Abstract
Observations in the 21 cm line of neutral hydrogen show that the normal spiral galaxy NGC 628 is encircled by an elliptical ring of gas. The outlying gas extends to three times the optical radius and lies in a plane which is inclined about 15 deg with respect to the plane of the bright inner disk. NGC 628 is isolated from other large galaxies which might aid in providing a tidal origin for the extended H I gas. It is possible that a pair of dwarf galaxies (UGC 1171/1176) may have had a close encounter with NGC 628 in the distant past, but there is some question as to whether they are sufficiently massive. On the other hand, if the structure that is observed now is a remnant of the galaxy's formation process, strict constraints are placed on the form of the gravitational potential in this outer region in order to prevent differential precession from smearing out the observed asymmetry. Specifically, longevity is insured only by a Keplerian potential; a logarithmic potential will lead to dissipation of the asymmetry in less than two-billion years.
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