A photometric study of pure disk galaxies

Pure disk galaxies are galaxies that form and evolve without a central bulge region. This morphology of galaxy is relatively unexplained and has yet to be successfully simulated using Lambda-Cold Dark Matter (ΛCDM) model parameters. The ΛCDM model is the standard framework from which astronomers and physicists understand and predict the Universe due to confirmed predictions such as the cosmic microwave background and the large scale structure of galaxy clusters. However, ΛCDM has yet to have a benchmark, observationally confirmed prediction on the galactic scale. This thesis is a study of eleven pure disk galaxies. Understanding this type of galaxy is very important in rectifying the incompatibility with the ΛCDM model. The method of analysis includes obtaining, cleaning and sky subtracting images from the Sloan Digital Sky Survey Data Release 7, deprojecting the images for a face on perspective, using g- and i-bands to construct color-index maps, using Fourier decompositions to create mode-dependent intensity ratio plots, surface density maps, mass-to-light ratio maps and surface brightness profiles, from which the radial scale length is derived. The future of this area of study is vital to understand a common feature of our Universe. Future studies can include looking for early supernova remnants or evidence of recent active galactic nuclei activity in young pure disk galaxies. Surveys and photometric analysis of edge-on pure disk galaxies may also reveal vital information to the origin and evolution of this class of galaxy. (Published By University of Alabama Libraries

The Spitzer Survey of Stellar Structure in Galaxies (S4^4G): Multi-component Decomposition Strategies and Data Release

The Spitzer Survey of Stellar Structure in Galaxies (S^4G) is a deep 3.6 and 4.5 μm imaging survey of 2352 nearby (<40 Mpc) galaxies. We describe the S^4G data analysis pipeline 4, which is dedicated to two-dimensional structural surface brightness decompositions of 3.6 μm images, using GALFIT3.0. Besides automatic 1-component Sérsic fits, and 2-component Sérsic bulge + exponential disk fits, we present human-supervised multi-component decompositions, which include, when judged appropriate, a central point source, bulge, disk, and bar components. Comparison of the fitted parameters indicates that multi-component models are needed to obtain reliable estimates for the bulge Sérsic index and bulge-to-total light ratio (B/T), confirming earlier results. Here, we describe the preparations of input data done for decompositions, give examples of our decomposition strategy, and describe the data products released via IRSA and via our web page (www.oulu.fi/astronomy/S4G_PIPELINE4/MAIN). These products include all the input data and decomposition files in electronic form, making it easy to extend the decompositions to suit specific science purposes. We also provide our IDL-based visualization tools (GALFIDL) developed for displaying/running GALFIT-decompositions, as well as our mask editing procedure (MASK_EDIT) used in data preparation. A detailed analysis of the bulge, disk, and bar parameters derived from multi-component decompositions will be published separately

Unveiling the Structure of Barred Galaxies at 3.6 \ensuremath\mum with the Spitzer Survey of Stellar Structure in Galaxies (S4^4G). I. Disk Breaks

We have performed two-dimensional multicomponent decomposition of 144 local barred spiral galaxies using 3.6 $\mu {\rm m}$ images from the Spitzer Survey of Stellar Structure in Galaxies. Our model fit includes up to four components (bulge, disk, bar, and a point source) and, most importantly, takes into account disk breaks. We find that ignoring the disk break and using a single disk scale length in the model fit for Type II (down-bending) disk galaxies can lead to differences of 40% in the disk scale length, 10% in bulge-to-total luminosity ratio (B/T), and 25% in bar-to-total luminosity ratios. We find that for galaxies with B/T $\geq$ 0.1, the break radius to bar radius, $r_{\rm br}/R_{\rm bar}$, varies between 1 and 3, but as a function of B/T the ratio remains roughly constant. This suggests that in bulge-dominated galaxies the disk break is likely related to the outer Lindblad Resonance (OLR) of the bar, and thus moves outwards as the bar grows. For galaxies with small bulges, B/T $<$ 0.1, $r_{\rm br}/R_{\rm bar}$ spans a wide range from 1 to 6. This suggests that the mechanism that produces the break in these galaxies may be different from that in galaxies with more massive bulges. Consistent with previous studies, we conclude that disk breaks in galaxies with small bulges may originate from bar resonances that may be also coupled with the spiral arms, or be related to star formation thresholds.Comment: Accepted for publication in ApJ, 21 pages, 13 figures, 2 table