Hart, Ross Edward
(2018)
The properties and origins of spiral structure across the galaxy population.
PhD thesis, University of Nottingham.
Abstract
Spiral galaxies are the most numerous type of massive galaxy in the low-redshift Universe. There have been a number of mechanisms proposed for their origin, including global density waves and swing amplified arms growing around local density perturbations. In order to understand the origin of arms fully, one requires a detailed study of all types of spiral structure. This thesis examines the global properties of the spiral galaxy population in large, complete samples of low-redshift galaxies. We utilise data from Galaxy Zoo, which provides visual classifications for a flux-limited sample of Sloan Digital Sky Survey (SDSS) galaxies, complete for m_r <17.0.
In order to study galaxy morphology, we developed techniques to measure the two fundamental properties to fully describe spiral arms: the arm number and pitch angle (how tightly wound the arms are). Accurate arm number measurements were obtained from the Galaxy Zoo 2 dataset. This required the development of a new method to remove the effect of redshift-dependent classification bias, an effect that galaxies viewed from further away have more difficult to distinguish features. To measure pitch angles, we used a tool called SPARCFIRE. This automatically detects spiral arms in input images, and measures a number of properties of the detected arms, including pitch angles. Accurately measuring pitch angles required the use of a machine learning algorithm trained on a subset of images checked by human inspection. Our method achieves a completeness of 75 per cent and contamination of 19 per cent in detecting real spiral arms in images.
Using these data, the demographics of spiral galaxies with different spiral arm numbers are compared. It is found that whilst all spiral galaxies occupy similar ranges of stellar mass and environment, many-armed galaxies display much bluer colours than their two-armed counterparts. I also combined UV and mid-IR photometry from GALEX and WISE to measure the rates and relative fractions of obscured and unobscured star formation in a sample of low-redshift SDSS spirals. Total star formation rate has little dependence on spiral arm multiplicity, but two-armed spirals convert their gas to stars more efficiently. I find significant differences in the fraction of obscured star formation: an additional ~ 10 per cent of star formation in two-armed galaxies is identified via mid-IR dust emission, compared to that in many-armed galaxies. The latter are also significantly offset below the IRX-beta relation for low-redshift star forming galaxies. I present several explanations for these differences versus arm number: variations in the spatial distribution, sizes or clearing timescales of star forming regions (i.e., molecular clouds), or contrasting recent star formation histories.
I also compare overall demographics with respect to arm pitch angle. A stellar mass-complete sample of ~ 6,000 SDSS spiral galaxies was selected. The star formation properties of galaxies vary significantly with arm number, but not pitch angle. We find that galaxies hosting strong bars have spiral arms substantially (4 − 6 degrees) looser than unbarred galaxies. Accounting for this, spiral arms associated with many-arm structures are looser (by 2 degrees) than those in two-arm galaxies. In contrast to this average trend, galaxies with greater bulge-to-total stellar mass ratios display both fewer and looser spiral arms. This effect is primarily driven by the galaxy disc, such that galaxies with more massive discs contain more spiral arms with tighter pitch angles. This implies that galaxy central mass concentration is not the dominant cause of pitch angle and arm number variations between galaxies, which in turn suggests that not all spiral arms are governed by classical density waves or modal theories.
Finally, I confront analytical predictions for swing amplified arms. By using a number of measured properties of galaxies, and scaling relations where there are no direct measurements, I model samples of SDSS and S4G spiral galaxies in terms of their relative halo, bulge and disc mass and size. Using these models, I test predictions of swing amplification theory with respect to directly measured spiral arm numbers from Galaxy Zoo 2. We find that neither a universal cored or cuspy inner dark matter profile can correctly predict observed number of arms in galaxies. However, by invoking a halo contraction/expansion model, a clear bimodality in the spiral galaxy population emerges. Approximately 40 per cent of unbarred spiral galaxies at z < 0.1 and M > 10 log(M_sun) have spiral arms that can be modelled by swing amplification. This population displays a significant correlation between predicted and observed spiral arm numbers, evidence that they are swing amplified modes. The remainder are dominated by two-arm systems for which the model predicts significantly higher arm numbers. These are likely driven by tidal interactions or other mechanisms.
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