Using Cumulative Number Densities to Compare Galaxies across Cosmic Time

TL;DR

This paper investigates the evolution of galaxy populations across cosmic time using cumulative number densities, accounting for scatter in mass accretion and mergers, and provides a practical tool for such analyses.

Contribution

It introduces a method to estimate median number density changes with redshift using abundance matching, improving galaxy evolution comparisons across large redshift ranges.

Findings

Galaxy progenitors and descendants evolve differently due to scatter in mass accretion.

A simple fit (+0.16 dex per unit Delta z) describes median number density evolution.

A public tool for calculating number density evolution and halo masses is provided.

Abstract

Comparing galaxies across redshifts at fixed cumulative number density is a popular way to estimate the evolution of specific galaxy populations. This method ignores scatter in mass accretion histories and galaxy-galaxy mergers, which can lead to errors when comparing galaxies over large redshift ranges (Delta z > 1). We use abundance matching in the LCDM paradigm to estimate the median change in number density with redshift and provide a simple fit (+0.16 dex per unit Delta z) for progenitors of z = 0 galaxies. We find that galaxy descendants do not evolve in the same way as galaxy progenitors, largely due to scatter in mass accretion histories. We also provide estimates for the 1-sigma range of number densities corresponding to galaxy progenitors and descendants. Finally, we discuss some limits on number density comparisons, which arise due to difficulties measuring physical quantities (e.g., stellar mass) consistently across redshifts. A public tool to calculate number density evolution for galaxies, as well as approximate halo masses, is available online.