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We present optical and infrared surface photometry of M99 (NGC 4254) at g, R, I, J, H and K-prime. M99 is a very interesting galaxy, both for its unusual three--arm spiral and for the exceptional strength of the spiral pattern, especially since it lacks an apparent companion.
We use these data to investigate whether the presence and strength of harmonic components of spiral structure are wavelength dependent, and hence if these components constitute features in the old stellar disk or in the gas and dust only. We also look for the location of resonances in the different bands. These results give us a clue to the nature of the triggering mechanism of spiral structure. Modal theory, for example, predicts that only lower order modes (m=1 and m=2) will survive in an old stellar disk. Likewise, swing amplification predicts that gas should form a stronger and more tightly wound spiral than the old stellar population. We can also distinguish between the coexistence of different independent modes and the driving of higher order spiral patterns by a dominant mode.
Our data allow us to investigate whether the passage of spiral density waves induces star formation. If this is the case, color gradients should occur across the spiral arms. Using the stellar population synthesis models of Charlot and Bruzual, we have identified optical/IR color indices that distinguish the effects of reddening from age; the Charlot--Bruzual models also predict that the color evolution across an arm is dramatic enough that these age gradients can be spatially resolved in a nearby spiral like M99.
We combine IR data tracing the molecular gas with HI data from the literature, in order to check if the star formation efficiency increases in the arms. This is yet another way of discriminating between a density wave producing spiral appearance through orbit crowding versus it organizing and even enhancing star formation.
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