Searching for dark clouds in the outer galactic plane I -- A statistical
  approach for identifying extended red(dened) regions in 2MASS
  [Abridged] Though the exact role of infrared dark clouds in the formation
process is still somewhat unclear, they seem to provide useful laboratories to
study the very early stages of clustered star formation. Infrared dark clouds
have been identified predominantly toward the bright inner parts of the
galactic plane. The low background emission makes it more difficult to identify
similar objects in mid-infrared absorption in the outer parts. This is
unfortunate, because the outer Galaxy represents the only nearby region where
we can study effects of different (external) conditions on the star formation
process. The aim of this paper is to identify extended red regions in the outer
galactic plane based on reddening of stars in the near-infrared. We argue that
these regions appear reddened mainly due to extinction caused by molecular
clouds and young stellar objects. The work presented here is used as a basis
for identifying star forming regions and in particular the very early stages.
We use the Mann-Whitney U-test, in combination with a friends-of-friends
algorithm, to identify extended reddened regions in the 2MASS all-sky JHK
survey. We process the data on a regular grid using two different resolutions,
60" and 90". The two resolutions have been chosen because the stellar surface
density varies between the crowded spiral arm regions and the sparsely
populated galactic anti-center region. We identify 1320 extended red regions at
the higher resolution and 1589 at the lower resolution run. The majority of
regions are associated with major molecular cloud complexes, supporting our
hypothesis that the reddening is mostly due to foreground clouds and embedded
objects.
