Delving Deeper into the Tumultuous Lives of Galactic Dwarfs: Modeling Star Formation Histories

TL;DR

This study models the star formation histories of Local Group dwarf galaxies using HST data and a phenomenological model, revealing insights into their evolution and the impact of recent star formation.

Contribution

It introduces a new approach combining HST-derived star formation histories with a dark matter halo assembly model, emphasizing recent star formation and stochastic effects.

Findings

Model accurately predicts radial distribution of dwarfs.

Recent star formation in last 2 Gyr is necessary for consistency.

Discrepancies include overestimated stellar masses and few ultra-faint dwarfs.

Abstract

The paucity of observed dwarf galaxies in the Local Group relative to the abundance of predicted dark matter halos remains one of the greatest puzzles of the LCDM paradigm. Solving this puzzle now requires not only matching the numbers of objects but also understanding the details of their star formation histories. We present a summary of such histories derived from the HST data using the color-magnitude diagram fitting method. To reduce observational uncertainties, we condense the data into five cumulative parameters -- the fractions of stellar mass formed in the last 1, 2, 5, and 10 Gyr, and the mean stellar age. We interpret the new data with a phenomenological model based on the mass assembly histories of dark matter halos and the Schmidt law of star formation. The model correctly predicts the radial distribution of the dwarfs and the fractions of stars formed in the last 5 and 10 Gyr. However, in order to be consistent with the observations, the model requires a significant amount of recent star formation in the last 2 Gyr. Within the framework of our model, this prolonged star formation can be achieved by adding a stochastic variation in the density threshold of the star formation law. The model results are not sensitive to late gas accretion, the slope of the Schmidt law, or the details of cosmic reionization. A few discrepancies still remain: our model typically predicts too large stellar masses, only a modest population of ultra-faint dwarfs, and a small number of dwarfs with anomalously young stellar populations. Nevertheless, the observed star formation histories of Local Group dwarfs are generally consistent the expected star formation in cold dark matter halos.