Black holes surrounded by Einstein clusters as models of dark matter fluid

Abstract

We construct a novel class of spherically symmetric and asymptotically flat black holes and naked singularities surrounded by anisotropic dark matter fluid with the equation of state (EoS) of the form $P_t=\omega \rho$. We assume that dark matter is made of weakly interacting particles orbiting around the supermassive black hole in the galactic center and the dark matter halo is formed by means of Einstein clusters having only tangential pressure. In the large distance from the black hole, we obtain the constant flat curve with the upper bound for the dark matter state parameter $\omega \lesssim 10^{-7}$. To this end, we also check the energy conditions of the dark matter fluid outside the black hole/naked singularity, the deflection of light by the galaxy, and the shadow images of the Sgr A$^\star$ black hole using the rotating and radiating particles. For the black hole case, we find that the effect of dark matter fluid on the shadow radius is small, in particular the angular radius of the black hole is shown to increase by the order $10^{-4} \mu$arcsec compared to the vacuum solution. For the naked singularity we obtain significantly smaller shadow radius compared to the black hole case. Finally, we study the stability of the S2 star orbit around Sgr A$^\star$ black hole under dark matter effects. It is argued that the motion of S2 star orbit is stable for values $\omega \lesssim 10^{-7}$, however further increase of $\omega $ leads to unstable orbits. Using the observational result for the shadow images of the Sgr A$^\star$ reported by the EHT along with the tightest constraint for $\omega$ found from the constant flat curve, we show that the black hole model is consistent with the data while the naked singularity in our model can be ruled out.