# The Assembly of the Virgo Cluster, Traced by its Galaxy Halos

## TL;DR

This paper investigates how the central dark matter density of galaxy halos relates to their infall history into the Virgo cluster, combining theoretical models, simulations, and observational data to show its potential as an assembly history indicator.

## Contribution

It demonstrates that halo density is a weak function of mass, strongly dependent on redshift, and can trace galaxy infall history into clusters using empirical, simulation, and observational analyses.

## Key findings

- Halo density depends weakly on mass and strongly on redshift.
- Subhalos maintain central density until near disruption.
- Galaxies with high halo density trace dense cluster regions.

## Abstract

Kinematic studies have produced accurate measurements of the total dark matter mass and mean dark matter density within the optical extent of galaxies, for large samples of objects. Here we consider theoretical predictions for the latter quantity, $\bar{\rho}_{dm}$, measured within the isophotal radius $R_{23.5}$, for isolated halos with universal density profiles. Through a combination of empirical scaling relations, we show that $\bar{\rho}_{dm}$ is expected to depend weakly on halo mass and strongly on redshift. When galaxy halos fall into larger groups or clusters they become tidally stripped, reducing their total dark matter mass, but this process is expected to preserve central density until an object is close to disruption. We confirm this with collisonless simulations of cluster formation, finding that subhalos have values of $\bar{\rho}_{dm}$ close to the analytic predictions. This suggests that $\bar{\rho}_{dm}$ may be a useful indicator of infall redshift onto the cluster. We test this hypothesis with data from the SHIVir survey, which covers a reasonable fraction of the Virgo cluster. We find that galaxies with high $\bar{\rho}_{dm}$ do indeed trace the densest regions of the cluster, with a few notable exceptions. Samples selected by environment have higher densities at a significance of 3.5-4$\sigma$, while samples selected by density are more clustered at 3-3.5$\sigma$ significance. We conclude that halo density can be a powerful tracer of the assembly history of clusters and their member galaxies.