How do dwarf galaxies acquire their mass & when do they form their stars?

TL;DR

This study uses a simple galaxy formation model on dark matter simulations to analyze how dwarf galaxies acquire mass and form stars, revealing that most stellar mass comes from gas accretion rather than mergers.

Contribution

It introduces a model combining halo data and star formation constraints to predict dwarf galaxy formation histories and their mass assembly processes.

Findings

Most dwarf galaxies acquire stellar mass through gas accretion, not mergers.

Star formation histories depend on halo circular velocity and IGM temperature evolution.

Predicted baryonic mass function lacks a maximum above 10^5.5 M_Sun.

Abstract

We apply a simple, one-equation, galaxy formation model on top of the halos and subhalos of a high-resolution dark matter cosmological simulation to study how dwarf galaxies acquire their mass and, for better mass resolution, on over 10^5 halo merger trees, to predict when they form their stars. With the first approach, we show that the large majority of galaxies within group- and cluster-mass halos have acquired the bulk of their stellar mass through gas accretion and not via galaxy mergers. We deduce that most dwarf ellipticals are not built up by galaxy mergers. With the second approach, we constrain the star formation histories of dwarfs by requiring that star formation must occur within halos of a minimum circular velocity set by the evolution of the temperature of the IGM, starting before the epoch of reionization. We qualitatively reproduce the downsizing trend of greater ages at greater masses and predict an upsizing trend of greater ages as one proceeds to masses lower than m_crit. We find that the fraction of galaxies with very young stellar populations (more than half the mass formed within the last 1.5 Gyr) is a function of present-day mass in stars and cold gas, which peaks at 0.5% at m_crit=10^6-8 M_Sun, corresponding to blue compact dwarfs such as I Zw 18. We predict that the baryonic mass function of galaxies should not show a maximum at masses above 10^5.5, M_Sun, and we speculate on the nature of the lowest mass galaxies.