The shape alignment of satellite galaxies in galaxy pairs in SDSS

TL;DR

This study reveals strong, environment-dependent shape alignments of satellite galaxies in galaxy pairs, providing insights into tidal influences and implications for weak lensing studies.

Contribution

It demonstrates the existence of significant radial and tangential satellite galaxy alignments in galaxy pairs, a phenomenon not previously confirmed in such environments.

Findings

Satellite galaxies show perpendicular alignment in facing regions.

Satellite galaxies align with the host in away regions.

Alignment strength depends on brightness, separation, and environment.

Abstract

It has been shown, both in simulations and observationally, that the tidal field of a large galaxy can torque its satellites such that the major axis of satellite galaxies points towards their hosts. This so-called `shape alignment' has been observed in isolated Milky Way-like galaxies but not in `Local Group'-like pairs. In this study, we investigate the shape alignment of satellite galaxies in galaxy pairs similar to the Local Group identified in the Sloan Digital Sky Survey Data Release 13 (SDSS DR13). By stacking tens of thousands of satellite galaxies around primary galaxy pairs, we find two statistically strong alignment signals. (1) The major axes of satellite galaxies located in the (projected) area between two primaries (the {\it facing} region) tend to be perpendicular to the line connecting the satellite to its host (tangential alignment), while (2) the major axes of satellite galaxies located in regions away from the other host (the {\it away} region) tend to be aligned with the line connecting the satellite to its host (radial alignment). These alignments are confirmed at $\sim5\sigma$ levels. The alignment signal increases with increasing primary brightness, decreasing pair separation, and decreasing satellite distance. The alignment signal is also found to be stronger in filamentary environments. These findings will shed light on understanding the mechanisms of how satellite galaxies are affected by the tidal field in galaxy pairs and will be useful for investigating galaxy intrinsic alignment in the analyses of weak gravitational lensing.