Shadows and lensing signatures of a rotating black hole in a Hernquist dark matter halo

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Original abstract

We investigate the optical properties of a rotating black hole immersed in a Hernquist dark matter halo. The spacetime is generated from a static Hernquist black hole through the noncomplexification version of the Newman-Janis procedure, yielding a Kerr-like geometry whose halo contribution is encoded in the radial function $Δ(r)$ \cite{AraujoFilho:2026hernquist}. We derive the null geodesic equations, effective potentials, radial acceleration, and representative three-dimensional photon trajectories around the event horizon and ergoregion. Using the separability of the Hamilton-Jacobi equation, we obtain the critical impact parameters of unstable spherical photon orbits and construct the shadow contours for a distant observer. The rotation parameter mainly shifts and distorts the shadow, whereas the Hernquist halo enlarges the photon capture region and increases the apparent shadow size. Comparing the area-equivalent shadow diameter with the Event Horizon Telescope measurements of Sgr A$^\ast$ and M87$^\ast$, we constrain the dimensionless halo parameter $\hatρ=M^2ρ$. The strongest restriction comes from Sgr A$^\ast$, giving $\hatρ\sim(2.7-3.8)\times10^{-3}$ at $1σ$ and $\hatρ\sim(4.1-5.2)\times10^{-3}$ at $2σ$. We also analyze strong- and weak-field gravitational lensing. In the strong-field regime, the halo shifts the unstable photon orbit and critical impact parameter, controlling the logarithmic deflection angle and the position of relativistic images. In the weak-field regime, the halo contributes already to the leading bending angle and enhances deviations from Kerr as $ρ$ grows. From the Einstein ring of ESO325-G004, we further obtain $0\leq\hatρ\lesssim0.00939$ at $1σ$ and $0\leq\hatρ\lesssim0.01963$ at $2σ$.

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