MACROMOLECULES, VERY SMALL GRAINS, AND LARGE DUST PARTICLES IN THE WHIRLPOOL GALAXY M51 AND ITS COMPANION - A UNIFIED VIEW

A combined image using optical and mid-infrared ISOCAM data of the gal axy M51 and its companion reveals a unified view of the distribution o f dust grains of all sizes. The image shows the large grains in extinc tion at blue light and the small grains in emission at 15 mu m. Much o f the emission from small grains coincides with extinction from large grains, indicating that dense cold clouds are surrounded by a warmer u ltraviolet-exposed envelope; other emission from small grains has no o bvious extinction counterpart, The diffuse gas in M51 is particularly striking: shell-like structures are common, the interarm clouds have s piral shapes, and the inner spiral dust lanes are remarkably symmetric . A circular shape to the gas spiral in the center of M51 suggests tha t there is a barrier for a wave mode. The dust lanes in the arms show sharp inner edges from shock fronts, dense, regularly spaced clumps wi th star formation, and feathered outer edges from disruption by star f ormation. This appearance strongly suggests that star formation is tri ggered in spiral arms by the gravitational collapse of shocked gas. Th e companion galaxy is barred and has a circumnuclear ring of dust that is similar to starburst rings at the inner Lindblad resonances of oth er barred galaxies; it has a radius of approximately 12 '', correspond ing to about 500 pc.