CO observations of the interacting edge-on galaxy NGC4631

The interacting edge-on galaxy NGC4631 was observed in the 12CO (J=1-0) line using the Nobeyama 45-m telescope. The molecular gas is strongly concentrated in the inner region of 1-kpc radius and shows a rigid rotation. Four intensity peaks of the CO emission have been identified along the major axis, located symmetrically with respect to the center and positionally coinciding with radio continuum peaks. Two possible models are considered to explain the CO peaks. (1) In the double-ring model, they are interpreted in terms of two rings of molecular hydrogen gas of radii 250 pc and 1 kpc. The rings are rotating at velocities of 20 and 80 kms-1, respectively, much slower than the galactic rotation (150 kms-1) of the outer disk, and no expanding motion has been detected. The rings are warped with respect to the optically determined major axis. The formation mechanism of the double gaseous rings in a bar potential caused by a tidal interaction with the companion galaxy NGC4656 is discussed. (2) In the fission model, the CO peaks are interpreted as being clumps of molecular gas formed as the result of a fission of the central gas disk which forms by a bar-induced gas inflow, where the bar is formed by a tidal encounter with the companion NGC4656. The deviation of radial velocities from rigid rotation is consistent with the fission model. (author)