Observational Signatures of Scalarized Black Holes in Gauss-Bonnet Gravity: Clean Shadow and Ringdown Delay

We investigate observational signatures of static spherically symmetric black holes with a scalar field coupled to the Gauss-Bonnet term. For βₐₘ = -5 × 10⁻³ and φₕ/MPl = 0.020, full ray-tracing yields a photon sphere at rₚₕ = 3.010M and impact parameter bₚₕ = 5.071M, giving a shadow diameter deviation of only $0.048%$ from Schwarzschild. The dynamical properties are significantly altered: the Lyapunov exponent drops to λ M = 0.112 versus $0.1910$ in General Relativity, increasing the instability timescale τ by $70.5%$. For M87* (M = 6.5 × 10⁹ M_), we predict τ = 249 ± 25 hours, a ringdown delay of ≈ 104 hours relative to GR. Crucially, sub-ring flux ratios Iₙ/I₀ ~ e-nπ/λ M are suppressed by factor >10⁵ for n ≥ 1, creating a "clean shadow" signature. We propose that ngEHT Phase-2 can test this model via light-curve autocorrelation of the $n=0$ ring: measuring τ/τKerr = 1.70 ± 0.10 favors this hairy solution, while detection of any sub-ring with I₁/I₀ > 10⁻⁹ or τ/τKerr = 1.00 ± 0.05 rules it out.