The Structure of Molecular Clouds: I - All Sky Near Infrared Extinction
  Maps
  We are studying the column density distribution of all nearby giant molecular
clouds. As part of this project we generated several all sky extinction maps.
They are calculated using the median near infrared colour excess technique
applied to data from the Two Micron All-Sky Survey (2MASS). Our large scale
approach allows us to fit spline functions to extinction free regions in order
to accurately determine the colour excess values. Two types of maps are
presented: i) Maps with a constant noise and variable spatial resolution; ii)
Maps with a constant spatial resolution and variable noise. Our standard Av map
uses the nearest 49 stars to the centre of each pixel for the determination of
the extinction. The one sigma variance is constant at 0.28mag Av in the entire
map. The distance to the 49th nearest star varies from below 1arcmin near the
Galactic Plane to about 10arcmin at the poles, but is below 5arcmin for all
giant molecular clouds (|b|< 30degr). A comparison with existing large scale
maps shows that our extinction values are systematically larger by 20% compared
to Dobashi et al. and 40% smaller compared to Schlegel et al.. This is most
likely caused by the applied star counting technique in Dobashi et al. and
systematic uncertainties in the dust temperature and emissivity in Schlegel et
al.. Our superior resolution allows us to detect more small scale high
extinction cores compared to the other two maps.
