Dr. Tomer Tal is awarded an NSF Astronomy and Astrophysics Postdoctoral Fellowship to carry out a program of research and education at the University of California, Santa Cruz. Precisely how galaxies initially form and how they change throughout their lifetimes are among the least understood problems in astrophysics. The properties of nearby galaxies depend strongly on their host environment with observed correlations among color, morphology, and local density. It remains unknown whether the physical characteristics of galaxies were determined at the time of their formation or are the result of ongoing evolution through environmentally induced processes. In this project the fellow will perform a statistical analysis of galaxy environments at the peak of cosmic star formation, when a large fraction of local galaxies were formed and assembled. He will apply a unique method that he demonstrated at intermediate galaxy distances and will extract luminosity functions for a large sample of galaxy groups and clusters at a redshift of z = 2, revealing the typical environments of the high redshift Universe. He will then employ these results to test whether the local correlations between galaxy properties and environment were already in place 10 billion years ago. To do so, the fellow will utilize the data sets of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and the 3D-HST survey, complementary surveys of the early Universe with the new infrared camera on the Hubble Space Telescope. This is the first time that such a study will be possible at redshift of z ~ 2, and it will unveil clues regarding the balance between internal and external processes at great distances.
The broader impacts of this project include a significant educational component. First, the fellow will develop and teach a high-school level extragalactic astronomy class as part of COSMOS, the California State Summer School for Mathematics and Science. Second, he will volunteer at the public observing nights hosted by the Santa Cruz Astronomy Club at the Harvey West Park Scout House. Both of these activities promote scientific literacy through direct involvement and contact with the public.
Galaxies in the universe are commonly found in groups and clusters, where galaxies interact with one another and with the baryonic and dark matter in the group. Over the tenure of this project we utilized statistical techniques to examine how different environments affect the life cycle of galaxies. Statistical measurements of galaxy properties can be used as a powerful tool to study galaxy populations that cannot be confidently identified individually. For this project we assembled data from a number of surveys and combined observations to study low and high redshift galaxies at low and high masses. We utilized a background subtraction approach to extract information from publicly available data sets, negating the need for the tremendous observational effort that would otherwise be required to study a sample of this size. We found that the environment plays an important role in the evolution of its member galaxies. Satellite galaxies undergo a different set of processes from group central galaxies, and they evolve along separate paths. Moreover, we found that star formation quenching begins at different times for central and satellite galaxies, mostly due to their immediate environments. We followed these trends since redshift of 2, when the universe was only a few billion years old. We also studied the distribution of galaxies in groups in the past 8 billion years and found that the distribution of satellite galaxies in group halos has stayed relatively constant over this time. This finding suggests that the environment plays a critical role in determining the distribution of galaxies in groups as well.
