Constraining the magnetic field strength and nonthermal electron injection of the M 87 jet. Left: Modeled spectral index distributions constrained by the observations. The distributions must lie in the gray shaded allowed region (see Sect. 4.4 for details). Colored lines are the modeled spectral index distributions. Different colors represent different initial magnetic field strengths, B_i. The dashed and dotted red line is a modeled spectrum index distribution with q = 10 and q = 11, respectively, with B_i = 0.3 G. Right: Parameter space (B_i, q). The black region is the allowed values of B_i and q constrained by the observations.