Metals Removed by Outflows from Milky Way Dwarf Spheroidal Galaxies

TL;DR

This study quantifies the metals expelled by Milky Way dwarf spheroidal galaxies, revealing they lost over 96% of their produced metals, with larger dSphs contributing more than smaller ones, and compares these findings to other galaxy observations.

Contribution

It provides the first detailed calculation of metal outflows from multiple dSphs and links their metal loss to galaxy size and starburst activity.

Findings

dSphs lost 96% to >99% of their produced metals

Fornax lost more metals than all smaller dSphs combined

Starburst in NGC 1569 aligns with the dSph ejected mass-stellar mass relation

Abstract

The stars in the dwarf spheroidal satellite galaxies (dSphs) of the Milky Way are significantly more metal-poor than would be expected from a closed box model of chemical evolution. Gas outflows likely carried away most of the metals produced by the dSphs. Based on previous Keck/DEIMOS observations and models, we calculate the mass in Mg, Si, Ca, and Fe expelled from each of eight dSphs. Essentially, these masses are the differences between the observed amount of metals present in the dSphs' stars today and the inferred amount of metals produced by supernovae. We conclude that the dSphs lost 96% to >99% of the metals their stars manufactured. We apply the observed mass function of Milky Way dSphs to the ejected mass function to determine that a single large dSph, like Fornax, lost more metals over 10 Gyr than all smaller dSphs combined. Therefore, small galaxies like dSphs are not significant contributors to the metal content of the intergalactic medium. Finally, we compare our ejected mass function to previous X-ray measurements of the metal content of the winds from the post-starburst dwarf irregular galaxy NGC 1569. Remarkably, the most recent starburst in that galaxy falls exactly on the ejected mass-stellar mass relation defined by the Milky Way dSphs.