Probing Hernquist dark matter with black hole shadows: A comprehensive study of various accretions

Abstract: The appearance of a black hole shadow, in both astronomical observations and theoretical analysis, depends critically on the properties of the surrounding accretion. In this study, we analyse the emission and observational properties of a Schwarzschild black hole surrounded by a Hernquist dark matter (DM) halo by considering three simplified accretion models, which have not been previously explored in such detail. Based on our findings, direct emission contributes markedly to the whole observed intensity for a Schwarzschild-Hernquist black hole encircled by a geometrically thin accretion disk. The corresponding regions and measured intensities of the lensing ring, the photon ring, and direct emission are sensitive to the parameters of the Hernquist DM halo. Apart from disk accretion, we also study two spherical accretion models: static and infalling. With increasing Hernquist DM parameters, both models exhibit a significant decrease in the measured intensity, yet the photon spheres are enlarged by an increase of $\sim2\%$ to $\sim30\%$. Furthermore, Doppler de-boosting due to the infalling accretion results in darker shadow images compared to static cases. By precising visualisations of the shadows, our results suggest that the observational characteristics of black hole shadows can serve as a powerful probe for distinguishing between different DM distributions around supermassive black holes.