| Abstract |
We perform a moments analysis of H I and H II 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 ~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. Hα 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 H II to H I widths is consistent with unity, even at low
projected line width. There may be a trend toward a decrease in the
ratio of H II to H I widths consistent with previous studies, but we
also observe greater scatter. While there is good agreement between H I
and H II first, second, and fourth moments, we find no positive
correlation between skew measured from H I and H II profiles. Detailed
analysis of the spatially resolved Hα distribution demonstrates
that H II global profile asymmetries are dominated by differences in the
gas distribution, not kinematic asymmetries. |