# PRIMUS: Clustering of Star-forming and Quiescent Central Galaxies at $0.2 < z < 0.9$

## TL;DR

This study measures the clustering of isolated primary galaxies at redshifts 0.2 to 0.9 to understand the role of central galaxies in galaxy evolution, revealing that quiescent centrals are more clustered than star-forming ones, influenced by halo relations and assembly bias.

## Contribution

It provides the first direct measurement of clustering differences between quiescent and star-forming central galaxies at intermediate redshifts using PRIMUS data.

## Key findings

- Quiescent central galaxies are more strongly clustered than star-forming centrals at z~0.35.
- Clustering differences are partly due to distinct stellar-to-halo mass relations and assembly bias.
- Isolation criteria introduce environmental incompleteness and bias in central galaxy samples.

## Abstract

Previous work has demonstrated that at a given stellar mass, quiescent galaxies are more strongly clustered than star-forming galaxies. The contribution to this signal from central, as opposed to satellite, galaxies is not known, which has strong implications for galaxy evolution models. To investigate the contribution from central galaxies, here we present measurements of the clustering of isolated primary (IP) galaxies, used as a proxy for central galaxies, at ${0.2<z<0.9}$ with data from the PRIMUS galaxy redshift survey. Using a sample of spectroscopic redshifts for $\sim60,000$ galaxies with ${M_*\gtrsim10^9~M_\odot}$ covering 5 deg$^2$ on the sky, we define IP galaxies using isolation cuts in spatial proximity and stellar mass of nearby galaxies. We find that at fixed stellar mass, quiescent IP galaxies are more strongly clustered than star-forming IP galaxies at $z\sim0.35$ $(10\sigma)$. Using mock galaxy catalogs based on recent halo occupation models of Behroozi et al. (2018) and designed to replicate the parameters of the PRIMUS survey dataset, we find that these clustering differences are due in part to quiescent central galaxies being more strongly clustered than star-forming central galaxies. This is consistent with either distinct stellar-to-halo mass relations for quiescent and star-forming central galaxies, and/or central galaxy assembly bias. We additionally use mock catalogs to assess the dependence of both incompleteness and satellite galaxy contamination in the IP galaxy samples on redshift, galaxy type, and stellar mass, and demonstrate how isolation criteria yield biased subsamples of central galaxies via {\it environmental incompleteness}, or the preferential exclusion of central galaxies in overdense environments.