AAS 207th Meeting, 8-12 January 2006
Session 188 Galaxy Anatomy: from Bars to Halos
Poster, Thursday, 9:20am-4:00pm, January 11, 2006, Exhibit Hall

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[188.10] The Stellar Populations of M33's Outer Regions

M. K. Barker, A. Sarajedini (Univ. of Florida), D. Geisler (Univ. de Concepcion), P. Harding (Case Western), R. Schommer (Cerro Tololo)

We present deep VI photometry of three fields in M33's outer regions obtained with HST/ACS. The radial distribution of stellar surface density declines exponentially with increasing galactocentric distance which is indicative of a disk population. However, we find evidence for a flattening of the distribution at the outermost observed radii which could indicate a transition from disk to halo or disk to thick-disk. The color-magnitude diagrams reveal a population with a mix of ages from ~ 100 Myr to over 2 Gyr. The red giant branch metallicity decreases from -1.1 to -1.3 across all three fields, a gradient consistent with that of the inner disk of M33. Finally, we present preliminary results on the star formation history of these fields using the technique of synthetic CMD fitting.

Support for this work (proposal number GO-9479) was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.

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