PI: Kate L. Barnes, Indiana University, email@example.com
Address: Department of Astronomy, 727 East 3rd St., Bloomington, IN 47405, USA
CoI: Liese van Zee, Indiana University
CoI: Evan D. Skillman, University of Minnesota
Title: Star Formation Activity and Stellar Populations in the Outer Disk of a Sample of ``Normal'' Spiral Galaxies
Abstract: Inside-out galaxy formation predicts that the delayed in-fall of gas in the outer disks of spiral galaxies should result in a young stellar population in the outer disk. Recent UV observations of galaxies with ``XUV'' disks may provide evidence supporting this model of disk assembly. However, extended UV disks are always found in galaxies with extended HI disks, where it is likely that the galaxies have had significantly different formation histories from galaxies with typical HI disks. To obtain a census of the stellar population in the outer disk of ``normal'' spiral galaxies, we have obtained ultra-deep H(alpha) and UV imaging of a sample of galaxies with typical HI-to- optical size ratios (i.e., ~1-1.5). In order to interpret our UV photometry set accurately, we require HII region spectra to obtain measurements of the chemical abundances and extinction. We have measured H(alpha) and UV fluxes and colors of these star forming complexes in order to quantify the current star formation rates and examine the underlying stellar populations in the outer disk. The outer disk star forming regions show a large range in FUV-NUV colors, and independent metallicity and extinction measurements are needed in order to ensure that these variations are due to the underlying stellar populations. Additionally, these spectra will allow us to significantly increase the number of galaxies for which abundance gradients in the extreme outer disk have been measured.
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