# Black Hole Space-time In Dark Matter Halo

## TL;DR

This paper derives analytical black hole space-time metrics within dark matter halos, revealing effects on horizons and cusps, and providing models for different dark matter types to aid astrophysical studies.

## Contribution

It presents the first analytical solutions for black holes in dark matter halos, including rotating cases, and analyzes their physical properties and observational implications.

## Key findings

- Dark matter halos cause small increases in black hole horizons.
- Cusps form in dark matter density profiles at small scales.
- Dark matter does not alter black hole singularities.

## Abstract

For the first time, we obtain the analytical form of black hole space-time metric in dark matter halo for the stationary situation. Using the relation between the rotation velocity (in the equatorial plane) and the spherical symmetric space-time metric coefficient, we obtain the space-time metric for pure dark matter. By considering the dark matter halo in spherical symmetric space-time as part of the energy-momentum tensors in the Einstein field equation, we then obtain the spherical symmetric black hole solutions in dark matter halo. Utilizing Newman-Jains method, we further generalize spherical symmetric black holes to rotational black holes. As examples, we obtain the space-time metric of black holes surrounded by Cold Dark Matter and Scalar Field Dark Matter halos, respectively. Our main results regarding the interaction between black hole and dark matter halo are as follows: (i) For both dark matter models, the density profile always produces "cusp" phenomenon in small scale in the relativity situation; (ii) Dark matter halo makes the black hole horizon to increase but the ergosphere to decrease, while the magnitude is small; (iii) Dark matter does not change the singularity of black holes. These results are useful to study the interaction of black hole and dark matter halo in stationary situation. Particularly, the "cusp" produced in the $0\sim 1$ kpc scale would be observable in the Milky Way. Perspectives on future work regarding the applications of our results in astrophysics are also briefly discussed.