Date of Award


Document Type

Honors Thesis (Open Access)


Colby College. Physics and Astronomy Dept.


Elizabeth McGrath


We used data from the GOODS-S field of the CANDELS survey in order to confirm previous studies that found that large fractions of high-redshift galaxies (z ∼ 2) are disk-dominated (McGrath et al. 2008; van der Wel et al. 2011). In the GOODS-S field we selected out all the massive quiescent disk galaxies by imposing a mass limit of M > 1010M and a redshift range of 0.5 ≤ z ≤ 2.5, and then choosing the quiescent red galaxies from a color-color plot. Once we had our sample, we did a qualitative visual classification of each galaxy and then a quantitative classification using the galaxy fitting program GALFIT. Our results from the fitting showed that 23 of the original 140 galaxies were classified as disk-dominated by GALFIT, and on further study we saw that most of these were at high-redshift. At a redshift of z ∼ 2 a significant fraction of galaxies showed strong disk components and 30% of them were disk-dominated. We also saw that the massive disk galaxies and the massive elliptical galaxies live in two different environments. The disk galaxies seem to live in less densely populated areas, which leads us to believe that there are two mechanisms for the creation of massive quiescent galaxies, one which creates the disks and one which creates the massive elliptical galaxies. For the disks, our observations imply a period of rapid star-formation in the early universe, but only after the majority of the gas had settled into a disk. The lower density environment and the disk nature of these galaxies leads us to favor cold streams over the major merger model of galaxy formation. For the ellipticals, which live in higher density environments, it is possible that major mergers of already aged stellar populations (e.g., dry mergers) could be the primary assembly mechanism.


high-redshift galaxies, quiescent galaxies, massive elliptical galaxies

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