Only marginal alignment of disc galaxies

TL;DR

This study investigates the alignment of disc galaxies' angular momentum and spiral-arm handedness, finding no significant correlations in SDSS data, and discusses implications for weak-lensing measurements and future surveys.

Contribution

It provides a rigorous error analysis of galaxy alignment signals, demonstrating that previous claims are not statistically significant and highlighting biases in ellipticity estimates.

Findings

No significant autocorrelation of galaxy orientations at 1 Mpc/h

Ellipticity estimates are biased by galactic bulges

Photometric redshifts are too imprecise for alignment studies

Abstract

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.