# Three Different Types of Galaxy Alignment within Dark Matter Halos

## TL;DR

This study investigates three types of galaxy alignment within dark matter halos using SDSS data, revealing preferential satellite orientations and implications for weak lensing.

## Contribution

It provides new measurements of galaxy alignments within halos, including radial and direct alignments, with distinctions between red and blue satellites.

## Key findings

- Red satellites show radial alignment within 0.7 Rvir.
- Blue satellites are isotropic in orientation.
- Weak direct alignment observed at very small scales.

## Abstract

Using a large galaxy group catalogue based on the Sloan Digital Sky Survey Data Release 4 we measure three different types of intrinsic galaxy alignment within groups: halo alignment between the orientation of the brightest group galaxies (BGG) and the distribution of its satellite galaxies, radial alignment between the orientation of a satellite galaxy and the direction towards its BGG, and direct alignment between the orientation of the BGG and that of its satellites. In agreement with previous studies we find that satellite galaxies are preferentially located along the major axis. In addition, on scales r < 0.7 Rvir we find that red satellites are preferentially aligned radially with the direction to the BGG. The orientations of blue satellites, however, are perfectly consistent with being isotropic. Finally, on scales r < 0.1 \Rvir, we find a weak but significant indication for direct alignment between satellites and BGGs. We briefly discuss the implications for weak lensing measurements.