The Large-scale Effect of Environment on Galactic Conformity

TL;DR

This study investigates how large-scale environment influences galactic conformity, revealing that environment significantly impacts galaxy activity and that existing models can explain these effects without new physics.

Contribution

It demonstrates that large-scale environment plays a dominant role in galactic conformity and shows that current semi-analytical models can account for these observations.

Findings

Neighbour galaxy activity depends more on environment than primary galaxy activity.

Passive primary galaxies are found in denser regions than active ones.

Semi-analytical models reproduce the observed conformity effects.

Abstract

We use a volume-limited galaxy sample from the SDSS Data Release 7 to explore the dependence of galactic conformity on the large-scale environment, measured on $\sim$ 4 Mpc scales. We find that the star formation activity of neighbour galaxies depends more strongly on the environment than on the activity of their primary galaxies. In under-dense regions most neighbour galaxies tend to be active, while in over-dense regions neighbour galaxies are mostly passive, regardless of the activity of their primary galaxies. At a given stellar mass, passive primary galaxies reside in higher density regions than active primary galaxies, leading to the apparently strong conformity signal. The dependence of the activity of neighbour galaxies on environment can be explained by the corresponding dependence of the fraction of satellite galaxies. Similar results are found for galaxies in a semi-analytical model, suggesting that no new physics is required to explain the observed large-scale conformity.