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T. K. Wyder (California Institute of Technology)
In this thesis, Hubble Space Telescope images in V and I of five fields in the Local Group dwarf irregular galaxy NGC 6822 were analyzed. Photometry was extracted from the data using a new program, HSTPHOT/MULTIPHOT. The field star color-magnitude diagrams were used to derive the star formation history. Star formation beginning 12-15 Gyr ago is best able to reproduce the observed distribution of stars along the red giant branch. NGC 6822 has most likely been forming stars at a fairly constant rate throughout its lifetime with no evidence for strong variations in the star formation rate with time. Within the past 500 Myr, there are noticeable differences in the star formation rates with three of the fields in the bar showing an increase in the recent star formation rate compared to the average past rate while two other fields that sample more of the outer regions show a decrease. This is consistent with stars forming as recently as 500 Myr ago not being spatially very well mixed throughout the galaxy.
Three of the fields target the star clusters Hubble VI, VII and VIII. Based upon cluster color-magnitude diagrams from which the field star contamination has been subtracted, I derived ages of 70 ±10 Myr for Hubble VI and 1.5 ± 0.2 Gyr for Hubble VIII. While the age is more uncertain in the case of Hubble VII, the available evidence argues that this cluster is probably similar to the old, metal-poor globular clusters in the Milky Way. Comparisons of the cluster ages and integrated magnitudes with models of cluster evolution imply that the two young clusters are an order of magnitude less massive than the older cluster Hubble VII. The half-light radii of these clusters are comparable to values for the inner Milky Way globulars and are on average smaller than clusters in the Magellanic Clouds.
The author(s) of this abstract have provided an email address for comments about the abstract: wyder@srl.caltech.edu