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Session 10 - Spiral Galaxies.
Display session, Monday, June 10
Great Hall,
Spiral density wave theory has had success describing the morphology and kinematics of two-armed grand design spiral structure. But what governs the morphology of multiple-armed spirals?
Recent work has suggested that multiple-armed spirals with grand design structure, such as M101, have underlying two-armed grand design spirals imposed on the disk, whose mass is carried mostly by old, red stars. The two-armed spiral would be evident in IR imaging, while visual imaging would show multiple arms. Under this paradigm, most of the visual spiral arms are the expression of emergent behavior in the Population I material that is responding to the two underlying ``driving'' arms.
We present V-band and K_s-band observations that show that the above paradigm does not apply to all multiple-armed spirals. In particular, we show examples of visually multiple-armed spirals for which the visual and IR morphologies are identical. We also show examples in which there is an underlying two-armed spiral in the IR, in support of the above paradigm. We explain the two cases in terms of differing stellar populations, with different star formation histories and initial mass functions.
