Prof. Zaritsky, in a collaboration including UA undergraduate and graduate students, is undertaking a large program to survey nearly a third of the sky for ultra-diffuse galaxies (UDGs). The largest of these galaxies appear to be analogs to the Large Magellanic Cloud that have only formed only 1 to 10% as many stars. As such, they are dark matter dominated at all radii and provide the first known examples of dark matter halos that have total masses of possibly up to 10^12 solar masses and which have been relatively undisturbed by baryonic processes. The program involves using the DESI preimaging data to identify candidates and then various techniques to estimate or measure redshifts, including the use of the Large Binocular Telescope.

Prof. Zaritsky, in collaboration with UA postdoctoral fellow Huanian Zhang, is utilizing the millions of spectra provided by the Sloan Digital Sky Survey in a novel approach to produce the first measurements ever of diffuse Halpha emission from intermediate temperature gas in the halos of nearby galaxies. The detection of Halpha emission in other halos enables directed surveys of nearby galaxies (because the gas is in emission bright background sources against which to measure absorption are no longer needed). This work opens up a new avenue for the exploration of this important, perhaps dominant, baryonic component of galaxies. As part of the Aspera satellite team, Prof. Zaritsky is also planning to explore the higher temperature phase of the circumgalactic gas. 

Prof. Zaritsky, in collaboration Prof. Charlie Conroy's team at Harvard, is compiling the largest sample of Milky Way halo stars at large distance with high dispersion spectra to address a variety of questions regarding the accretion history of the Milky Way and probe the gravitational potential of the Milky Way at large distances. The survey is roughly 60% complete and the team has already published several key results regarding the nature of structure in the halo.