In an earlier study of the spiral galaxy M33 , we photometrically identified arcs or outer spiral arms of intermediate age ( 0.6 Gyr - 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 M33 .
The target stars have estimated visual magnitudes as faint as V \sim 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 M33 ; the target stars there are also identified as carbon stars .
We also study the non-stellar PAH dust morphology of M33 secured using IRAC on board the Spitzer Space Telescope .
The Spitzer 8 \mu m 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 M33 .
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 .