Search For Star Cluster Age Gradients Across Spiral Arms of Three LEGUS Disk Galaxies

TL;DR

This study investigates the presence of age gradients across spiral arms in three galaxies to test the stationary density wave theory, finding evidence in one galaxy but not in others, suggesting different spiral arm formation mechanisms.

Contribution

It provides observational evidence supporting the stationary density wave theory in NGC 1566 and challenges its applicability to M51a and NGC 628, highlighting alternative formation processes.

Findings

NGC 1566 shows a significant age gradient consistent with the stationary density wave theory.

M51a does not exhibit an age gradient, indicating different spiral arm formation mechanisms.

No offset observed in NGC 628's spiral arms, suggesting non-stationary formation processes.

Abstract

One of the main theories for explaining the formation of spiral arms in galaxies is the stationary density wave theory. This theory predicts the existence of an age gradient across the arms. We use the stellar cluster catalogues of the galaxies NGC 1566, M51a, and NGC 628 from the Legacy Extragalactic UV Survey (LEGUS) program. In order to test for the possible existence of an age sequence across the spiral arms, we quantified the azimuthal offset between star clusters of different ages in our target galaxies. We found that NGC 1566, a grand-design spiral galaxy with bisymmetric arms and a strong bar, shows a significant age gradient across the spiral arms that appears to be consistent with the prediction of the stationary density wave theory. In contrast, M51a with its two well-defined spiral arms and a weaker bar does not show an age gradient across the arms. In addition, a comparison with non LEGUS star cluster catalogues for M51a yields similar results. We believe that the spiral structure of M51a is not the result of a stationary density wave with a fixed pattern speed. Instead, tidal interactions could be the dominant mechanism for the formation of spiral arms. We also found no offset in the azimuthal distribution of star clusters with different ages across the weak spiral arms of NGC 628.