THE PHOTOMETRIC AND KINEMATIC STRUCTURE OF FACE-ON DISK GALAXIES. II. INTEGRATED LINE PROFILE CHARACTERIZATION AND THE ORIGIN OF LINE PROFILE ASYMMETRY

We perform a moments analysis of HI and HII global line profiles for 33 nearly face-on disk galaxies for the threefold purpose of rationalizing and interpreting line profile indices in the literature, presenting robust moment definitions with analytic error functions calibrated against Monte Carlo simulation, and probing the physical origin of line profile asymmetries. The first four profile moments serve as viable surrogates for the recession velocity, line width, asymmetry, and profile shape, respectively. The first three moments are superior, by a factor of similar to 2 in precision, to related quantities defined in the literature. First and third profile moments are related; skew can be used to calculate more accurate recession velocities from global profiles. Second and fourth profile moments are linked, corresponding to the known trend that narrow (but well resolved) line widths tend to be more Gaussian. Ha kurtosis also appears correlated with the spatially resolved line width of the ionized gas. No systematics appear between various measures of line width and true rotation speed other than the wide range of normalizations, which we calibrate. This conclusion and calibration, however, is highly sample dependent. The ratio of HII to HI widths is consistent with unity, even at low projected line width. There may be a trend toward a decrease in the ratio of HII to HI widths consistent with previous studies, but we also observe greater scatter. While there is good agreement between Hi and Hii first, second, and fourth moments, we find no positive correlation between skew measured from HI and HII profiles. Detailed analysis of the spatially resolved Ha distribution demonstrates that HII global profile asymmetries are dominated by differences in the gas distribution, not kinematic asymmetries.