Matter and dynamics of the local volume through tip of the red giant branch distances

Abstract

Studies at high redshift leverage look-back time to probe the history of the universe. These are complemented by detailed examinations of nearby objects and the clues they reveal about their past. The Hubble Space Telescope (HST) has opened the Local Volume (within 10 Mpc) to studies of resolved stellar populations, and a significant fraction of these ∼103 local galaxies have been observed with either the Wide-Field Planetary Camera 2 or the Advanced Camera for Surveys. The observations reveal information about the galaxies' star formation histories as well as their distances, which are key to understanding their past interactions with other galaxies. Using a combination of searches of the HST archive and observations from my collaborators' programs, I have identified and run photometry on ∼300 local galaxies. The criteria for a set of observations' inclusion in the sample are that they are of sufficient depth and are taken in filters appropriate for measuring an accurate distance based on the Tip of the Red Giant Branch method. Color-magnitude diagrams and distance estimates from these observations are published online. I use these distances, along with spectroscopic radial velocities, in a numerical application of the Principle of Least Action to develop scenarios representing the galaxies' motion from early times, and use these scenarios to constrain the mass of the M81 group. The results suggest that the group may contain more dark matter than has been previously inferred. Next, in pursuing the other thread of the galaxies' histories, I synthesize the color-magnitude diagram of the nearby dwarf irregular UGC4879 to estimate its star formation rate over time, finding both young and ancient stars but no evidence for ones of intermediate age. Building upon the study of UGC4879, I examine the stellar content of ∼90 dwarf galaxies from the sample by focusing on the relative numbers and colors of the red and asymptotic giants observed in their diagrams. These tests show excellent discrimination by morphological type, as well as hint at intriguing trends in star formation and enrichment that merit further investigation.