Date of Graduation
Doctor of Philosophy in Physics (PhD)
Second Committee Member
Third Committee Member
Fourth Committee Member
Density-wave Theory, Galaxy Evolution, Galaxy Structure, Spiral Galaxy, Spiral Pitch Angle
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.
Pour-Imani, Hamed, "Strong Evidence for the Density-wave Theory of Spiral Structure from a Multi-wavelength Study of Disk Galaxies" (2018). Theses and Dissertations. 2864.