The Tail of Late-forming Dwarf Galaxies in $\Lambda$CDM

TL;DR

This study combines an analytical model and hydrodynamical simulations to show that a small fraction of dwarf galaxies in a $\Lambda$CDM universe form unusually late, providing insights into their formation history and cosmological implications.

Contribution

It introduces a model explaining late-forming dwarf galaxies based on halo growth and critical mass thresholds, validated by high-resolution simulations.

Findings

A small fraction of dwarf galaxies form late ($z<3$) in $\Lambda$CDM.

Late formation correlates with halos crossing a time-dependent critical mass.

Most observed dwarf galaxies are unlikely to be forming for the first time today.

Abstract

We use a robust analytical model together with a high-resolution hydrodynamical cosmological simulation to demonstrate that in a $\Lambda$ cold dark matter ($\Lambda$CDM) universe, a small fraction of dwarf galaxies inhabiting dark matter (DM) halos in the mass range $3\times 10^{9} \lesssim M_{200} / M_{\odot} \lesssim 10^{10}$ form unusually late ($z<3$) compared to the bulk population of galaxies. These galaxies originate from the interplay between the stochastic growth of DM halos and the existence of a time-dependent DM halo mass below which galaxies do not form. The formation epoch of the simulated late-forming galaxies traces remarkably well the time when their host DM halos first exceeded a nontrivial (but well-understood) time-dependent critical mass, thus making late-forming dwarfs attractive cosmological probes with constraining power over the past growth history of their host halos. The agreement between our model and the simulation results demonstrates that the population of simulated late-forming dwarfs is a robust cosmological outcome and largely independent of the specific galaxy formation model included in the simulations provided: (1) the universe underwent cosmic reionization before $z_{\rm re} \sim 8$; (2) star formation proceeds in gas that self-gravitates; and (3) galaxy formation is largely restricted to atomic-cooling halos before $z_{\rm re}$. The scarcity of massive late-forming dwarfs expected in $\Lambda$CDM implies that the great majority of bright, metal-poor, and actively star-forming dwarfs observed in our local universe--the most obvious candidates for these late-forming galaxies--cannot be undergoing their formation for the first time at the present day in a $\Lambda$CDM universe.