(a) Optical depth produced by nanoparticles in a column ﹩{N}_{{\rm{H}}}={10}^{21}\,{\mathrm{cm}}^{-2}﹩ of CNM. Each curve is for nanoparticles of the indicated size with total mass relative to H ﹩{X}_{\mathrm{gr}}=4\times {10}^{-4}﹩ and ﹩\mu =2000\,{\rm{D}}﹩. (b) Absorption signal for assumed synchrotron background of Equation (18).