Formation of dwarf galaxies in major gas-rich disc–disc mergers

Abstract

ABSTRACT Galaxies usually follow a mass–metallicity relation, where higher mass galaxies are typically more metal-rich than lower mass galaxies. Yet, tidal dwarf galaxies are outliers to this relation. These kinds of dwarfs are formed in galactic mergers. Since their material comes from the parent galaxies, they are typically more metal-rich than regular dwarfs. However, galaxies were far less enriched when the Universe was younger. One can ask if tidal dwarfs that formed at high redshift could be chemically distinguished from regular dwarfs. To answer this question, we performed a series of numerical simulations of gas-rich galaxy mergers at high redshift. We then identified the dwarf galaxies that formed in these mergers, and studied the evolution of their metal content. While the initial abundance of metals in the progenitors was low, the merger allows rapid enrichment and all tidal dwarf galaxies end up with high abundances. Their mass–metallicity relation is well fitted by the relation $12+\log ({\rm O/H})=5.47+0.415\log (M_*/\,{\rm {M}_\odot})$, putting them well above the observed relation for local dwarfs galaxies. We conclude that tidal dwarfs should be outliers to the mass–metallicity relation, no matter the epoch of formation.