Supermassive black hole in NGC 4649 (M60) with a dark matter halo: impact on shadow measurements and thermodynamic properties

Abstract

We investigate black hole (BH) solutions embedded in a dark matter (DM) halo, modeled as extensions of the Schwarzschild metric. The DM density profile is constrained by Hubble Space Telescope (HST) data, stellar dynamics, and globular cluster (GC) measurements of the elliptical galaxy NGC 4649 (M60). Using this profile, we construct two distinct spacetime solutions characterized by the black hole mass (M), critical velocity (Vc), and core radius (a), all reducing to the Schwarzschild case in the limit Vc = 0 and a = 0. Our results show that the DM halo modifies essential BH features, such as the event horizon radius and spacetime curvature, as reflected in the Kretschmann scalar. We also derive an approximate analytical expression for the BH shadow radius, which increases slightly due to the halo's influence. Comparisons with two observational datasets further validate the analysis. Thermodynamic properties are examined across the two models. In the first, a generalized Smarr formula is obtained via two additional variables, γ and a. The second incorporates halo effects through Vc and a. These results underscore the role of DM in shaping both geometric and thermodynamic aspects of BHs.