Dynamical Influences in Flocculent Galaxies: an Examination of Structure and Star Formation

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Flocculent spiral galaxies are those spiral galaxies which show 'patchy, fleece-like' spiral structure that can be traced only over small azimuthal angles. In contrast, 'grand design' spirals have continuous, two-arm spirals which cover much of the optical disk of the galaxy. While detailed studies of grand design spirals have provided abundant evidence for the presence of spiral density waves, very little is known about the formation of structure in flocculent spirals, in particular about the distribution of the gas components (atomic and molecular) with respect to the stellar potential and star forming regions. One of the primary goals of this dissertation is to fill this void. Thus, this dissertation presents a comparative study of the distribution of gas and stars of nearby flocculent spirals, in order to make a direct comparison with similar studies in grand design spirals. Near infrared broadband imaging reveals kiloparsec-scale spiral structure in four nearby (D < 20 Mpc) flocculent galaxies (NGC 2403, NGC 3521, NGC 4414, and NGC 5055). NGC 4414 appears the most flocculent, exhibiting a chaotic, four-arm structure in the inner disk. NGC 5055, one of the prototypes for flocculent spirals, shows a symmetric, two-arm spiral in the inner disk. NGC 4414 and NGC 5055 were chosen for further study, and the distribution of CO, HI, and Hα emission were compared with the spiral structure observed in the near infrared. There is strong evidence for density waves in NGC 5055. HI surface densities are enhanced along the near infrared spiral, and variations in HI and Hα velocity fields suggest density wave streaming motions of 10-25 km s-1. The arm enhancements in NGC 4414 are too large to be easily explained by local processes in a differentially rotating disk, but there is only a weak correlation of the gas distribution with spiral structure, and no clear evidence of density wave streaming. On smaller scales, Giant Molecular Associations are detected in both NGC 4414 and NGC 5055. In addition, there is weak evidence of enhanced star formation efficiency along the near infrared arms in both galaxies. This study suggests that some flocculent galaxies have a significant contribution from a large-scale organizing influence.

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