About 10% of rich clusters of galaxies are strong emitters of far-infrared (FIR) radiation; the 100 micron to 60 micron flux ratios are consistent with emission from cool dust. Since dust grains in the harsh environment of the intracluster medium should be destroyed on timescales of about 10^8 years, this emission poses a puzzle for us. I am the principle investigator on an Infrared Space Observatory (ISO) project to solve this puzzle through multi-filter far-infrared mapping of clusters, and I intend to use SIRTF to continue this work in the future. I use x-ray and optical observations of these clusters to understand what processes are important in heating the dust. There is little evidence of heating by starlight, so it is likely that the primary source of energy for the dust in clusters is collisions between the dust particles and the energetic electrons in the intracluster medium.
I also use observations of clusters of galaxies to constrain interesting cosmological parameters such as Omega, the ratio of the true density of the universe to the closure density. The baryon mass of the universe is highly constrained by Big Bang nucleosynthesis, so Omega can be estimated if we know the baryon fraction of the universe. The baryon fraction of clusters of galaxies can be determined through the use of X-ray observations to calculate their gravitational binding masses and of X-ray and optical observations to calculate their baryon masses. The ROSAT PSPC was well-suited to such observations for low-temperature, relatively nearby clusters, and observations of many such clusters are available in the ROSAT data archive. The forthcoming AXAF satellite will enable me to extend my sample to larger redshifts and lower cluster temperatures. About 40\% of elliptical galaxies have strong FIR emission, although the scenario for dust distribution and heating may be different than in clusters of galaxies. I am also involved in an ongoing multi-wavelength project to study these galaxies that uses IRAS, ISO, x-ray, and optical observations. We have proposed to use ISOCAM to determine the distribution of polycyclic aromatic hydrocarbons (PAHs) as a tracer of dust to determine whether dust heating is dominated by ultraviolet photons in the galaxy core or by the hot ISM throughout the galaxy. PAHs are more fragile than dust in the harsh ISM, so the presence and distribution throughout the galaxy of their signature at 6.2, 7.7 and 11.3 micron would indicate that the gas is important in heating the dust. If the PAH emission is concentrated in the core, the dust could be in a central disk of cold gas, and the dust may be heated solely by starlight.
Interactions between the x-ray emitting hot gas in elliptical galaxies and radio emission from active nuclei in these galaxies can be detected with high-resolution x-ray observations. In some cases we see a bar of x-ray emission roughly perpendicular to the radio emission. The radio jet may blow the hot gas out of its path, leaving a deficit of x-ray emission in that region, or the hot gas may be cooling down into a disk that feeds the central engine. In order for the gas to form a disk, it must have angular momentum that it cannot release, so the galaxy should be rotating; many ellipticals do not rotate at all. If this geometry is observed in galaxies that rotate strongly much more often than in galaxies that do not rotate at all, it would imply that the x-ray bar is in an edge-on rotating disk.