Evidence in favour of density wave theory through age gradients observed in star formation history maps and spatially resolved stellar clusters

Quasi-stationary density wave theory predicts the existence of an age gradient across the spiral arms with a phase crossing at the corotation radius. We have examined evidence for such age gradients using star formation history (SFH) maps derived from LIGHTNING, a spectral energy distribution fitting procedure, and by using spatially resolved stellar clusters. Three galaxies from the LEGUS survey were used to analyse the azimuthal offsets of spatially resolved stellar clusters. Kernel density estimation plots of azimuthal cluster distance offsets reveal prominent central peaks and secondary peaks on the positive side, relative to the density wave for NGC 5194 and NGC 5236. These secondary downstream peaks in the cluster distributions show overall evidence for an age gradient. NGC 628 shows secondary peaks on both sides of the density wave. The cluster distributions also show an increasing spatial spread with age, consistent with the expectation that they were born in the density wave. SFH maps of 12 nearby galaxies were analysed using SPIRALITY, a MATLAB-based code, which plots synthetic spiral arms over FITS images. The SFH maps reveal a gradual decrement (tightening) in pitch angles with increasing age. By analysing the pitch angle differences between adjacent age bins using the error function, the average of the probabilities shows a 69 per cent +/- 25 per cent chance that the pitch angle values decrease (tighten) with increasing age. Thus, we see a tightening of the spiral pattern in galaxies, both when segregating stellar populations specifically by age or more generally by colour, as was shown in our previous studies.