NGC 4631 is a considerably disturbed edge-on starburst galaxy, similar in many respects to M82 and NGC 253. It offers a diverse range of unusual features, many of which are the result of galactic interactions. The galaxy is undergoing complex tidal interactions with its close neighbor NGC 4627, a dwarf elliptical about 3 arc minutes NW of the nucleus, and NGC 4656, an edge on spiral about 30 arc minutes SE of the galaxy. Radio telescopes have detected large tidal spurs of molecular gas reaching out from NGC 4631 to its two companion galaxies confirming the physical relationship. Optical images reveal the optical counterpart of the tidal spurs in the form of a faint light bridge to NGC 4627.
Many peculiar features of the galaxy are the direct result of strong tidal influences in particular the vigorous star formation occurring throughout its observable disk. Unlike the well defined dust lane of most spiral galaxies, NGC 4631 possesses a chaotic and patchy distribution of dust, concentrated towards the center and in some regions extending well above the plane of the galaxy by as much as 4000 light years. The dust has been dispersed by immense blown out shells of molecular gas known as supershells. Two of the most impressive supershells yet discovered have been detected in NGC 4631by radioastronomers. The largest one located in the eastern half of the galaxy spans some 10,000 light years and is being blown outward by the winds of one of the largest OB associations known. The kinetic energy of a shell this large implies the input of some 10,000 to 50,000 OB stars believed to have formed in a massive starburst 20 million years ago.
NGC 4631 is well known for its extensive gaseous halo formed from outflows of hot gas arising from its disk, and confined by a strong magnetic field that envelopes the entire galaxy. Even further out than the halo a high energy corona of multimillion degree gas has been detected at distances up to 26,000 light years away from the galactic plane. X-ray observations have confirmed the existence of the corona which lends support to the belief that coronas may exist around many galaxies most likely as a consequence of intense star formation.
Vigorous star formation is occurring throughout the observable disk of NGC 4631 but is especially concentrated near the center where a Giant HII region designated CM 67 exists. Studies of CM 67 suggest that it formed at the end of a large barlike structure in the center of the galaxy which makes NGC 4631 the nearest large barred spiral to the Milky Way.