The detailed spiral structure in the outer Galactic disk is still poorly kno
wn, and for
several Galactic directions we still have to rely on model extrapolations.
One of these regions is the fourth Galactic quadrant, in the sector comprised
Vela and Carina (270o ≤ l ≤ 300o) where -apart from the conspicuous
branch of the Carina Sagittarius arm- no spiral arms have been detected so far
in the optical
beyond l ~270o.
By means of deep UBVI photometry, we search for spiral features in known
windows. U photometry, although observationally demanding, constitutesa
tool to detect and characterize distant aggregates of young stars, and allows
firmer distance estimates. We have studied a direction close to the tangent (
to the Carina arm, in an attempt to detect optical spiral tracers far beyond
the Carina branch,
where radio observations and models predictions seem to indicate the presence
of the extension
of the Perseus and Norma-Cygnus spiral arms in the fourth quadrant.
Along this line of sight, we detect three distinct groups of young stars. Two of them, at
distances of 2.5 and 6.0 kpc, belong to the Carina spiral arm
(which is crossed twice
in this particular direction). Interestingly, the latter is here detected for
the first time.
The third group, at a distance of ~12.7 kpc, is likely a
part of the Perseus arm which lies beyond the Carina arm, and constitutes the
detection of this arm in the fourth Galactic quadrant. The position of this
feature is compatible
both with HI observations and model predictions. We furthermore present
evidence that this
extremely distant group, formerly thought to be a star cluster (Shorlin 1),
is in fact a diffuse
young population typical of spiral features. In addition, our data-set does
as claimed in the
literature, the possible presence of the Monoceros Ring toward this direction.
This study highlights how multicolor optical studies can be effective to pro
be the spiral structure
in the outer Galactic disk. More fields need to be studied in this region of
the Galaxy to better
constrain the spiral structure in the fourth Galactic quadrant, in particular
the shape and extent
of the Perseus arm, and, possibly, to detect the even more distant Norma-Cygnus