Matter environments around black holes: geodesics, light rings and ultracompact configurations

TL;DR

This paper studies how surrounding matter, modeled as dark matter distributions, influences black hole spacetime features, geodesic structures, and gravitational wave signals, revealing significant environmental effects on observable phenomena.

Contribution

It provides a comprehensive analysis of matter effects on black hole geometry, geodesics, and ringdown signals using self-consistent models with various density profiles, including ultracompact configurations.

Findings

Environmental matter shifts ISCO inward and light rings outward.

Additional light rings and stable orbits can emerge at high compactness.

Matter effects produce observable signatures like echoes in gravitational wave signals.

Abstract

Astrophysical black holes are invariably embedded in matter environments whose gravitational influence can alter key strong-field features of the spacetime. In this work, we investigate the impact of spherically symmetric dark-matter distributions on black hole geometry, geodesic structure, and ringdown phenomenology. Modeling the surrounding matter through Einstein clusters, we construct self-consistent spacetimes for three widely used density profiles - the Hernquist, Navarro-Frenk-White (NFW), and Jaffe models - and examine how their near-horizon behavior modifies the location and stability of circular timelike and null geodesics, including the innermost stable circular orbit (ISCO) and light rings. In the low-compactness regime, we derive analytical expressions showing that environmental effects generically shift the ISCO inward and the principal light ring outward, leading to parametric deviations in their associated orbital frequencies and Lyapunov exponents. At higher compactness, we explore the emergence of additional light rings, marginally stable orbits, and secondary horizons, identifying the regions of parameter space in which these ultracompact configurations arise. Using time-domain evolutions of scalar perturbations, we demonstrate how such structures can imprint characteristic signatures on the ringdown signal, including long-lived trapped modes and echo-like modulations associated with multiple potential barriers. Our results provide a unified framework for assessing environmental effects around black holes and highlight the importance of matter-induced corrections for interpreting upcoming electromagnetic and gravitational-wave observations.