Infalling ultra-faint dwarfs as emissaries of the Axiverse

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.

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Publication Details

Journal
Astronomy and Astrophysics
Published
2025-11-27
DOI
https://doi.org/10.1051/0004-6361/202556196
Primary Topic
Space Science and Extraterrestrial Life
Type
article
Field-Weighted Citation Impact
0.00

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article

Infalling ultra-faint dwarfs as emissaries of the Axiverse

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Infalling ultra-faint dwarfs as emissaries of the Axiverse

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article en

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.

Astronomy and AstrophysicsVol. 704
Ikerbasque (ES), Centre National de la Recherche Scientifique (FR), Institute of Astronomy and Astrophysics, Academia Sinica (TW), Asia University (TW), Lawrence Berkeley National Laboratory (US), National Taiwan University (TW), University of the Basque Country (ES), Université Paris Cité (FR), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), Laboratoire AstroParticule et Cosmologie (FR), Sorbonne Université (FR), Kavli Institute for Particle Astrophysics and Cosmology (US), Center for Astrophysics Harvard & Smithsonian (US), Institute for Advanced Study (DE), Flatiron Health (United States) (US), Flatiron Institute (US), Nazarbayev University (KZ), Massachusetts Institute of Technology (US), University of Hong Kong (HK)
National Science Foundation, Smithsonian Institution, U.S. Department of Energy, Harvard University, Hong Kong University of Science and Technology, European Regional Development Fund, Agencia Estatal de Investigación, Lawrence Livermore National Laboratory
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