Infalling ultra-faint dwarfs as emissaries of the Axiverse
Pozo A. Broadhurst T. Luu H.N. Smoot G. Umetsu K. Chiueh T. Schive H.-Y. Emami R. Hernquist L. Mocz P. Vogelsberger M.
1 December 2025EDP Sciences
Astronomy and Astrophysics
2025#704
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.
dark matter
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DIPC, UPV/EHU, Basque Country, San Sebastian, 48080, Spain
University of the Basque Country UPV/EHU, Department of Theoretical Physics, Bilbao, 48080, Spain
Ikerbasque, Basque Foundation for Science, Bilbao, E-48011, Spain
Hong Kong University of Science and Technology, Institute for Advanced Study, Department of Physics, IAS TT & WF Chao Foundation, Hong Kong
Energetic Cosmos Laboratory, Nazarbayev University, Nursultan, Kazakhstan
Paris Centre for Cosmological Physics, APC, AstroParticule et Cosmologie, Université de Paris, CNRS/IN2P3, CEA/lrfu, Université Sorbonne Paris Cité, 10, rue Alice Domon et Leonie Duquet, Paris, 75205, France
Department of Physics, Lawrence Berkeley National Laboratory, University of California, Berkeley, 94720, CA, United States
ASIAA, Taipei, 10617, Taiwan
Department of Physics, National Taiwan University, Taipei, 10617, Taiwan
Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, 02138, MA, United States
Center for Computational Astrophysics, Flatiron Institute, 162 5th Ave, New York, 10010, United States
Department of Physics, Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, 02139, MA, United States
DIPC
University of the Basque Country UPV/EHU
Ikerbasque
Hong Kong University of Science and Technology
Energetic Cosmos Laboratory
Paris Centre for Cosmological Physics
Department of Physics
ASIAA
Department of Physics
Center for Astrophysics
Center for Computational Astrophysics
Department of Physics
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