Asymmetry in high-precision global H I profiles of isolated spiral galaxies

New high signal-to-noise ratio 21 cm H I line profiles have been obtained for 104 galaxies with the Green Bank 43 m telescope. The primary sample is composed of isolated spirals with no known optical companions within a 1 degrees radius and a median ratio of optical diameter to beamwidth of 0.17. An effort was made to ensure linearity of baseline fitting and precise flux density calibration to better than 5%. Two quantitative measures of asymmetry are applied to assess the occurrence of lopsidedness in the global H I profiles. In agreement with previous estimates, half the galaxies show significant H I profile asymmetries. The lopsidedness cannot be explained by pointing offsets but, rather, must result from noncircular motions. confusion with unidentified companions within the telescope beam, or true distortions in the H I distribution.