The Milky Way’s Peculiar Primordial Halo: A Shallow Core with a Steep Decline
Li et al. (2025) use Gaia data and a numerical “reverse–compression” method to infer the Milky Way’s primordial dark matter halo. They find an unusual structure: a shallow inner core and a steep outer decline, unlike halos predicted by standard cold dark matter models. Neither baryonic feedback nor alternative dark matter models fully explains this combination, suggesting gaps in current theories of dark matter or galaxy formation.
Tracing the Galactic Past: Chemical Clues from the Milky Way’s Faint Companions
Cheng Xu and collaborators used APOGEE data to study the chemical makeup of four dwarf galaxies orbiting the Milky Way. They found that galaxy mass influences how elements like magnesium and iron evolve over time, with larger galaxies retaining alpha elements longer. In Fornax, they discovered nitrogen-rich stars likely from disrupted globular clusters, offering clues about early star formation and galactic evolution.
Peering Past the Galactic Bar: Uncovering a Hidden Spiral Arm in the Milky Way
Simran Joharle and collaborators analyzed red clump stars near the Milky Way’s center using motion and dust data from the VVV survey. They found that one group of stars lies farther away and moves differently, consistent with Galactic rotation. Slightly higher extinction toward this group suggests it belongs to a spiral arm beyond the Galactic bar, providing new insight into the Galaxy’s hidden structure.
Unraveling the Milky Way’s Past: Tagging Stellar Substructures with Chemistry and Motion
Kristopher Youakim and Karin Lind used a new chemo kinematic tagging method combining stellar motions and chemical compositions to trace the Milky Way’s merger history. Using data from over 5000 stars, they identified known structures like Gaia Sausage Enceladus and Sequoia, linked many globular clusters to past mergers, and revealed new connections such as between the Orphan Chenab stream and Grus II dwarf galaxy.
Tracing the Ghosts of Clusters: StarStream Reveals Hidden Stellar Streams in the Milky Way
Yingtian Chen and colleagues used their new algorithm, StarStream, to uncover 87 stellar streams from globular clusters in Gaia data, doubling the known number. The method detects even irregular, misaligned streams, revealing that many clusters are actively losing stars. Measured mass loss rates show that low-mass, extended clusters like Palomar 5 are nearing tidal disruption, offering fresh insights into the Milky Way’s evolution.
Mapping the Hidden Streams of the Milky Way: Correcting Bias in Dark Matter Searches
Boone et al. (2025) develop a method to correct biases in stellar stream observations caused by uneven survey conditions in the Dark Energy Survey. Using synthetic stars from the Balrog tool, they refine measurements of stellar densities, demonstrating the method on the Phoenix stream. Their corrections remove false patterns and improve dark matter studies, offering an essential approach for future deep surveys like LSST.
Mapping the Motion of the Milky Way’s r-Process Stars
Pallavi Saraf and collaborators studied how r-process-enhanced stars, those rich in heavy elements formed by rapid neutron capture, move through the Milky Way. Using Gaia data and orbital simulations, they found these stars are almost evenly split between the disk and halo. Most have uncertain origins, though halo stars are more likely accreted. Similar chemical patterns across regions suggest r-process enrichment occurred under comparable conditions throughout the Galaxy.
Tracing the Milky Way’s Past with HDBSCAN: Finding the Ghosts of Ancient Galaxies
Andrea Sante and collaborators test the HDBSCAN clustering algorithm to trace the Milky Way’s merger history using Auriga simulations. By optimizing parameters and using a 12-dimensional feature space, they show HDBSCAN reliably identifies recent stellar streams but struggles with older, well-mixed debris. Contamination from stars formed inside the Milky Way further limits recovery, though cluster purity remains high.
Do Most Stars Form in Clusters? A New Look at Our Galaxy’s Star Birthplaces
Quintana and collaborators used new Gaia data to show that most stars in the Milky Way likely form in compact clusters. Their calculations suggest that at least half, and probably over 80%, of stars are born this way, much higher than past estimates. This supports the clustered star formation model, though many clusters dissolve quickly, leaving stars spread across the Galaxy.
Survivors and Zombies: How the Milky Way Built Its Satellite Family
Pathak and collaborators use high-resolution simulations to study why some dwarf galaxies around the Milky Way survive while others are destroyed. They find that survival depends on mass, time of infall, and orbit: massive satellites usually disrupt before quenching, while tiny ultra-faint dwarfs quench early but endure. Disrupted galaxies often kept forming stars until the moment of destruction, helping to explain the mix of surviving satellites and stellar debris in the Milky Way halo.
Galactic Encounters: What TNG50 Reveals About the Milky Way’s Dance with Sagittarius
Using the TNG50 simulation, researchers studied galaxy interactions similar to that between the Milky Way and Sagittarius. They found that such encounters rarely disturb the host galaxy’s vertical stellar motions or trigger star formation, unless the galaxy was already unusually cold or inactive. Most Milky Way-like discs were already perturbed, raising questions about how common this disequilibrium is in the universe.
Is the Milky Way Really Slowing Down? A Closer Look at the Galaxy’s Rotation Curve
Klacka and Šturc argue that recent claims of a declining Milky Way rotation curve result from using incorrect equations suited for flat disks, not spherical systems. When the correct spherical models are applied, the rotation curve appears flat, consistent with other spiral galaxies, suggesting no unusual drop in velocity or dark matter content.
Unwinding the Mystery of the Phase Spiral in the Milky Way
Widmark et al. map the phase spiral, a vertical motion pattern of stars in the Milky Way, using Gaia data. They find that its structure is smooth and consistent across the disk, suggesting a global, rather than local, origin. The winding time varies with location, raising questions about the Galaxy’s dynamical history and hinting at complex gravitational processes at play.
A Pulsar Clue: Finding a Hidden Clump of Dark Matter Near the Sun
Chakrabarti et al. report the first detection of a dark matter sub-halo near the Sun using pulsar timing data. By analyzing excess acceleration in binary pulsars, they infer a compact dark object with a mass around 10 million solar masses. This finding supports ΛCDM predictions and opens a new method for probing dark matter in our Galaxy.
Tracing the Milky Way’s Past: How Globular Clusters Reveal the History of the Gaia-Sausage-Enceladus Merger
Fernando Aguado-Agelet and colleagues studied 13 globular clusters linked to the Gaia-Sausage-Enceladus (GSE) merger to trace the Milky Way’s history. They found most clusters follow a clear age-metallicity pattern, with two distinct star-formation bursts about 2 billion years apart, likely triggered by GSE’s interaction with the Milky Way. Two clusters probably formed in the Milky Way, and two others may belong to different mergers.
Inside-Out Chemistry: Unveiling the Metal Distribution in Simulated Milky Way-Mass Galaxies
Iza et al. use simulations from the Auriga Project to study how metals are distributed in galaxies like the Milky Way. They find that older, metal-poor stars dominate the halo, while younger, iron-rich stars are found in the disc. The bulge shows intermediate properties, supporting an inside-out galaxy formation scenario.
Tracing the Twists and Turns of a Galaxy Like Ours: What Simulations Reveal About the Milky Way's Dynamic Heart
This study uses a detailed simulation to explore the Milky Way’s structure, showing that spiral arms are dynamic, short-lived features shaped by gas flows rather than fixed patterns. The inner bar drives much of this motion, influencing where arms form and dissolve. These findings explain why the Galaxy’s spiral structure is difficult to pin down and suggest a more chaotic process behind star formation.
A New Map of Our Galactic Neighborhood: The DECam Field of Streams
Ferguson and Shipp present a new map of the Milky Way’s stellar halo using DECam data, revealing numerous stellar streams and substructures. By selecting old, metal-poor stars at various distances, they highlight how our galaxy was built from smaller systems. Their work sets the stage for even deeper surveys with the upcoming LSST.
A Chemical Portrait of the Milky Way’s Heart: Mapping the Elements of the Nuclear Stellar Disc
Ryde et al. analyze nine stars in the Milky Way’s Nuclear Stellar Disc, measuring 18 chemical elements using infrared spectroscopy. Their results show strong chemical similarities between the NSD, Nuclear Star Cluster, and inner bulge, suggesting shared evolutionary histories. Sodium stands out with uniquely high levels, possibly linking the NSD to metal-rich clusters like Liller 1.
Ghosts and Companions of the Milky Way: What Dwarf Galaxies Tell Us About Galaxy Formation
Grimozzi et al. used simulations to compare gas in disrupted and surviving dwarf galaxies around the Milky Way. They found that disrupted dwarfs, accreted earlier, have lower metallicity and higher [Mg/Fe], reflecting bursty star formation. These chemical differences reveal how timing influences galaxy evolution in the Milky Way’s past.