Two extreme H I supershells in the disturbed edge-on galaxy NGC 4631

Abstract

Using Westerbork Synthesis Radio Telescope maps of 21 cm emission from NGC 463 1, we have discovered two very large supershells of atomic gas in the disk of this nearby, disturbed edge-on galaxy. The larger shell has a diameter of about 3 kpc and a swept up mass of roughly 1-2X10(8) M .. There is some evidence for expansion, with a velocity of 45 km s-1. Above a break at the top of the shell, there is a prominent ''worm'' of mass about 4X10(7) M .. The smaller shell has a diameter of 1.8 kpc and shows better evidence for expansion, at 35 km s-1, and has a mass of about 5-10X10(7) M .. The kinetic energy of the larger shell is roughly 2-5X10(54) erg, and the necessary input energy from supernovae and stellar winds to explain its current parameters is 2-5X10(55) erg, implying an association containing 10 000-35 000 supernova-producing OB stars! The smaller shell has energy parameters a few times lower. These would be extremely rich OB associations for a galactic disk if the supershells did indeed form in this way. A more attractive explanation as far as energetics are concerned is that small companions or massive gas clouds hit the disk of the galaxy and formed the shells. Unfortunately, there is no clear independent evidence for such collisions, and the near-complete ringlike appearance of the shells may be a problem for this explanation. The larger shell can be seen in the Halpha image of Rand et al. [ApJ, 396, 97 (1992)] as a bright, circular-shaped disturbance. Both shells are coincident with x-ray sources.