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 galaxy M51 and its companion reveals a unified view of the distribution of dust grains of all sizes. The image shows the large grains in extinction at blue light and the small grains in emission at 15 mu m. Much of the emission from small grains coincides with extinction from large grains, indicating that dense cold clouds are surrounded by a warmer ultraviolet-exposed envelope; other emission from small grains has no obvious extinction counterpart, The diffuse gas in M51 is particularly striking: shell-like structures are common, the interarm clouds have spiral shapes, and the inner spiral dust lanes are remarkably symmetric. A circular shape to the gas spiral in the center of M51 suggests that 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 with star formation, and feathered outer edges from disruption by star formation. This appearance strongly suggests that star formation is triggered in spiral arms by the gravitational collapse of shocked gas. The companion galaxy is barred and has a circumnuclear ring of dust that is similar to starburst rings at the inner Lindblad resonances of other barred galaxies; it has a radius of approximately 12 '', corresponding to about 500 pc.