H II regions, extinction, and IC 342 - A new view of the galactic neighborhood

The galactic extinction of a heavily reddened spiral galaxy can be gauged from observations of the Balmer decrement of constituent giant H II regions and measurements of annular-averaged H I column densities. Application of the technique to IC 342 (l = 138.2^deg^, b = + 10.6^deg^) reveals that A_B_ = 3.05 +/- 0.25. Thus, this galaxy is nearly as luminous as the Milky Way and only 1.9 times farther away from M31. The gravitational acceleration of M31 by IC 342 is between 15% and 25% of that due to the Milky Way. It appears that the Milky Way should be regarded dynamically as a member of a quartet of giant galaxies lying roughly in a line in the supergalactic plane.