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The dwarf galaxy Triangulum (M33) presents an interesting testbed forstudying stellar halo formation: it is sufficiently massive so as to havelikely accreted smaller satellites, but also lies within the regime wherefeedback and other "in-situ" formation mechanisms are expected to play a role.In this work, we analyse the line-of-sight kinematics of stars across M33 fromthe TREX survey with a view to understanding the origin of its halo. We splitour sample into two broad populations of varying age, comprising 2032 "old" redgiant branch (RGB) stars, and 671 "intermediate-age" asymptotic giant branch(AGB) and carbon stars. We find decisive evidence for two distinct kinematiccomponents in both old and intermediate-age populations: a low-dispersion (~22km/s) disk-like component co-rotating with M33's HI gas, and a significantlyhigher-dispersion component (~50-60 km/s) which does not rotate in the sameplane as the gas and is thus interpreted as M33's stellar halo. Whilekinematically similar, the fraction of stars associated with the halo componentdiffers significantly between the two populations: this is consistently ~10%for the intermediate age population, but decreases from ~34% to ~10% as afunction of radius for the old population. We additionally find evidence thatthe intermediate-age halo population is systematically offset from the systemicvelocity of M33 by ~25 km/s, with a preferred central LOS velocity of ~-155km/s. This is the first detection and characterisation of an intermediate-agehalo in M33, and suggests in-situ formation mechanisms, as well as potentiallytidal interactions, have helped shaped it.

TREX: Kinematic Characterisation of a High-Dispersion Intermediate-Age Stellar Component in M33