Growing the First Galaxies' Merger Trees

TL;DR

This paper introduces a new, efficient method for generating constrained galaxy merger trees that target specific high-redshift systems, improving predictions of their growth histories and final halo masses.

Contribution

The authors develop a novel framework combining Press-Schechter theory with Brownian bridges to efficiently generate merger trees with desired properties, implemented in an open-source model.

Findings

High-redshift JWST galaxy candidates likely merge around 2 Gyr after observation.

These galaxies reside in haloes exceeding 10^14 solar masses today.

The method is thousands of times more efficient than previous approaches.

Abstract

Modelling the growth histories of specific galaxies often involves generating the entire population of objects that arise in a given cosmology and selecting systems with appropriate properties. This approach is highly inefficient when targeting rare systems such as the extremely luminous high-redshift galaxy candidates detected by JWST. Here, we present a novel framework for generating merger trees with branches that are guaranteed to achieve a desired halo mass at a chosen redshift. This method augments extended Press Schechter theory solutions with constrained random processes known as Brownian bridges and is implemented in the open-source semi-analytic model $\texttt{Galacticus}$. We generate ensembles of constrained merger trees to predict the growth histories of seven high-redshift JWST galaxy candidates, finding that these systems most likely merge $\approx 2~\mathrm{Gyr}$ after the observation epoch and occupy haloes of mass $\gtrsim 10^{14}~M_{\mathrm{\odot}}$ today. These calculations are thousands of times more efficient than existing methods, are analytically controlled, and provide physical insights into the evolution of haloes with rapid early growth. Our constrained merger tree implementation is publicly available at http://github.com/galacticusorg/galacticus.