  The dwarf galaxy Triangulum (M33) presents an interesting testbed for
studying stellar halo formation: it is sufficiently massive so as to have
likely accreted smaller satellites, but also lies within the regime where
feedback 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 from
the TREX survey with a view to understanding the origin of its halo. We split
our sample into two broad populations of varying age, comprising 2032 "old" red
giant branch (RGB) stars, and 671 "intermediate-age" asymptotic giant branch
(AGB) and carbon stars. We find decisive evidence for two distinct kinematic
components in both old and intermediate-age populations: a low-dispersion (~22
km/s) disk-like component co-rotating with M33's HI gas, and a significantly
higher-dispersion component (~50-60 km/s) which does not rotate in the same
plane as the gas and is thus interpreted as M33's stellar halo. While
kinematically similar, the fraction of stars associated with the halo component
differs significantly between the two populations: this is consistently ~10%
for the intermediate age population, but decreases from ~34% to ~10% as a
function of radius for the old population. We additionally find evidence that
the intermediate-age halo population is systematically offset from the systemic
velocity of M33 by ~25 km/s, with a preferred central LOS velocity of ~-155
km/s. This is the first detection and characterisation of an intermediate-age
halo in M33, and suggests in-situ formation mechanisms, as well as potentially
tidal interactions, have helped shaped it.
