  We present a new technique for the statistical evaluation of the Tully-Fisher
relation (TFR) using spectral line stacking. This technique has the potential
to extend TFR observations to lower masses and higher redshifts than possible
through a galaxy-by-galaxy analysis. It further avoids the need for individual
galaxy inclination measurements.
  To quantify the properties of stacked HI emission lines, we consider a
simplistic model of galactic disks with analytically expressible line profiles.
Using this model, we compare the widths of stacked profiles with those of
individual galaxies. We then follow the same procedure using more realistic
mock galaxies drawn from the S3-SAX model (a derivative of the Millennium
simulation). Remarkably, when stacking the apparent HI lines of galaxies with
similar absolute magnitude and random inclinations, the width of the stack is
very similar to the width of the deprojected (= corrected for inclination) and
dedispersed (= after removal of velocity dispersion) input lines. Therefore,
the ratio between the widths of the stack and the deprojected/dedispersed input
lines is approximately constant - about 0.93 - with very little dependence on
the gas dispersion, galaxy mass, galaxy morphology, and shape of the rotation
curve.
  Finally, we apply our technique to construct a stacked TFR using HIPASS data
which already has a well defined TFR based on individual detections. We obtain
a B-band TFR with a slope of $-8.5\pm0.4$ and a K-band relation with a slope of
$-11.7\pm0.6$ for the HIPASS data set which is consistent with the existing
results.
