Introducing a new, robust galaxy finder algorithm for simulations

TL;DR

This paper introduces a new phase-space based galaxy finder algorithm for simulations, improving the robustness and accuracy of galaxy identification especially in dense regions, and compares it with existing methods.

Contribution

A novel scale-free, shape-independent galaxy finder algorithm using phase-space dispersion tensors, implemented in VELOCIraptor, enhancing structure detection in simulations.

Findings

Improved galaxy property measurements in dense environments.

Systematic differences in galaxy properties compared to real-space finders.

Overall statistical distributions remain consistent despite individual discrepancies.

Abstract

Identifying galaxies in hydrodynamical simulations is a difficult task, particularly in regions of high density such as galaxy groups and clusters. We present a new scale-free shape-independent algorithm to robustly and accurately identify galaxies in simulation, implemented within the phase-space halo-finder code VELOCIraptor. This is achieved by using the full phase-space dispersion tensor for particle assignment and an iterative adjustment of search parameters, which help us overcome common structure finding problems. We apply our improved method to the Horizon-AGN simulation and compare galaxy stellar masses ($M_*$), star formation rates (SFR) and sizes with the elaborate configuration-space halo finder, HaloMaker. Galaxies living in halos with $> 1$ galaxy are the most affected by the shortcomings of real-space finders, with their mass, SFR, and sizes being $> 2$ times larger (smaller) in the case of host (satellite) galaxies. Thus, our ability to measure minor/major merger rates and disentangle environmental effects in simulations can be generally hindered if the identification of galaxies is not treated carefully. Though large systematic differences are obtained on a one-to-one basis, the overall Galaxy Stellar Mass Function, the Star Formation Rate Function and mass-size relations are not greatly affected. This is due to isolated galaxies being the most abundant population, dominating broad statistics.