Constraints on asteroid-mass primordial black holes in dwarf galaxies using Hubble Space Telescope photometry

¹ Service de Physique Théorique, Université Libre de Bruxelles, Boulevard du Triomphe, CP225, 1050 Brussels, Belgium
² Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Ave, New York, NY 10010, USA
³ Department of Physics and Astronomy, Ghent University, Krijgslaan 281, 9000 Ghent, Belgium
⁴ Department of Astronomy, The University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA
⁵ Department of Physics & Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA

^⋆ Corresponding author; nicolas.esser@ulb.be

Received: 21 March 2025
Accepted: 17 May 2025

Abstract

Primordial black holes (PBHs) in the asteroid-mass range remain a viable and, until now, unconstrained dark matter (DM) candidate. If such PBHs exist, they could be captured by stars in DM-dominated environments with low velocity dispersion, such as ultra-faint dwarf galaxies (UFDs). The capture probability increases with the stellar mass, and captured PBHs would rapidly destroy their host stars. As a result, the presence of PBHs in UFDs would alter their stellar mass functions. Using photometric observations of three UFDs from the Hubble Space Telescope, we show that it is unlikely that their mass functions have been significantly modified by PBHs, and we place constraints on the PBH abundance. In the UFD Triangulum II, PBHs around 10¹⁹ g are excluded at the 2σ (3σ) level from constituting more than ∼55% (∼78%) of the DM, while the possibility that PBHs represent the entirety of the DM is excluded at the 3.7σ level.