Optical appearance of black holes surrounded by a dark matter halo

Abstract

Black holes in General Relativity are described by space-time metrics that are simpler in comparison to non-vacuum compact objects. However, given the universality of the gravitational pull, it is expected that dark matter accumulates around astrophysical black holes, which can have an impact in the overall gravitational field, especially at galactic centers, and induce non-negligible effects in their observational imprints. In this work, we study the optical appearance of a spherically symmetric black hole both when orbited by isotropically emitting light sources and when surrounded by a (geometrically and optically thin) accretion disk, while immersed in a dark matter halo. The black hole geometry plus the dark matter halo come as a solution of Einstein's field equations coupled to an anisotropic fluid whose density component follows a Hermquist-type distribution. We analyze in some depth the circular geodesic structure in both perturbative and non-perturbative regimes, investigating particular possible consequences for the structure of accretion disks. Despite this, however, even in situations in which the geodesic description differs profoundly from the isolated black hole case, we find minor modifications to the primary and secondary tracks of the isotropic orbiting sources, and to the width, location, and relative luminosity of the corresponding photon rings as compared to the Schwarzschild black hole at equal black hole mass and emission models. This shows that physical structures are crucial for understanding black hole images and points the limitations of drawing conclusions from more artificial imaging profiling. More profoundly, this fact points towards troubles distinguishing between both geometries using present observations of very-long baseline interferometry.