Only marginal alignment of disc galaxies
  Testing theories of angular-momentum acquisition of rotationally supported
disc galaxies is the key to understand the formation of this type of galaxies.
The tidal-torque theory tries to explain this acquisition process in a
cosmological framework and predicts positive autocorrelations of
angular-momentum orientation and spiral-arm handedness on distances of 1Mpc/h.
This disc alignment can also cause systematic effects in weak-lensing
measurements. Previous observations claimed discovering such correlations but
did not account for errors in redshift, ellipticity and morphological
classifications. We explain how to rigorously propagate all important errors.
Analysing disc galaxies in the SDSS database, we find that positive
autocorrelations of spiral-arm handedness and angular-momentum orientations on
distances of 1Mpc/h are plausible but not statistically significant. This
result agrees with a simple hypothesis test in the Local Group, where we find
no evidence for disc alignment. Moreover, we demonstrate that ellipticity
estimates based on second moments are strongly biased by galactic bulges,
thereby corrupting correlation estimates and overestimating the impact of disc
alignment on weak-lensing studies. Finally, we discuss the potential of future
sky surveys. We argue that photometric redshifts have too large errors, i.e.,
PanSTARRS and LSST cannot be used. We also discuss potentials and problems of
front-edge classifications of galaxy discs in order to improve estimates of
angular-momentum orientation.
