Infrared cirrus, what have we learned?

IRAS discovered thermal emission from infrared cirrus in 1983 and COBE has extended observations of these interstellar dust clouds to additional IR wavelengths. IR studies of individual high galactic latitude clouds are combined with sensitive optical measurements of scattered light from the same clouds. At least 4 distinctly different types of grains are required to explain the complex relationships revealed by combining observations of various samples of clouds made on different spatial scales and in a number of spectral bands. In addition to the dominant 22 K component seen most prominently at 100 μm, many clouds are bright at 12 and 25 μm, indicating dust at temperatures well above 100 K. Optical studies suggest at least two more types of grains are needed to explain the measured blue and red colors and the highly variable ratio of blue light to l00 μm thermal emission that is observed. We show a new display of 100 μm cirrus on a global scale and we determine the power density in a typical cloud to spatial scales of a few arcsec.