Ghosts and Companions of the Milky Way: What Dwarf Galaxies Tell Us About Galaxy Formation
Galaxies like the Milky Way didn’t form all at once—they were built over time by absorbing smaller galaxies, a process known as hierarchical galaxy formation. This idea, part of the standard Lambda Cold Dark Matter (ΛCDM) model, predicts that many of the stars and gas in the MW today came from smaller systems that merged into it. Grimozzi et al. focus on two types of dwarf galaxies: disrupted ones that fully merged into the MW in the past, and surviving satellites that still orbit the galaxy today. Using simulations, they study how the gas in these dwarfs—particularly the gas that can still form stars—reveals clues about when and how these galaxies were accreted.
Simulating the Milky Way’s Past
The researchers used the ARTEMIS suite of simulations, which model the formation of MW-mass galaxies in great detail. These simulations incorporate many processes important for galaxy evolution, including star formation, chemical enrichment, and feedback from supernovae. The authors examined the chemical properties of the cold gas in both disrupted and surviving dwarfs, paying close attention to two key quantities: gas-phase metallicity (how enriched the gas is with heavy elements like oxygen) and α-enhancement (measured by the ratio of magnesium to iron, [Mg/Fe]). These properties help trace the history of star formation in each galaxy.
Disrupted Dwarfs Are Chemically Distinct
One of the main results is that, at the same stellar mass, disrupted dwarfs have lower gas-phase metallicities and higher [Mg/Fe] than surviving dwarfs. This suggests that the disrupted galaxies were accreted earlier, when the universe was younger and had fewer heavy elements. Because they formed stars in short, intense bursts, these dwarfs were enriched quickly with α-elements but had less time to accumulate iron. In contrast, surviving satellites likely experienced more prolonged star formation, leading to a different chemical makeup.
Timing Matters
To understand the role of when these galaxies were accreted, the authors introduced two time indicators: the redshift of accretion for disrupted dwarfs, and the redshift when half of the stars were formed for survivors. They found that earlier-accreted dwarfs had lower metallicities and higher [Mg/Fe] values. This timing explains why disrupted dwarfs show more variation (scatter) in their chemical properties than surviving ones, which were generally accreted later and have more uniform histories.
Chemical Clues in the Gas
Another striking feature is how [Mg/Fe] varies with oxygen abundance. Both types of dwarfs follow a trend: as the gas becomes more oxygen-rich, [Mg/Fe] decreases. However, there’s a noticeable “knee” in this relation—a turning point where this decrease becomes much sharper. This feature is tied to the balance between different types of supernovae: Type II supernovae (from massive stars) enrich gas quickly with α-elements, while Type Ia supernovae (from older stars) add more iron later on. The timing of star formation thus shapes the chemical signature left in the gas.
Star-Forming Gas as a Key Ingredient
Grimozzi et al. also investigated how the amount of star-forming gas influences metallicity and [Mg/Fe]. For disrupted dwarfs, those with more cold gas had lower metallicity and higher [Mg/Fe], supporting the idea that these galaxies were still forming stars when they merged. For surviving dwarfs, the trend was less clear, but galaxies with depleted gas tended to have lower α-enhancement. This suggests that longer star-forming histories gradually used up their gas and diluted the α-elements.
Putting the Pieces Together
The authors conclude that the gas in dwarf galaxies—especially in those that no longer exist as separate objects—can tell us a lot about the formation history of galaxies like the MW. By comparing disrupted and surviving dwarfs, they reveal how differences in when and how these galaxies were accreted affect their present-day chemical properties. These findings mirror earlier work on stars but provide new insights through the lens of gas, which is crucial for understanding how galaxies grow and evolve.
Why It Matters
For students and early astronomy enthusiasts, this study shows how simulations and chemical analysis come together to uncover the MW’s past. Even though some dwarf galaxies no longer exist, their remnants—both stars and gas—hold clues to how our galaxy assembled over billions of years. This research underscores the power of cosmological simulations to explore what we can’t observe directly and highlights the importance of small galaxies in shaping the universe as we see it today.
Source: Grimozzi
