Date of Graduation

8-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor

Daniel Kennefick

Committee Member

William Oliver

Second Committee Member

Vincent Chevrier

Third Committee Member

Claud Lacy

Fourth Committee Member

Julia Kennefick

Keywords

Density-wave Theory, Galaxy Evolution, Galaxy Structure, Spiral Galaxy, Spiral Pitch Angle

Abstract

The density-wave theory of spiral structure, though first proposed as long ago as the mid-1960s by C.C. Lin and F. Shu (Lin & Shu, 1964; Bertin & Lin, 1996; Shu, 2016), continues to be challenged by rival theories, such as the manifold theory. One test of these theories which has been proposed is that the pitch angle of spiral arms for galaxies should vary with the wavelength of the image in the density-wave theory, but not in the manifold theory. The reason is that stars are born in the density wave but move out of it as they age. In this dissertation, I combined large sample size with a wide range of wavelengths to investigate this issue. For each galaxy, I used wavelength FUV151nm, u-band, H-α, optical wavelength B-band and infrared 3.6 and 8.0 µm. I measured the pitch angle with the 2DFFT and Spirality codes (Davis et al., 2012; Shields et al., 2015). I find that the B-band and 3.6 µm images have smaller pitch angles than the infrared 8.0 µm image in all cases, in agreement with the prediction of the density-wave theory. I also find that the pitch angle at FUV and H-α are close to the measurements made at 8.0 µm. The Far-ultraviolet wavelength at 151nm shows very young, very bright UV stars still in the star-forming region (they are so bright as to be visible there and so short-lived that they never move out of it). I find that for both sets of measurements (2DFFT and Spirality) the 8.0 µm, H-α and ultraviolet images agree in their pitch angle measurements, suggesting that they are, in fact, sensitive to the same region. By contrast, the 3.6 µm and B-band images are uniformly tighter in pitch angle measurements than these wavelengths, suggesting that the density-wave picture is correct.

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