Stellar structures in the outer regions of M33
  We present Subaru/Suprime-Cam deep V and I imaging of seven fields in the
outer regions of M33. Our aim is to search for stellar structures corresponding
to extended HI clouds found in a recent 21-cm survey of the galaxy. Three
fields probe a large HI complex to the southeastern (SE) side of the galaxy. An
additional three fields cover the northwestern (NW) side of the galaxy along
the HI warp. A final target field was chosen further north, at a projected
distance of approximately 25 kpc, to study part of the large stellar plume
recently discovered around M33. We analyse the stellar population at R > 10 kpc
by means of V, I colour magnitude diagrams reaching the red clump. Evolved
stellar populations are found in all fields out to 120' (~ 30 kpc), while a
diffuse population of young stars (~ 200 Myr) is detected out to a
galactocentric radius of 15 kpc. The mean metallicity in the southern fields
remains approximately constant at [M/H] = -0.7 beyond the edge of the optical
disc, from 40' out to 80'. Along the northern fields probing the outer \hi
disc, we also find a metallicity of [M/H] = -0.7 between 35' and 70' from the
centre, which decreases to [M/H] = -1.0 at larger angular radii out to 120'. In
the northernmost field, outside the disc extent, the stellar population of the
large stellar feature possibly related to a M33-M31 interaction is on average
more metal-poor ([M/H] = -1.3) and older (> 6 Gyr). An exponential disc with a
large scale-length (~ 7 kpc) fits well the average distribution of stars
detected in both the SE and NW regions from a galactocentric distance of 11 kpc
out to 30 kpc. The stellar distribution at large radii is disturbed and,
although there is no clear correlation between the stellar substructures and
the location of the HI clouds, this gives evidence for tidal interaction or
accretion events.
