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Abstract

We present a detailed study of spiral structure and lopsidedness in four LEGUS disk galaxies. We first search for a possible age gradient of star clusters across the spiral arms, as predicted by the spiral density wave theory. We find a clear age sequence across the grand-design arms of NGC 1566, indicating that the density wave theory is the primary mechanism for the formation of spiral arms in this galaxy. In contrast, we find no offset in the location of star clusters with different ages in M51a and NGC 628, suggesting that tidal interactions and swing amplification theory might be the dominant scenarios to generate spiral patterns in these galaxies, respectively. Later, we search for a link between the asymmetric distribution of the star clusters along the spiral arms and the physical properties of NGC 3344, NGC 1566, NGC 628, and M51a. For this purpose, we construct 2D spatially resolved stellar mass and SFR surface density maps of the galaxies applying a pixel-by-pixel spectral energy distribution fitting technique. We do not observe any lopsidedness in the distribution of stellar mass surface density of the two arms in our target galaxies. On the contrary, our derived resolved SFR surface density maps of NGC 3344, NGC 1566, and the inner regions of M51a confirm the presence of lopsidedness between the two arms in a similar fashion to the distribution of the young star clusters. In the case of NGC 628, we do not observe any correlation between the lopsidedness in the distribution of star clusters and its SFR surface density