Tracing the Stellar Mass in M51

We present optical and IR surface photometry of M51 (NGC 5194) at B, V, R, I, J, K, and CO(2.3 μm). These data are used to establish whether K band (2.2 μm) 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 the optical depths to be ∼0.5 in the K band. For these optical depths the K band flux is attenuated by only ≲10% even in the dust lanes. From monitoring the gravity-sensitive CO(2.3 μm) index across the spiral arms we find that young, red supergiants do not distort significantly the K band image except in one small patch. OB associations are visible in the K band images but only cover a very small fraction of the spiral arms. On this basis, we conclude that K band images of face-on galaxies do trace the massive disk star population and allow a mapping of the azimuthal variation in the surface mass density of the stellar disk. In M51 we find the surface mass density contrast (arm/interarm) to range from 1.8 to 3, comparable to results from N-body simulations of the galaxy's tidal encounter with NGC 5195. This density contrast is larger than the light contrast in I band images, where the spiral arm crest is affected by dust extinction. The spiral arm amplitudes in M51 clearly show smooth, strong radial variations, with a maximum at ∼130" and minima at 45" and 170". These variations may arise from interference of a pre-existing spiral pattern with the tidally induced spiral arms. An ongoing K band imaging study of a sample of spiral galaxies will yield a more representative picture of the role of bars and massive spiral arm features.