2009.02343
Age dissection of the vertical breathing motions in Gaia DR2: evidence for spiral driving
Ghosh, et al
Gaia DR2 has revealed breathing motions in the Milky Way, with stars on both sides of the Galactic mid-plane moving coherently towards or away from it. The generating mechanism of these breathing motions is thought to be spiral density waves. Here we test this hypothesis. Using a self-consistent, high-resolution simulation with star formation, and which hosts prominent spirals, we first study the signatures of breathing modes excited by spirals. In the model, the breathing modes excited by the spiral structure have an increasing amplitude with distance from the mid-plane, pointing to an internal cause for them. The same behaviour is present also in the Gaia data. We then show that, at fixed height, the breathing motion amplitude decreases with age. We demonstrate that the Gaia data exhibit the same trend, which strengthens the case that the observed breathing modes are driven by spiral density waves.
2009.04471
An excess of small-scale gravitational lenses observed in galaxy clusters
Meneghetti, et al
Cold dark matter (CDM) constitutes most of the matter in the Universe. The interplay between dark and luminous matter in dense cosmic environments like galaxy clusters is studied theoretically using cosmological simulations. Observed gravitational lensing is used to test and characterize the properties of substructures - the small-scale distribution of dark matter - in clusters. An apt metric, the probability of strong lensing events produced by dark matter substructure, is devised and computed for 11 galaxy clusters. We report that observed cluster substructures are more efficient lenses than predicted by CDM simulations, by more than an order of magnitude. We suggest that hitherto undiagnosed systematic issues with simulations or incorrect assumptions about the properties of dark matter could explain our results.
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