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Paper text:
roughly 50% of bright spiral galaxies show asymmetry in their global hi profiles ( haynes et al .
such asymmetries may arise from noncircular motions , lopsided gas distributions , or unresolved companions . by examining resolved optical rotation curves ( rc s )
, we can isolate the kinematic contribution to the asymmetry .
although optical rc s do not extend as far as _ resolved _ hi data , courteau ( 1997 ) points out that global hi profiles sample primarily the kinematics of the inner disk , and high quality optical data generally perform equally well .
we analyze only rc s extending to at least 1.3@xmath2 , the peak velocity position for a pure exponential disk .
we also require that typical outer velocities reach @xmath3 @xmath4 , deliberately introducing a bias against the smallest and most face - on galaxies . although this bias is not in the spirit of the minimum - bias parent survey ( the nearby field galaxy survey , jansen et al .
2000 ) , our velocity cut serves to focus attention on galaxies with clear bulk motion , while still including a wide range of types , and also luminosities as faint as m@xmath5 .
the final sample comprises 113 galaxies , excluding one agn .
we adopt a quantitative asymmetry measure akin to the photometric asymmetry index of abraham et al .
( 1996 ) . reflecting the rotation curve about its origin , we compute the asymmetry within 1.3@xmath2 as the average absolute deviation between the two sides , @xmath6 .
( asymmetries for @xmath7@xmath2 are more difficult to compare due to the variable spatial extent of the data . )
measured asymmetries depend critically upon the choice of origin , so we shift the origin to numerically minimize the asymmetry . we constrain the spatial coordinate of the origin to remain within the one - sigma error bars of our determination of the continuum peak position , but we allow the velocity coordinate to vary freely .
the final asymmetry is expressed as a percentage of the typical outer velocity @xmath8 .
figure  1 shows the distribution of inner asymmetry values by morphology .
about 23% of our sample galaxies have asymmetry @xmath15% .
this number appears to be smaller than the hi result , consistent with the idea that we are isolating kinematic asymmetries from lopsidedness in the hi distribution , but direct comparison is difficult given the differences in asymmetry measurement technique .
some of the asymmetries we see reflect peculiar gas dynamics with no stellar counterpart , as illustrated in figure  2 , suggesting that the fraction of galaxies with asymmetric gravitational potentials is probably smaller than 23% .
we analyze asymmetries in the gas rotation curves of 113 galaxies drawn from the nearby field galaxy survey , spanning a broad range of luminosities and morphologies . 
 if the origin of the rotation curve is chosen to minimize the asymmetry , then @xmath023% of sample galaxies show asymmetry @xmath15% within 1.3@xmath2 . 
 however , not all gas kinematic asymmetries indicate asymmetries in the underlying potential .