The edge of the young Galactic disc
  In this work we report and discuss the detection of two distant diffuse
stellar groups in the third Galactic quadrant. They are composed of young
stars, with spectral types ranging from late O to late B, and lie at
galactocentric distances between 15 and 20 kpc. These groups are located in the
area of two cataloged open clusters (VdB-Hagen~04 and Ruprecht~30), projected
towards the Vela-Puppis constellations, and within the core of the Canis Major
over-density. Their reddening and distance has been estimated analyzing their
color-color and color-magnitude diagrams, derived from deep $UBV$ photometry.
The existence of young star aggregates at such extreme distances from the
Galactic center challenges the commonly accepted scenario in which the Galactic
disc has a sharp cut-off at about 14 kpc from the Galactic center, and
indicates that it extends to much greater distances (as also supported by
recent detection of CO molecular complexes well beyond this distance). While
the groups we find in the area of Ruprecht~30 are compatible with the Orion and
Norma-Cygnus spiral arms, respectively, the distant group we identify in the
region of VdB-Hagen~4 lies in the external regions of the Norma-Cygnus arm, at
a galactocentric distance ($\sim$20 kpc) where no young stars had been detected
so far in the optical.
