Searchfor star cluster age gradients across spiral arms of three LEGUS disc galaxies

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.

Propagation of star formation at sub-kiloparsec scales

“…This is not surprising for peculiar NGC 5585. Our results support the result of Shabani et al (2018), who did not find a noticeable age gradient across spiral arms in NGC 628. We identified star clusters from catalogue of Shabani et al (2018) and compared the obtained ages for common objects.…”

“…Our results support the result of Shabani et al (2018), who did not find a noticeable age gradient across spiral arms in NGC 628. We identified star clusters from catalogue of Shabani et al (2018) and compared the obtained ages for common objects. Estimates showed a good agreement between our ages, t, and ages found in Shabani et al (2018), t ref :…”

Into the Darkness: Classical and Type II Cepheids in the Zona Galactica Incognita

“…There are several factors that can contribute to the lack of a clearer spiral structure in the classical Cepheids' distribution, apart from the non-uniform incompleteness of the sample. Most importantly, the Cepheids show a wide age range, and the older they are, the more they drift away from the the spiral arms due to the difference between the standard Galactic rotation and the spiral arms' pattern speed expected from density wave theory (e.g., Dobbs & Baba 2014;Shabani et al 2018), and observed by Hou & Han (2015). On the other hand, young Cepheids are not present in our sample in sufficient numbers to trace spiral arms.…”

The evolution of pitch angles of spiral arms

“…This picture is supported by recent observations from Galaxy Zoo results showing that there is no link between spiral arms pitch angle and central galaxy concentration, which is inconsistent with a modal theory of spiral arms (Hart et al 2017). Shabani, et al (2018) also analyse age gradients in 3 galaxies and only find evidence of a fixed density wave in a galaxy with a strong bar. In contrast, after analysing the properties of a large sample of spiral galaxies, ) (see also ) do find correlations of pitch angle with galaxy morphology, which lead them to favour the idea that spiral arms are manifestations of standing modes of propagating spiral waves within the galaxy (Lin & Shu 1964).…”