Dark Matter Surrounded Quartic Square-root Horndeski Black Hole: Thermodynamics, Optical Properties and Quasinormal Oscillations

Abstract: In this work, we study a special form of Horndeski solutions, viz. quartic "square-root" Horndeski black hole immersed in a perfect-fluid dark-matter halo, by examining its thermodynamics, null geodesic shape, optical shadow, and quasinormal ringdown spectrum. The model is characterized by three parameters, namely $\beta$, $\eta$ (non-minimal Horndeski coupling parameters), and $b$ (perfect fluid dark matter parameter), which collectively determine horizon properties and observational effects. To study thermodynamic stability, we used the specific heat and free energy arguments, with which we demonstrated that small horizon states are locally stable but are never globally preferred. Analytic solutions of null geodesics reveal the radius of the photon sphere and the critical impact parameter, proving that increases in the dark matter parameters and the Horndeski parameter $\beta$ enlarge both the photon sphere and the subsequent shadow, whereas increase in the Horndeski coupling $\eta$ causes a mild diminishing in the shadow radius. Numerical ray tracing verifies these qualitative trends in the apparent shadow. Using the 6$^{th}$ - order WKB approximation method, we also calculate the scalar quasinormal modes and determine that oscillation frequencies and damping rates behave oppositely according to the sign and magnitude of each parameter. By comparing the shadow radius with the recent Event Horizon Telescope constraints on the Sgr A*, we find a narrow window of parameter space that agrees with observed data. In other words, the coupling parameters should be very small. These measurements restrict modified gravity impacts within realistic astrophysical contexts.