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Open AccessArticle

EHT Constraint on the Ultralight Scalar Hair of the M87 Supermassive Black Hole

by Pedro V. P. Cunha 1,2,3,*,†, Carlos A. R. Herdeiro 1,2,3,† and Eugen Radu 1,2,†
Departamento de Matemática da, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal
Centre for Research and Development in Mathematics and Applications (CIDMA), Campus de Santiago, 3810-183 Aveiro, Portugal
Centro de Astrofísica e Gravitação—CENTRA, Departamento de Física, Instituto Superior Técnico—IST, Universidade de Lisboa—UL, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Universe 2019, 5(12), 220;
Received: 20 September 2019 / Revised: 19 November 2019 / Accepted: 23 November 2019 / Published: 27 November 2019
(This article belongs to the Special Issue Gravitational Lensing and Optical Geometry: A Centennial Perspective)
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 a 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 μ 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. View Full-Text
Keywords: ultralight particles; black hole shadow; event horizon telescope ultralight particles; black hole shadow; event horizon telescope
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Cunha, P.V.P.; Herdeiro, C.A.R.; Radu, E. EHT Constraint on the Ultralight Scalar Hair of the M87 Supermassive Black Hole. Universe 2019, 5, 220.

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