M51 Stripped To Its Bones

We present optical and IR surface photometry of M51 (NGC~5194) at B~V~R~I~J~K and CO$(2.3\mu )$. These data are used to establish whether K-band ($2.2\mu$) images of spiral galaxies provide reliable maps of stellar surface mass density features such as massive spiral arms or bars. The main distorting agents in the mapping at shorter wavelengths are dust extinction and luminous young stars. From modeling the color changes across the main dust lanes in M51, we find that the K-band flux is attenuated by only $\ltorder 10\%$ even in the dust lanes. From monitoring the gravity-sensitive CO$(2.3\mu )$ index across the spiral arms we find that young, red supergiants do not distort significantly the K-band image except in one small patch. On this basis, {\it we conclude that K band images do trace the massive disk star population}. In M51 we find the surface mass density contrast (arm/inter-arm) to range from 1.8 to 3, comparable to results from N-body simulations of the galaxy's tidal encounter with NGC~5195. The spiral arm amplitude in M51 also shows smooth, strong radial variations, with a maximum at $\sim 130\asec$ and minima at $45\asec$ and $170\asec$.