2009MNRAS.395.1640R

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. 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 and (ii) maps with a constant spatial resolution and variable noise. Our standard A_V map uses the nearest 49 stars to the centre of each pixel for the determination of the extinction. The 1σ variance is constant at 0.28mag A_V in the entire map. The distance to the 49th nearest star varies from below 1 arcmin near the Galactic plane to about 10 arcmin at the poles, but is below 5 arcmin for all giant molecular clouds (|b|<30°). A comparison with existing large-scale maps shows that our extinction values are systematically larger by 20 per cent compared to Dobashi et al. and 40 per cent 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 as compared to the other two maps.