Using galaxy pairs as cosmological tracers

TL;DR

This paper investigates how galaxy pairs can be used as tracers of cosmological expansion, highlighting the effects of local density and peculiar velocities on the Alcock-Paczynski test.

Contribution

It demonstrates that only low-mass, isolated galaxy pairs reliably trace the universe's expansion, emphasizing the importance of pair selection for cosmological measurements.

Findings

Low-mass, isolated pairs best trace expansion

Peculiar velocities significantly affect pair-based measurements

Bound pairs do not carry cosmological information

Abstract

The Alcock-Paczynski (AP) effect uses the fact that, when analyzed with the correct geometry, we should observe structure that is statistically isotropic in the Universe. For structure undergoing cosmological expansion with the background, this constrains the product of the Hubble parameter and the angular diameter distance. However, the expansion of the Universe is inhomogeneous and local curvature depends on density. We argue that this distorts the AP effect on small scales. After analyzing the dynamics of galaxy pairs in the Millennium simulation, we find an interplay between peculiar velocities, galaxy properties and local density that affects how pairs trace cosmological expansion. We find that only low mass, isolated galaxy pairs trace the average expansion with a minimum "correction" for peculiar velocities. Other pairs require larger, more cosmology and redshift dependent peculiar velocity corrections and, in the small-separation limit of being bound in a collapsed system, do not carry cosmological information.