Synchrotron spectral index and its relationship with star formation and magnetic field strength. Top: Synchrotron spectral index obtained at different frequency intervals (α_{[0.1 − 1.4]}, α_{[1.4 − 4.8]} and α_{[0.1 − 4.8]}) versus star formation surface density Σ_SFR (M_⊙ yr⁻¹ kpc⁻²). Lines show bisector OLS fits. Middle: Same spectral indices versus the total magnetic field strength. Bottom: Synchrotron spectral index α_{[1.4 − 4.8]} versus magnetic field after compensating both for the effect of the SFR.