The ACS LCID Project: On the origin of dwarf galaxy types: a manifestation of the halo assembly bias?

TL;DR

This study explores how the early formation conditions of dwarf galaxies influence their types, suggesting that initial environment density plays a key role in their evolutionary paths and current morphology.

Contribution

It introduces a classification of dwarf galaxies into fast and slow types based on their star formation histories and links these types to their early formation environments.

Findings

Fast dwarfs experienced a brief, dominant early star formation event.

Slow dwarfs formed stars gradually over time, with less early activity.

Environmental density at formation influences dwarf galaxy evolution.

Abstract

We discuss how knowledge of the whole evolutionary history of dwarf galaxies, including details on the early star formation events, can provide insight on the origin of the different dwarf galaxy types. We suggest that these types may be imprinted by the early conditions of formation rather than being only the result of a recent morphological transformation driven by environmental effects. We present precise star formation histories of a sample of Local Group dwarf galaxies, derived from colour-magnitude diagrams reaching the oldest main-sequence turnoffs. We argue that these galaxies can be assigned to two basic types: fast dwarfs that started their evolution with a dominant and short star formation event, and slow dwarfs that formed a small fraction of their stars early and have continued forming stars until the present time (or almost). These two different evolutionary paths do not map directly onto the present-day morphology (dwarf spheroidal vs dwarf irregular). Slow and fast dwarfs also differ in their inferred past location relative to the Milky Way and/or M31, which hints that slow dwarfs were generally assembled in lower density environments than fast dwarfs. We propose that the distinction between a fast and slow dwarf galaxy reflects primarily the characteristic density of the environment where they form. At a later stage, interaction with a large host galaxy may play a role in the final gas removal and ultimate termination of star formation.