High Redshift Galaxy Morphology in the Hubble Space Telescope Frontier Fields
Brittany Tompkins ’17, Leah Jenks, Colgate University ’17, and Debra Elmegreen (Astronomy)
One question in the study of galaxies is how their disks evolve. Locally, galaxies like the Milky Way contain both a thin (younger) and a thick (older) disk component. At high redshifts, only the thick disk component is observed. Here, we access archival data from the HST Frontier Fields to examine the thick disks of distant galaxies as a function of redshift and mass. Using the Abell 2744 Parallel and MACSJ0416-1.2403 Parallel fields, we identified and analyzed a total of 149 edge-on galaxies. The mass and redshift of each galaxy were given by photometric data from the Frontier Fields catalogues. Galaxies were classified into four categories (clumpy, spiral, transition, and spheroidal) based on their shapes and radial light distributions. Since the ages of stars are correlated with different colors, we fit sech2 functions to the vertical light profiles of each galaxy in three passbands (blue, visual, and infrared) to determine the scale heights. We used the scale heights to calculate vertical color gradients, which we measured to be redder outward for all galaxy types. This result suggests that older stars are located further from the midplane. We found a positive correlation between scale height and galaxy mass for all categories, a trend which was also seen in local galaxies with ~10 times higher mass. The ratio of the scale height to the galaxy radius was relatively constant with redshift, while the average galaxy radius increased with decreasing redshift. These results suggest that galaxies change proportionately in length and height as they grow, and that thick disks form early on in galaxy evolution.