The shock-induced star formation sequence resulting from a constant spiral pattern speed

We utilize a suite of multiwavelength data, of 9 nearby spirals, to analyze the shock-induced star formation sequence, that may result from a constant spiral pattern speed. The sequence involves tracers as the HI, CO, 24um, and FUV, where the spiral arms were analyzed with Fourier techniques in order to obtain their azimuthal phases as a function of radius. It was found that only two of the objects, NGC 628 and NGC 5194, present coherent phases resembling the theoretical expectations, as indicated by the phase shifts of CO-24um. The evidence is more clear for NGC 5194, and moderate for NGC 628. It was also found that the phase shifts are different for the two spiral arms. With the exception on NGC 3627, a two-dimensional Fourier analysis showed that the rest of the objects do not exhibit bi-symmetric spiral structures of stellar mass, i.e., grand design spirals. A phase order inversion indicates a corotation radius of ~ 89" for NGC 628, and ~ 202" for NGC 5194. For these two objects, the CO-Halpha phase shifts corroborate the CO-24um azimuthal offsets. Also for NGC 5194, the CO-70um, CO-140um, and CO-250um phase shifts indicate a corotation region.