Catherine Zucker

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Compact Groups

Dominated by dark matter and swirling with large quantities of diffuse gas, compact groups may provide key clues to the overall structure of the universe. As their name suggests, compact groups are dense associations of three or more gravitationally bound galaxies with relatively low velocity dispersions, meaning they have prolonged gravitational interactions. They are among the densest systems of galaxies currently known in the universe, making them an ideal environment in which to study physical processes such as the formation of central super massive black holes, super star clusters, and dwarf galaxies. Compact groups also play a critical role in understanding galaxy evolution, or the change in star formation and gas processing that occurs over a galaxy's lifetime. Despite this important role, compact groups are understudied compared to larger, more massive galaxy clusters or smaller, more isolated galaxy pair mergers. More difficult to characterize than either clusters or mergers, they present a significant challenge to scientists: unlike clusters, compact groups evolve on rapid dynamical time scales, and unlike galaxy pair mergers, their interactions are typically harder to quantify.

In order to assess the impact of the compact group environment on star formation and galaxy evolution, my advisor, Dr. Kelsey Johnson, studied the mid-infrared colors of these galaxies and discovered an underdensity between compact group galaxies that are actively star-forming and those that are relatively quiescent. These results indicate a rapid transformation of galaxy properties, apparent in the existence of an underdensity in mid-infrared color space. This underdensity, occupied by relatively few moderately star-forming galaxies, is not seen in comparison samples of field galaxies, interacting pairs, or the center of the Coma Cluster (see Walker et al. 2010, Walker et al. 2012). The lack of galaxies with moderate levels of specific star formation, or stellar mass build-up, implies that compact group galaxies experience accelerated evolution.

This is where my research comes in! Using all-sky WISE data, I will mirror the analyses of Johnson and Walker, with the goal of identifying additional galaxies in the mid-infrared underdensity. This allows us to expand the sample from 49 compact groups to 169 compact groups, constituting 669 member galaxies. This will in turn allow me to better address the questions: What are the dominant processes that influence galaxy evolution in the compact group environment, and what implications does this have for the formation of hierarchical structure in the earlier universe?

My research is ongoing. Check back in the future for updates!