code: def density_2d(self, x, y, rho0, gamma, center_x=0, center_y=0):
          
          x_ = x - center_x
          y_ = y - center_y
          r = np.sqrt(x_**2 + y_**2)
          sigma = np.sqrt(np.pi) * special.gamma(1./2*(-1+gamma))/special.gamma(gamma/2.) * r**(1-gamma) * rho0
          return sigma
docs: projected density
          :param x:
          :param y:
          :param rho0:
          :param a:
          :param s:
          :param center_x:
          :param center_y:
          :return: