Keck spectroscopy and Spitzer space telescope analysis of the outer disk of the Triangulum spiral galaxy M 33

In an earlier study of the spiral galaxy M 33, we photometrically identified arcs or outer spiral arms of intermediate age (0.6-2 Gyr) carbon stars precisely at the commencement of the HI-warp. Stars in the arcs were unresolved, but were likely thermally-pulsing asymptotic giant branch carbon stars. Here we present Keck I spectroscopy of seven intrinsically bright and red target stars in the outer, northern arc in M 33. The target stars have estimated visual magnitudes as faint as V ∼ 25^m. Absorption bands of CN are seen in all seven spectra reported here, confirming their carbon star status. In addition, we present Keck II spectra of a small area 0.5 degree away from the centre of M 33; the target stars there are also identified as carbon stars. We also study the non-stellar PAH dust morphology of M 33 secured using IRAC on board the Spitzer Space Telescope. The Spitzer 8 μm image attests to a change of spiral phase at the start of the HI warp. The Keck spectra confirm that carbon stars may safely be identified on the basis of their red J - K _s colours in the outer, low metallicity disk of M 33. We propose that the enhanced number of carbon stars in the outer arms are an indicator of recent star formation, fueled by gas accretion from the HI-warp reservoir.