Stellar-stream wrinkles linked to dark matter formed without subhaloes
In FIRE-2 simulations of roughly 15,000 stellar streams, spurs, kinks and gaps arose from the host galaxy's evolving gravity without adding small dark-matter subhaloes. Samples returned from Ryugu separately connect ammonium-bearing clays and other nitrogen species to the drying of late brines in its parent body. One result sets a baseline for dark-matter inference; the other offers a measurement-led interpretation of early Solar System chemistry.
Science··Morning
The host galaxy's gravity wrinkled streams too
The Astrophysical Journal study followed roughly 15,000 globular-cluster streams inside four Milky Way-mass haloes from the FIRE-2 cosmological simulations. The models included the changing gravity of the galactic disk, halo and large-scale structures, while deliberately excluding small dark-matter subhaloes and giant molecular clouds. Even so, about three-quarters of the streams developed spurs, kinks or cocoon-like structures. The smoothest streams still carried changes in width, overdensities and gaps along their tracks. Shapes resembling the GD-1 spur and the ATLAS–Aliqa Uma kink emerged without a subhalo encounter.[1]
Orbit shaped how smooth the trace remained
Pericentric distance, the closest point of an orbit to the galactic center, was the strongest predictor of stream shape. Roughly 15 kiloparsecs separated mostly smooth streams from more disturbed ones, while circular orbits beyond 20 kiloparsecs produced the smoothest examples. Only about 70 of roughly 15,000 streams lacked detectable wiggles and density structure at every scale. The authors describe this as a host-driven complexity floor in the absence of dark-matter subhaloes. This baseline implies that a gap in one stream cannot be securely attributed to a dark-matter subhalo from shape alone; the host galaxy's gravity and orbital history can also produce similar structures. A real subhalo encounter remains another possible explanation, but these simulations deliberately excluded small subhaloes. Future surveys will need many streams plus orbital and kinematic information.[1]
Ryugu's last brines concentrated nitrogen
Nature Astronomy researchers examined regolith returned from Ryugu by Hayabusa2 using infrared and X-ray spectroscopy and electron microscopy. They found ammonium-bearing phyllosilicates, carbon–nitrogen triple-bonded species and sodium nitrate in the same regions as sodium carbonate. That association supports the interpretation that nitrogen species became concentrated and ammonium entered clay minerals as late brines disappeared in the parent body. The team proposes that reactive ammonia and other nitrogen species may have persisted until aqueous activity ceased. The measurements come from Ryugu grains; the extension to prebiotic chemistry on icy bodies such as Ceres rests on analogy.[2]
