How Do Ultra-Faint Dwarf Galaxies Get Their Metals?

In the past couple of decades, astronomers have discovered many ultra-faint dwarf (UFD) galaxies -- tiny, faint collections of stars orbiting the Milky Way. These galaxies are especially interesting because they are old and small, making them useful for learning about the early universe and how galaxies form. However, simulations that try to model UFDs struggle to match what astronomers actually observe -- particularly when it comes to the amount of heavy elements (“metals”) in these galaxies. The paper by Wheeler et al. explores what determines the metallicity (the proportion of heavy elements) of UFDs: is it the metals already present in the gas they form from, or does it come from processes within the galaxies themselves?

Models and Data

The authors used a combination of computer simulations and semi-analytical models. They tracked how small galaxies grow in the environments of Milky Way-like galaxies, using halo evolution data from “Caterpillar” simulations. They also employed the GRUMPY model to calculate how gas is accreted, how stars form, and how metals are produced and lost. They varied two main ingredients: the metallicity of gas in the intergalactic medium (IGM -- the gas floating between galaxies) and the strength of outflows (winds that blow metals out of the galaxy). They compared their models to observational data of UFDs from the Local Volume Database and other studies.

The Role of the IGM

One might think that the metallicity of a UFD comes mainly from the metals already in the gas it accretes from the IGM. To test this idea, the team looked at high-resolution simulations of Milky Way environments to measure how enriched the IGM was at early times (redshifts 5–10, or when the universe was only a few hundred million years old). They found that, in all these simulations, only a tiny fraction of the IGM gas had metallicities higher than [Fe/H] ≈ −4 (a very low level of enrichment). Even the models that included the effects of Population III stars (the very first stars) did not produce much enriched gas. When they plugged these low IGM metallicities into their GRUMPY model, the resulting UFDs had metallicities much lower than what is actually observed.

The Role of Outflows

Since the IGM didn’t explain the observed metallicities, the authors turned to internal processes. As stars form and die, they release metals into the surrounding gas. However, supernovae and stellar winds can also blow gas and metals out of the galaxy, reducing the amount of metals that stay behind. This process is described by the “mass loading factor” (η), which measures how much gas is ejected compared to how much star formation happens. The team varied the maximum value of η in their models and found that by adjusting η to fall between about 200 and 2000, they could reproduce the range of metallicities observed in UFDs. In other words, the metallicity seems to depend more on how much of the metals the galaxy manages to keep rather than how enriched the gas it starts with is.

Comparing Models to Observations

Wheeler et al. also compared the detailed distribution of metallicities in stars within UFDs -- the so-called metallicity distribution function (MDF) -- to observations of five well-studied UFDs. They found that all their models, even with different assumptions about IGM metallicity and η, produced MDFs that matched observations reasonably well. This is partly because the observational data itself is noisy and sparse, making it hard to tell small differences between models.

Discussion and Conclusions

The paper concludes that the metallicity of UFDs is not set primarily by the metals in the IGM they accrete but rather by the balance between producing metals and losing them through outflows. This suggests that simulations of UFDs need to better account for how feedback processes work in tiny galaxies to match observations. Even though Population III stars contribute to early enrichment, their impact on UFD metallicity appears to be minimal.

Final Thoughts

Wheeler et al. provide an important step in understanding the faintest galaxies we can observe. Their study shows that the internal physics of UFDs -- particularly how effectively they hold onto their metals -- is key to explaining their metallicity. This has implications not only for modeling galaxy formation but also for using UFDs to learn about dark matter and the early universe.

Source: Wheeler