Polarized image of a rotating black hole surrounded by a cold dark matter halo

TL;DR

This study investigates how a cold dark matter halo influences the polarized image of a rotating black hole with an emitting ring, revealing subtle effects on polarization intensity and EVPA that depend on halo parameters and magnetic field configuration.

Contribution

It provides a detailed analysis of the impact of cold dark matter halos on polarized black hole images, considering various magnetic and velocity configurations, which was not previously explored.

Findings

CDM halo density affects polarization similarly to halo radius.

Polarization intensity decreases with increasing CDM parameters for equatorial magnetic fields.

The EVPA change becomes more complex with the presence of a CDM halo.

Abstract

We have studied the polarized image of an equatorial emitting ring around a rotating black hole surrounded by a cold dark matter (CDM) halo. Results show that the CDM halo density has the similar effects of the halo's characteristic radius on the polarized image for the black hole. The effects of the CDM halo on the polarized image depend on the magnetic field configuration, the fluid velocity and the observed inclination. With the increase of the CDM halo parameters, the observed polarization intensity decreases when the magnetic field lies in equatorial plane, but in the case where the magnetic field is perpendicular to the equatorial plane, the change of the observed polarization intensity with CDM halo also depends on the position of the emitting point in the ring. The change of the electric vector position angle (EVPA) with the CDM halo becomes more complicated. Our results also show that the influence of the CDM halo on the polarized image is generally small, which are consistent with the effects of dark matter halo on black hole shadows. These results could help to further understand dark matter from black hole images.