Dependence of relative central surface density (ΔΣ₁) on spiral arm strength (2 + log s_arm). The bin color encodes the galaxy number in each bin. Σ₁ is the stellar surface density within the central 1 kpc, while the log ΔΣ₁ is calculated via log ΔΣ₁ = logΣ₁ + 0.275(log M_*)² − 6.445log M_* + 28.059, which can be used to distinguish pseudo bulges (log ΔΣ₁ <  0) from classical bulges (log ΔΣ₁ ≥ 0). The Pearson correlation coefficient is denoted at the top.