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We present optical and IR surface photometry of M99 (NGC 4254) at g, r_S i, Jand K'. We also present a K' image of M51 (NGC 5194) for comparison. Fourierdecomposition of the disk light reveals that the radial distribution of powerdepends on wavelength, which in turn implies that the spiral structure tracedin the visual (i.e. young population I and dust) is different from the onedetected at 2 microns (i.e. old stellar disk). We observe radial modulation ofthe power and a dependency of power with wavelength that are consistent withmodal theory of spiral structure.  A central motivation for our research is the fundamental idea of density wavetheory that the passage of a spiral density wave triggers star formation. Wehave found a stellar population age gradient consistent with this scenario in areddening-free, red supergiant-sensitive, Q-like photometric parameter at 6 kpcgalactocentric distance across one of the arms of M99. We rule out that thechange in this parameter, Q(r_SJgi), across the arm is mainly due to dust. Thedifference in Q(r_SJgi) going from the interarm regions to the arms alsoindicates that arms cannot be due exclusively to crowding of stellar orbits.  We present the first measurement of Omega_p, the angular speed of the spiralpattern, and of the location of the corotation radius, derived from the driftvelocity of the young stars away from their birth site. The measured Q(r_SJgi)implies a star formation rate for M99 within the range of 10-20 M_odot/yr; adisk stellar mass surface density of ~80 M_odot/pc^2; and a maximumcontribution of ~20 percent from red supergiants to the K' light in a smallregion, and much smaller on average. We measure a K' arm--interarm contrast of2-3, too high for M99 to be a truly isolated galaxy.

Tracing the Dynamics of Disk Galaxies with Optical and Infrared Surface Photometry: Color Gradients in M99