1994A&A...285..833R

We present WSRT observations of 21-cm line emission from the disturbed, edge-on galaxy NGC 4631. Maps at resolutions 12"x22", 35", and 45"x89" are presented, and these are used in conjunction with position-velocity cuts through the various data cubes in order to understand the structure of the disk and the many tidal spurs which protrude from this interacting galaxy. We also study the two companions NGC 4656 and NGC 4627, and three dwarfs in the system with H I detections. We first attempt to get a rough idea of the structure of the disk of NGC 4631. The data can be well modelled by a differentially rotating disk with a steep fall-off beyond 4 kpc, and an outer disk restricted to the radius range 7-15 kpc. The latter component could simply be a set of spiral arms. Radial motions are probably present in the central few arcminutes. The inner disk bends upwards with increasing distance from the center on both the east and west sides. Strong tides may be responsible. The H I layer at high resolution shows many disturbances, a large extent parallel to the minor axis, and a ragged appearance along its edges. The velocity structure of the gas far from the major axis of NGC 4631 suggests that most of this emission is due to the galaxy being not quite edge-on, a rather large (500-1000 pc) scale-height in the outer galaxy, and the connection of one of the spurs with the disk. A few high latitude features with a probable inner disk origin are found, and these are more likely to be due to gas raised out of the plane by stellar winds and supernovae. However, outer galaxy star formation may contribute to the large H I scale-height there. No gas can be unambiguously associated with the dwarf elliptical companion, NGC 4627. It is difficult to identify any of the spurs as gas that once belonged to this galaxy (despite recent signs of star formation), as in the model of Combes. Modelling of the tidal encounters should now be redone, since more tidal debris has been discovered and an optical velocity of NGC 4627 has been determined. Position-velocity diagrams parallel to the major axis of the edge-on companion, NGC 4656, reveal a ring-like structure in the inner 15 kpc. Furthermore, the high and low-velocity sides of the ring arise from opposite sides of the midplane. Two possible explanations are given for this structure. First, NGC 4656 may consist primarily of two loosely wrapped tidal arms, viewed not quite edge-on. Second, NGC 4656 may be a ring galaxy similar to the Cartwheel. The south-west side of the disk shows many "worms" (or vertical filaments) with velocities consistent with an outer disk origin. The most prominent worm, however, is on the north-east side, above the center of the major axis. Its mass is ∼3 x 10⁷ M_☉. At very marginal signal-to-noise ratios, the worm bends around to form a complete loop.