Ultralight dark matter mixed with primordial black holes

Abstract

Dark matter candidates span many orders of magnitude in mass, from ultralight bosonic fields to massive compact objects. In this work, we connect these two extremes by investigating ultralight dark matter (ULDM) mixed with primordial black holes (PBHs). We study mixed ULDM-PBH halos by separating the continuum PBH contribution from the shot-noise fluctuation generated by discrete PBHs. The continuum contribution enters the averaged Schr\"odinger-Poisson background, while the discreteness contribution is treated as a perturbation that induces ULDM eigenmode transitions and soliton heating. The two contributions have distinct parametric dependencies: continuum effects scale with PBH fraction, whereas discreteness-driven transition rates scale with the product of PBH fraction and individual PBH mass in the perturbative regime. For a fiducial mixed halo with ULDM particle mass $10^{-22}\,\mathrm{eV}$, virial mass of order $10^{10}\,M_{\odot}$, and PBH fraction $1\%$, the continuum PBH component modifies the background density, gravitational potential, and low-lying ULDM eigenvalues only at the sub-percent level. Nevertheless, this percent-level continuum PBH contribution produces a tens-of-percent response in the coherent soliton region, changing the radial mode participation by about $20\%$. For stellar-mass PBHs, the discrete shot-noise fluctuation induces extremely slow ULDM mode transitions, with the fastest low-lying multiplet transition having a timescale of order $10^9\,\mathrm{Gyr}$ for solar-mass PBHs. In this regime, the leading PBH effect is the continuum contribution, while discrete PBH shot noise is dynamically negligible on galactic timescales.