Metal-poor Galaxies in the Local Universe

TL;DR

This paper reviews the properties, formation, and evolution of metal-poor dwarf galaxies in the local universe, emphasizing their metallicity relations, environmental effects, and role in galaxy formation history.

Contribution

It provides a comprehensive synthesis of observational and theoretical insights into the metallicity and evolution of local dwarf galaxies, highlighting their diversity and significance.

Findings

Metallicity correlates with luminosity and mass in galaxies.

Dwarf galaxies show deviations from the typical metallicity-luminosity relation.

Ultra-faint dwarf spheroidals may have contributed to the Galactic halo formation.

Abstract

A galaxy's mean metallicity is usually closely correlated with its luminosity and mass. Consequently the most metal-poor galaxies in the local universe are dwarf galaxies. Blue compact dwarfs and tidal dwarfs tend to deviate from the metallicity-luminosity relation by being too metal-poor or too metal-rich for their luminosity, respectively. A less pronounced offset separates dwarf spheroidal (dSph) and dwarf irregular galaxies, making the former too metal-rich for their luminosity, which indicates different formation conditions for these two types of dwarfs. While environment (photo-evaporation through local re-ionization by massive galaxies, tidal and ram pressure stripping) govern the observed morphology-distance relation, intrinsic properties (in particular total mass) play a decisive role in dwarf galaxy evolution with respect to the time and duration of star formation and the amount of enrichment. The metallicity distribution functions of nearby dwarfs can be understood taking pre-enrichment, gas infall, and winds into account. Many dwarfs show evidence for inhomogeneous, localized enrichment. Ultra-faint dSphs, which may have formed their metal-poor stars at high redshift via H2 cooling, show an overabundance of metal-deficient stars as compared to the (inner) Galactic halo, but may, along with classical dSphs, have contributed significantly to the build-up of the outer halo. The abundance ratios measured in the irregular Large Magellanic Cloud are consistent with the postulated early accretion of irregulars to form the inner Galactic halo.