EHT constraint on the ultralight scalar hair of the M87 supermassive black hole

TL;DR

This paper uses Event Horizon Telescope data to place constraints on the presence of ultralight scalar hair around the M87 black hole, finding that such hair, if it exists, is consistent with current observations within error margins.

Contribution

It introduces a method to constrain ultralight scalar hair on supermassive black holes using EHT shadow observations, considering dynamical viability and superradiance effects.

Findings

EHT observations weakly constrain scalar hair around M87

Typical dynamically developed hairy black holes are compatible with EHT data

Scalar field mass around 10^{-20} eV remains plausible for M87 black hole

Abstract

Hypothetical ultralight bosonic fields will spontaneously form macroscopic bosonic halos around Kerr black holes, via superradiance, transferring part of the mass and angular momentum of the black hole into the halo. Such process, however, is only efficient if resonant: when the Compton wavelength of the field approximately matches the gravitational scale of the black hole. For a complex-valued field, the process can form a stationary, bosonic field-black hole equilibrium state - a black hole with synchronised hair. For sufficiently massive black holes, such as the one at the centre of the M87 supergiant elliptic galaxy, the hairy black hole can be robust against its own superradiant instabilities, within a Hubble time. Studying the shadows of such scalar hairy black holes, we constrain the amount of hair which is compatible with the Event Horizon Telescope (EHT) observations of the M87 supermassive black hole, assuming the hair is a condensate of ultralight scalar particles of mass $\mu\sim 10^{-20}$ eV, as to be dynamically viable. We show the EHT observations set a weak constraint, in the sense that typical hairy black holes that could develop their hair dynamically, are compatible with the observations, when taking into account the EHT error bars and the black hole mass/distance uncertainty.