The Milky Way galaxy grew into its current form with the help of smaller galaxies over time, which it has "consumed" or merged with. Astronomers are able to pick out which stars in the Milky Way came from other galaxies by identifying certain features, like the eccentricities of their galactic orbits and how many heavier elements they contain. Properties of some of the merged galaxies can then be determined when astronomers find collections of stars with similar features.

A group of astronomers recently studied a sample of 20 stars they believe formed together in a dwarf galaxy they call "Loki" that merged with the Milky Way during its early evolution. The study, published in Monthly Notices of the Royal Astronomical Society, shows that these stars are metal-poor, but distinct from other metal-poor stars found in the halo of the Milky Way.

Metal-poor stars as building blocks

The earliest stars formed in the universe were made up of hydrogen and helium. These stars fused hydrogen and helium into heavier elements, which then made up later generations of stars. These later generations then fused their elements into even heavier elements, and so on. Astronomers refer to stars with relatively small amounts of heavier elements, like iron, as "metal-poor." Galaxies made up of these stars acted as building blocks in the early universe.

"These building blocks merged together at early epochs, dispersing their stellar, gaseous, and dark matter content into the forming proto-galaxy. Therefore, the most metal-poor stars coming from the early galactic assembly are supposed to populate the inner regions of the Milky Way, while those accreted later might be dispersed in the outer halo," the study authors explain.

Surveys of stars in the Milky Way have found very metal-poor stars, but most are in the halo, not the galactic plane. Some evidence suggests that retrograde planar stars can only originate from early Milky Way assembly, while prograde orbiting stars were added by later accreted systems.

A dwarf galaxy hidden in the galactic plane

The new study investigated chemical properties of a group of 20 metal-poor stars from the Milky Way's galactic plane. The group contained both prograde and retrograde stars, all with fairly high eccentricities. The stars' chemical abundances were compared to those of halo stars, dwarf galaxies, and simulated populations.

The team found that chemical signatures from the group indicated enrichment from high-energy supernovae, hypernovae, fast-rotating massive stars, and neutron star mergers, but no white dwarf explosions. This meant that the origin of these stars was likely a short-lived, energetic dwarf galaxy. These chemical signatures were similar in both prograde and retrograde stars, suggesting similar origins.

The study authors say their results indicate that these stars came from a distinct origin, compared to metal-poor stars in the halo. They write, "These targets, with the exception of one star, show a narrower dispersion in the [X/Fe] than that of the halo and of the bulge at the same [Fe/H]. The [X/Fe] dispersions of our targets are very similar to that of a closed system, and smaller than in the case of two formation sites with different chemical enrichment."

Retrograde and prograde orbits from one system?

The astronomers raised the question of whether stars with prograde orbits may have come from a different system than those with retrograde orbits. But, they don't think this is the case. They say that the mass of the relevant star and gas material from their models reflects that of a single system instead, in particular, the mass of a dwarf galaxy.

"Alternatively, if our sample originated in a pair of systems, the simplest case would be one for the prograde and one for the retrograde stars. The pair of systems would share a very similar, if not identical, chemical history and evolution, as suggested by the small MAD and by the GCE model. The total baryonic mass would be twice the case of the single-system scenario."

The authors note that the sample for this study was small, but future larger, homogeneous spectroscopic surveys, such as WEAVE and 4MOST will help clarify the origins of planar metal-poor stars.

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