Extreme Metallicity Dwarf Galaxies in IllustrisTNG

TL;DR

This study uses the IllustrisTNG simulation to investigate the origins of extremely metal-deficient dwarf galaxies, revealing that their low metallicity results from recent gas inflow/outflow interactions rather than being relics of the first galaxies.

Contribution

It demonstrates that XMDs are not primordial but are shaped by recent gas dynamics, emphasizing inflow/outflow interactions as key to their low metallicity in simulations.

Findings

XMDs at z=0 are not first galaxies but recent gas inflow events.

Inflow/outflow interactions strongly correlate with dwarf galaxy metallicity.

XMDs have decreased star formation rates over 1-5 Gyr and more extended young stellar populations.

Abstract

The use of extremely metal-deficient dwarf galaxies (XMDs) as nearby analogs for high-redshift protogalaxies is generating renewed interest due to recent JWST observations studying these protogalaxies. However, the existence of a population of unenriched galaxies at $z\sim0$ raises fundamental questions about how galaxies with such pristine gas reservoirs could be formed. To address these questions we study XMDs in the IllustrisTNG simulation. We find that XMDs at $z=0$ are not relics of the first galaxies, but dwarf galaxies that experience a dramatic $\sim0.3$ dex drop in their gas-phase metallicity in the past few Gyr. We investigate possible causes of this drop in metallicity including high gas fractions, outflow efficiency or inflow/outflow rates, unique environments, pristine inflow metallicity, and inflow/SFR interactions. Of these, we find that inflow/outflow interactions, parameterized by the cumulative regional SFR experienced by inflows, has the strongest correlation with dwarf galaxy metallicity and XMD formation. In other words, inefficient gas enrichment during the short time between its accretion from the CGM and the initiation of star formation is the most important cause of XMD formation in the simulation. Observationally, we identify differences in star formation history between XMDs and non-XMDs (with XMDs having significantly decreased star formation rates on $1-5$ Gyr timescales) and differences in galaxy size (with XMDs having a more extended young stellar population) as the primary differences between the two populations. These results highlight the importance of inflow enrichment efficiency as a possible driver of dwarf galaxy metallicities.