  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.
