Unique insights into the building of galactic disks may be gleaned from the Triangulum Galaxy, Messier 33. We identify spectacular arcs of intermediate age (0.6 Gyr - 2 Gyr) stars in the low-metallicity outer disk of the Triangulum Spiral The northern arc spans ∼120 degrees in azimuth and up tp 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. 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 seven spectra reported in this review talk, confirming their carbon star status. In addition, we present Keck II spectra of a small area 0.5 degree (5 scale lengths) away from the centre of M33; the target stars there are also identified as carbon stars. We next analyse images of M33 secured with the Spitzer Space Telescope. We seek for signatures of turbulence at the location of the arcs and the HI warp. To this end, power spectra are performed on the IRAC images, from the Fourier mode m=1 up to the Nyquist limit (10 parsecs). The power spectra reveal that the carbon stars arcs indeed contain the signature of turbulence expected as gas accretes onto the outer disk of M33. There is also a change of phase in the Fourier m=2 component of the PAH emission. While earlier studies have alluded to gas accretion as a driving mechanism of galactic evolution (AA, 394, L35, 2002), the Triangulum Galaxy serves as a Rosetta Stone in this regard. The presence and location of the carbon star arcs attests to a scenario wherein low-metallicity gas continues to be accreted at the HI warp. M33 serves as an excellent example how the disks of spiral galaxies in our Universe are built: as dynamically open systems. The Triangulum Spiral Galaxy grows from the inward, outward in full accord with state-of-the-art cosmological simulations of galaxy formation wherein disk formation is halted until a redshift of approximately unity.
Galaxy Evolution across the Hubble Time
- Pub Date:
- May 2007
- Invited review paper presented at IAU Simposium 235, Galaxy Evolution Across the Hubble Time, Prague. To be published by Cambridge University Press, eds. F. Combes &