The Building of Galactic Disks: Insights from the Triangulum Spiral   Galaxy Messier 33
  The 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 approx. 120 degrees in azimuth and up to 5 arcmin in width. The arcs are located 2-3 disk scale lengths from the galaxy centre (where 1 disk scale length is equivalent to 0.1 degrees in the V-band) and lie precisely where there is a warp in the HI profile of M33. Warps and infall are inextricably linked (Binney, 1992). We present spectroscopy of candidate stars in the outer northern arc, secured using the Keck I telescope in Hawaii. The target stars have estimated visual magnitudes as faint as V ~ 25m. Absorption bands of CN are seen in all spectra reported in this review talk, confirming their carbon star status. Also presented are PAH emissivity radial profiles generated from IRAC observations of M33 using the Spitzer Space Telescope. A dramatic change of phase in the m=2 Fourier component is detected at the domain of the arcs. M33 serves as an excellent example how the disks of spiral galaxies in our Universe are built: as dynamically open systems, growing from the inward, outward. 
D.L. Block, I.Puerari, G.G. Fazio, A. Stockton, G. Canalizo, K.C.   Freeman, T.H. Jarrett, F. Combes, Robert Groess, G. Wort...