A Dynamic Galaxy: Stellar Age Patterns Across the Disk of M101

Using deep, narrowband imaging of the nearby spiral galaxy M101, we present stellar age information across the full extent of the disk of M101. Our narrowband filters measure age-sensitive absorption features such as the Balmer lines and the slope of the continuum between the Balmer break and 4000 Å break. We interpret these features in the context of inside-out galaxy formation theories and dynamical models of spiral structure. We confirm the galaxy's radial age gradient, with the mean stellar age decreasing with radius. In the relatively undisturbed main disk, we find that stellar ages get progressively older with distance across a spiral arm, consistent with the large-scale shock scenario in a quasi-steady spiral wave pattern. Unexpectedly, we find the same pattern across spiral arms in the outer disk as well, beyond the corotation radius of the main spiral pattern. We suggest that M101 has a dynamic, or transient, spiral pattern with multiple pattern speeds joined together via mode coupling to form coherent spiral structure. This scenario connects together the radial age gradient inherent to inside-out galaxy formation with the across-arm age gradients predicted by dynamic spiral arm theories across the full radial extent of the galaxy.