Year: 2025
Type: article
Abstract: Recent discoveries of ultra-faint dwarf galaxies (UFDs) infalling onto the Milky Way, namely Leo K & M at r ≃ 450 kpc, considerably strengthens the case that UFDs constitute a distinct galaxy class that is inherently smaller and fainter, and metal-poorer than the classical dwarf spheroidals (dSph). This distinction is at odds with the inherent continuity of galaxy halo masses formed under scale-free gravity for any standard dark-matter (DM) model. Here, we show that distinct galaxy classes do evolve in cosmological simulations of multiple light bosons representing the “Axiverse” proposal of string theory, where a discrete mass spectrum of axions is generically predicted to span many decades in mass. In this context, the observed UFD class we show corresponds to a relatively heavy boson of 3 × 10 −21 eV, including Leo K & M, whereas a lighter axion of 10 −22 eV comprises the bulk of DM in all larger galaxies including the dSphs. Although Leo M is larger in size than Leo K, we predict its velocity dispersion to be smaller ( ≃ 1.7 km/s) than that of Leo K ( ≃ 4.5 km/s) because of the inverse de Broglie scale dependence on momentum. This scenario can be definitively tested using millisecond pulsars close to the Galactic center, where the Compton frequencies of the heavy and light bosons imprint monotone timing residuals that may be detected by the Square Kilometre Array (SKA) on timescales of approximately one week and four months, respectively.
Source: Astronomy and Astrophysics
Institutions: University of the Basque Country, Donostia International Physics Center, Ikerbasque, Centre National de la Recherche Scientifique, Lawrence Berkeley National Laboratory, Hong Kong University of Science and Technology, Université Paris Cité, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, Laboratoire AstroParticule et Cosmologie, Sorbonne Paris Cité, Institut National de Physique Nucléaire et de Physique des Particules, Nazarbayev University, University of California, Berkeley, Institute of Astronomy and Astrophysics, Academia Sinica, National Taiwan University, Center for Astrophysics Harvard & Smithsonian, Flatiron Institute, Massachusetts Institute of Technology
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W4415056084
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