Infalling ultra-faint dwarfs as emissaries of the Axiverse

¹ DIPC, Basque Country UPV/EHU, E-48080 San Sebastian, Spain
² University of the Basque Country UPV/EHU, Department of Theoretical Physics, Bilbao E-48080, Spain
³ ASIAA, Taipei 10617, Taiwan
⁴ Department of Physics, National Taiwan University, Taipei 10617, Taiwan
⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Center for Computational Astrophysics, Flatiron Institute, 162 5th Ave, New York, NY 10010, USA
⁷ Ikerbasque, Basque Foundation for Science, Bilbao E-48011, Spain
⁸ Hong Kong University of Science and Technology, Institute for Advanced Study and Department of Physics, IAS TT & WF Chao Foundation Professor, Hong Kong
⁹ Energetic Cosmos Laboratory, Nazarbayev University, Nursultan, Kazakhstan
¹⁰ 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, 75205 Paris CEDEX 13, France
¹¹ Department of Physics and Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA
¹² Department of Physics, Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

^⋆ Corresponding authors: alvaro.pozolarrocha@bizkaia.eu; tom.j.broadhurst@gmail.com

Received: 1 July 2025
Accepted: 1 October 2025

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.