A new study sheds light on how dark matter halos influence the physical properties and observational characteristics of accretion disks surrounding black holes. Led by the Yunnan Observatories of the Chinese Academy of Sciences, in collaboration with Guizhou University, the research was recently published in The European Physical Journal C.

In 2019, the Event Horizon Telescope (EHT) captured the first image of the supermassive black hole in the M87 galaxy, revealing a bright emission ring encircling a central shadow. The ring originates from synchrotron radiation in the hot accretion flow, while the shadow denotes the region where photons are trapped. The size, shape, and brightness pattern of these features depend on the black hole's mass, spin, viewing angle, and the properties of the surrounding matter—carrying key insights into accretion dynamics, magnetic fields, and the potential presence of dark matter halos.

The new study systematically investigates the radiative properties and observational images of geometrically thin accretion disks in strong gravitational fields. These disks surround Schwarzschild black holes embedded within Dehnen-type dark matter halos. The findings confirm that dark matter distribution exerts a measurable impact on accretion processes and the observation of black hole shadows.

Building on the classical Novikov–Thorne model, the research team analyzed changes in the accretion disk's energy flux, temperature distribution, and emission spectrum when a black hole is embedded in a dark matter halo with specific density parameters (ρ_s) and scale radii (r_s).

Results show that as ρ_s and r_s increase, the overall radiation intensity and temperature of the accretion disk decrease significantly. The underlying physical mechanism lies in the additional gravitational potential provided by dark matter: this weakens the gravitational binding energy released during accretion, causing the disk to cool and dim.

A key discovery is that dark matter dramatically alters the observational image of black holes in strong gravitational fields. As ρ_s and r_s rise, both the primary and secondary images of the accretion disk expand outward horizontally and vertically—an effect that becomes particularly pronounced at high inclination angles. This implies that the apparent size of the black hole's shadow and photon rings grows due to the presence of the dark matter halo.

The study further indicates that, under fixed conditions of black hole mass, accretion rate, and observation angle, dark matter halo properties leave observable, distinct signatures on the accretion disk's thermodynamic behavior and visual morphology. These signatures manifest as two key signals: first, a systematic reduction in the accretion disk's radiative flux and temperature under identical conditions; second, an anisotropic increase in the apparent size of the black hole's shadow and photon ring, especially notable in systems with high inclination angles.

These theoretical predictions offer a new method for detecting dark matter density distribution around supermassive black holes at galactic centers using high-resolution observational instruments like the EHT. By comparing EHT imaging and spectral data with theoretical models that incorporate dark matter effects, future observations are expected to effectively constrain or detect the density profile of dark matter in galactic centers.

The research received support from the Basic Science Center of the National Natural Science Foundation of China, the National Key R&D Program of China, and other funding sources.

Full apparent images of thin accretion disks at different inclination angles. (Image by LI Zhi)