Superradiance of charged black holes embedded in dark matter halos

Abstract

Astrophysical environments are ubiquitous in the Universe; from accretion disks around black holes to galactic dark matter halos, distributions of astrophysical material veil the vast majority of Cosmos. Including environmental effects in strong-field gravity and astrophysics is, therefore, a rather tantalizing task in the quest for novel gravitational-wave phenomena. Here, we examine how environments affect the high-energy process of superradiance. In particular, we study the amplification of charged scalar waves under the expense of the electrostatic energy contained in a charged black hole that is embedded in an observationally-motivated, and qualitatively generic, dark matter halo. We find that the superradiant amplification of massless charged scalar fields scattering off environmentally-enriched charged black holes can be equally efficient to those occurring in vacuum charged black holes. This occurs due to the fact that the sole interplay between the scalar wave and the black hole is the electromagnetic interaction. The addition of mass on the charged scalar waves leads to a rapid suppression of superradiant amplification. This transpires to a great extent due to the `friction' that the mass introduces to the black hole potential. Nevertheless, for sufficiently large scalar masses, the amplification factors can be also subdominantly affected by the compactness of the halo. This occurs because the gravitational interaction between the dense halo and the wave's mass grows, thus further suppressing the superradiant amplification.