and Katz (1989) is employed neglecting the self-gravity of galaxies in tidal disruptions by massive primaries of small dwarf galaxies. The evolution of the galactic debris is examined showing that the gas and stars are effectively segregated with the stars forming sharp-edged features and the gas forming disks in the nucleus of the primary. Star formation is theorized to occur in the nuclei of shell galaxies due to shocks in the gas, and the added presence of a black hole can lead to an active galactic nucleus. The simulation is of interest to the interpretation of such observations as those of NGC1275 in which dormant elliptical galaxies are activated by mergers between large galaxies and gas-rich satellites.

Hernquist, Lars↗

NASA NTRS1990

Preliminary results of radiation measurements from the marine stratus FIRE experiment

During the marine stratocumulus phase of the First International Satellite Cloud Climatology Regional Experiment (FIRE) in July 1987, researchers acquired radiative flux data from a variety of instruments which were flown on the ER-2 high altitude aircraft. The spectral coverage ranged from the near UV to beyond 40 microns. A survey and selected preliminary analyses of these measurements are presented. The specific instruments used in the experiment were chosen primarily for measuring quantities of specific interest for marine stratocumulus fields. However, testing and evaluation of instrumentation and techniques to be used in the future inland cirrus experiment was also an important consideration. Details of the instruments and the significance of what they measure are given.

Valero, Francisco P. J.↗