Imprints of Dark Matter on the Shadow and Polarization Images of a Black Hole Illuminated by Various Thick Disks

TL;DR

This paper investigates how perfect fluid dark matter (PFDM) around a Schwarzschild black hole affects shadow and polarization images generated by two thick-disk models. By solving the null geodesics and a covariant radiative transfer equation for thermal synchrotron emission, the study reveals that the PFDM parameter $η$ enlarges the photon ring and central shadow, while the observer’s inclination $θ_o$ induces symmetry-breaking and vertical distortions, especially for anisotropic emission. The analysis contrasts a phenomenological RIAF-like disk with the Hou disk (BAAF) model, finding that disk thickness and emission anisotropy influence the prominence of higher-order images and the visibility of the inner horizon silhouette, with polarization patterns tracking brightness and reflecting spacetime signatures. Overall, the work demonstrates that horizon-scale intensity and polarization from thick disks can serve as probes of the underlying spacetime geometry near the horizon, offering potential observational handles on PFDM effects alongside model-dependent astrophysical complexities.

Abstract

Based on two distinct thick accretion flow disk models, such as a phenomenological RIAF-like model and an analytical Hou disk model, we investigate the impact of relevant parameters on the visual characteristics of the Schwarzschild black hole (BH) surrounded by perfect fluid dark matter (PFDM). We impose a general relativistic radiative transfer equation to determine the synchrotron emission from thermal electrons and generate horizon-scale images. In the RIAF-like model, we notice that the corresponding photon ring and central dark region are expanded with the aid of the PFDM parameter $η$, with brightness asymmetries originating at higher inclination angles and closely tied to flow dynamics and emission anisotropy. The fundamental difference between isotropic and anisotropic radiation is that anisotropy introduces vertical distortions in the higher-order images, resulting in an elliptical appearance. For the Hou disk model, the observed images produce narrower rings and dark interiors, while polarization patterns trace the brightness distribution and changes with the variations of the inclination angle and PFDM parameter $η$, which reflects the spacetime signature. All these results indicate that the observed intensity and polarization characteristics in the framework of thick disk models may serve as valuable probes of underlying spacetime geometry and the accretion-dynamics close to the horizon.

Figures (8)

• Figure 1: Effects of $\eta$ and $\theta_o$ on the RIAF model with isotropic radiation at the observation frequency of $230\,\mathrm{GHz}$, with the accretion flow in the infalling motion.
• Figure 2: The intensity cuts with $\theta_o=70^\circ$ and the accretion flow in the infalling motion. The red, green, and blue curves correspond to $\eta=0.1,\,0.3,\,0.5$, respectively.
• Figure 3: Effects of $\eta$ and $\theta_o$ on the RIAF model with anisotropic radiation at $230\,\mathrm{GHz}$, with the accretion flow in the infalling motion.
• Figure 4: The intensity cuts with $\theta_o=70^\circ$ and the accretion flow in the infalling motion. The red, green, and blue curves correspond to $\eta=0.1,\,0.3,\,0.5$, respectively.
• Figure 5: Effects of $\eta$ and $\theta_o$ on the HOU disk model at the observation frequency of $230\,\mathrm{GHz}$, with the accretion flow is the infalling motion.