Ancient Relics in the Milky Way: The DECam MAGIC Survey Uncovers the Galaxy’s Most Metal-Poor Stars

Vinicius Placco and collaborators report on the discovery and analysis of six stars in the farthest reaches of our Galaxy that contain extraordinarily little metal content, five “extremely metal-poor” and one “ultra metal-poor” star with [Fe/H] = –4.12. Using data from the Mapping the Ancient Galaxy in CaHK (MAGIC) survey, which employs a special calcium-sensitive filter on the Dark Energy Camera (DECam), the team confirmed that such photometric techniques can reliably identify these ancient stellar fossils.

The First Stars and Their Descendants

The paper opens with a discussion of the Universe’s chemical evolution. The first generation of stars, known as Population III stars, contained no metals, elements heavier than hydrogen or helium. When these massive stars exploded, they seeded space with metals that later generations incorporated. Stars with very low metallicity, such as those observed here, serve as “time capsules” preserving records of the early Universe’s composition. Many such stars, known as CEMP (carbon-enhanced metal-poor) stars, show unusually high carbon levels. These chemical signatures help astronomers infer what the first supernovae were like and how they shaped the Galaxy’s evolution.

Selecting Ancient Stars from the Galactic Halo

The researchers identified candidate stars from the MAGIC catalog, which contains about 21 million stellar sources. By combining narrowband Ca II HK photometry with broadband DECam data, they estimated each star’s metallicity. Only candidates fainter than [Fe/H] ≈ –3.0 and located over 30 kiloparsecs from the Sun were selected for spectroscopic follow-up using the MIKE spectrograph on the 6.5 m Magellan-Clay Telescope. Six of the seven observed stars were confirmed to have [Fe/H] ≤ –3.0, validating the photometric selection method. These objects are part of the Milky Way’s distant halo, a sparsely populated region containing some of the Galaxy’s oldest stars.

Measuring Atmospheres and Chemical Fingerprints

To analyze each star’s atmosphere, Placco’s team derived effective temperatures, surface gravities, and microturbulent velocities using Gaia and 2MASS photometry. They then determined chemical abundances for up to 16 elements, from carbon to barium, using high-resolution spectra. The resulting data showed consistent patterns typical of very old stars. One standout, J0433−5548, has an extreme carbon enhancement ([C/Fe] = +1.73) and likely formed from gas enriched by just a single Population III supernova. Its abundance pattern matches theoretical yields from a roughly 11 solar-mass explosion, providing a rare glimpse into the earliest stages of chemical enrichment in the Universe.

Confirming Photometric Precision and Galactic Origins

By comparing photometric and spectroscopic metallicities, the authors found that the MAGIC metallicity estimates are generally accurate to within 0.1 dex, except for the carbon-rich J0433−5548, where strong molecular carbon absorption slightly skewed results. Kinematic modeling revealed that these stars belong to the outer halo, orbiting the Milky Way at distances of 35–55 kiloparsecs. Some may have originated in the Magellanic system or the Gaia–Sausage/Enceladus merger, while one aligns with the Sagittarius stream, evidence of ongoing galactic accretion events.

Tracing the Universe’s First Supernovae

The study highlights how ultra metal-poor stars act as “chemical fossils.” By comparing the detailed abundances of J0433−5548 to supernova yield models, the team found it most consistent with enrichment from a low-mass (∼11 M⊙) Population III progenitor that exploded with modest energy. This supports theoretical predictions that not all first-generation stars were extremely massive and that their diverse explosions seeded the interstellar medium unevenly, producing the chemical diversity observed today.

A Window into Cosmic Origins

Placco and collaborators conclude that narrowband photometric surveys like MAGIC are powerful tools for identifying the oldest stars in the Milky Way. The combination of DECam imaging, Gaia data, and Magellan spectroscopy demonstrates how astronomers can uncover and analyze these stellar relics with increasing efficiency. Each new ultra metal-poor star discovered helps refine our understanding of how the earliest stars lived, died, and set the stage for galaxies like our own.

Source: Placco