Nebular Astrophysics

Diffuse matter in space that is irradiated by hot stars, energized by shock waves, or otherwise ionized and heated, will appear as a "nebula" with a spectrum that displays emission lines. The analysis of the resulting spectrum can reveal a tremendous amount of information about the physical properties of the emitting cloud, including its temperature, density, speed of motion, and composition. The addition of absorption line measurements for sources with strong ultraviolet continuum sources can be a powerful additional means of studying interstellar gas.

Shields is involved in a variety of projects that employ the tools of nebular analysis, while also finding ways to refine the application of spectral diagnostics. His previous and ongoing work includes studies of quasars and active galaxies, starbursts and HII regions, and emission-line regions in the centers of elliptical galaxies and galaxy clusters.

An example application is the study of a remarkable system of emission-line filaments associated with NGC 1275, the central galaxy in the Perseus cluster. Former graduate student Bassem Sabra (now on the faculty at Notre Dame University of Lebanon), working with Shields and Alex Filippenko (UC-Berkeley), examined the detailed energetics of this system. Most existing models for powering such filaments fall short of explaining the observations, with the remaining evidence pointing to the hot intracluster medium and its magnetic field as important sources of energy.
Composite spectrum of the NGC 1275 filaments, from Sabra et al. 2000 (data from the Lick Observatory 3-meter telescope).


Joe Shields

Updated: 2005 December 13