The prodigious warp of NGC4013

Non-planar distortions, or warps, in the outer gas layers of galaxies are more the rule than the exception, although the degree of warping may differ drastically from one galaxy to another1-3. H I radio synthesis observations reveal that the edge-on Sbc4 galaxy NGC4013 has the largest regular H I warp so far observed. It extends to a large height above the plane of the galaxy, and begins abruptly at just the radius where photometry indicates the end of the luminous disk. Furthermore, at precisely this position, the rotational velocity is seen to drop by 25 km s-1. These observations can only be explained by a situation in which not only the disk-light distribution, but also the disk-mass distribution, suddenly approach zero at the radius of the warp onset. On the basis of a modelling of the H I gas distribution, we conclude that our line of sight to NGC4013 is, by coincidence, one which shows the warp most clearly