Using Cumulative Number Densities to Compare Galaxies Across Cosmic Time

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 galaxygalaxy mergers, which can lead to errors when comparing galaxies over large redshift ranges (∆z > 1). We use abundance matching in the ΛCDM paradigm to estimate the median change in cumulative number density with redshift and provide a simple fit (+0.16 dex per unit ∆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 1σ range of cumulative number densities corresponding to galaxy progenitors and descendants. Finally, we discuss some limits on cumulative number density comparisons, which arise due to difficulties measuring physical quantities (e.g., stellar mass) consistently across redshifts. A public tool to calculate cumulative number density evolution for galaxies, as well as approximate halo masses, is available online.

Baryonic solutions and challenges for cosmological models of dwarf galaxies

“…There is substantial overlap on the space spanned by different simulations, which is encouraging given the different codes and hydrodynamical solvers involved. In general, models approximately follow the extrapolations (dotted/dashed lines) from abundance matching relations 76,77 calculated from more massive galaxies. However, in detail, the slope and the scatter for the stellar mass -halo mass relation may differ for each simulation.…”

“…Central/field (non-satellite) dwarf galaxies in a sample of state-of-the-art cosmological simulations of galaxy formation are presented, as well as abundance matching models of ref. 77 and 76 . Panel (a): Central/field dwarf galaxies, beyond a MW-mass host halo, in simulations of MW-like or Local Group-like environments from various models; APOSTLE, L1 resolution 44,71 from the EAGLE project 72 shown as red crosses; the Latte and ELVIS suites 45,65 of FIRE-2 simulations 73 shown as blue circles; Auriga 74 shown as green triangles; NIHAO-UHD 41 as orange squares; and DC Justice League 66 as purple stars.…”

“…Note that the Marvel Suite simulates a zoom-in volume with several isolated dwarf halos and therefore falls in-between the definitions for the left and right panels. Solid lines show abundance matching relations in the regime where they are constrained and switch to dotted 76 and dashed 77 where they are extrapolated. The yellow shaded regions in both panels indicate the ultrafaint dwarf regime.…”