H I shells in the outer Milky Way

We present results of a method for an automatic search for shells in 3D data cubes and apply it to the Leiden-Dwingeloo survey of the northern Milky Way. In the 2nd Galactic quadrant, where identifications of structures are not substantially influenced by overlapping, we find nearly 300 structures. The Galactic distribution of shells has an exponential profile in the radial direction with a scale length of kpc. In the z direction, one half of the shells are found at distances smaller than 500 pc. We also calculate the energies necessary to create the shells: there are several structures with energies greater than but only one with an energy exceeding . Their size distribution, corrected for distance effects, is approximated by a power-law with an index . Our identifications provide a lower limit to the filling factor of shells in the outer Milky Way: and .