# Hunting for Galaxies and Halos in simulations with VELOCIraptor

## TL;DR

VELOCIraptor is a new, highly parallel galaxy and halo finder that can identify substructures, including tidally disrupted objects and stellar halos, revealing new insights into subhalo distributions within larger halos.

## Contribution

The paper introduces VELOCIraptor, a novel parallel code capable of identifying subhalos and disrupted objects, with improved detection of substructure deep within host halos compared to existing methods.

## Key findings

- VELOCIraptor detects subhalos with negligible density contrast.
- Large subhalos are more common near the center of host halos.
- VELOCIraptor's results align with HBT+ in well-resolved halos.

## Abstract

We present VELOCIraptor, a massively parallel galaxy/(sub)halo finder that is also capable of robustly identifying tidally disrupted objects and separate stellar halos from galaxies. The code is written in c++11, use the MPI and OpenMP API's for parallelisation, and includes python tools to read/manipulate the data products produced. We demonstrate the power of the VELOCIraptor (sub)halo finder, showing how it can identify subhalos deep within the host that have negligible density contrasts to their parent halo. We find a subhalo mass-radial distance dependence: large subhalos with mass ratios of $\gtrsim10^{-2}$ are more common in the central regions that smaller subhalos, a result of dynamical friction and low tidal mass loss rates. This dependence is completely absent in (sub)halo finders in common use, which generally search for substructure in configuration space, yet is present in codes that track particles belonging to halos as they fall into other halos, such as HBT+. VELOCIraptor largely reproduces the dependence seen without tracking, finding a similar radial dependence to HBT+ in well resolved halos from our limited resolution fiducial simulation.