Investigating effects of dark matter on photon orbits and black hole shadows

Abstract

We consider Kerr black holes (BHs) surrounded by perfect dark fluid matter (PFDM), with an additional parameter (k) because of PFDM, apart from mass (M) and rotation parameter (a) — the rotating PFDM BHs. We analyse the photon orbits around PFDM BHs and naked singularities (NSs) and emphasize the effect of PFDM on photon boomerangs. Interestingly, the azimuthal oscillations first increase and then decrease for retrograde orbits, whereas they first decrease and then increase for prograde orbits, with increasing k. Unlike in the Kerr NSs, photon boomerangs can form around rotating PFDM NSs. We use the Event Horizon Telescope (EHT) observational results for Schwarzschild shadow deviations of M87* and Sgr A*, δM87∗=−0.01±0.17 and δSgrA∗=−0.08−0.09+0.09(VLTI),−0.04−0.10+0.09(Keck), to report the upper bounds on the PFDM parameter: 0≤k≤0.0792M and kmax∈[0.0507M,0.0611M] respectively. Together with the EHT bounds on the shadows of Sgr A∗ and M87∗, our analysis concludes that a substantial part of the rotating PFDM BH parameter space agrees with the EHT observations. Thus, one must consider the possibility of the rotating PFDM BHs being strong candidates for the astrophysical BHs.