Block, D LPuerari, IFazio, G. G.Stockton, ACanalizo, GFreeman, KennethJarrett, Thomas HCombes, FGroess, RWorthey, GGehrz, R. D.Woodward, Chick EPolomski, E. F.2015-12-07August 14-9780521863445http://hdl.handle.net/1885/27138The Triangulum Spiral Galaxy Messier 33 offers unique insights into the building of a galactic disk. We identify spectacular arcs of intermediate age (0.6 Gyr - 2 Gyr) stars in the low-metallicity outer disk. The northern arc spans ∼120 degrees in azimuKeywords: Galaxies: evolution - galaxies: spiral - galaxies: individual (M33 = NGC598)200710.1017/S17439213060049962015-12-07