The discrepancy in the mid-infrared continuum and the features of polycyclic aromatic hydrocarbons for Spitzer and Herschel SWIRE-field galaxies

On the basis of observations by Spitzer and Herschel, we present and analyse the correlations between various monochromatic infrared (IR) luminosities for star-forming galaxies, selected from two northern Spitzer Wide-area InfraRed Extragalactic Survey (SWIRE) fields. The 24- and 70-mu m luminosities (L[24] and L[70]), which are dominated by the continuum of very small grains (VSGs) and warm dust in thermal equilibrium, respectively, correlate tightly with ongoing star formation. The contribution from cool dust excited by evolved stars also increases as the wavelength increases in the far-infrared (FIR) wavelength range. The spectral features of ionized polycyclic aromatic hydrocarbons (PAHs) around rest-frame 8 mu m are excited by the moderated radiation field related to evolved stars as well, rather than by the intensive radiation field related to young stars. Even though the carriers of PAHs could be treated as types of VSG with a smaller scale, the radiation condition between PAHs and classic VSGs seems to be significantly different. The formulae to calculate the total infrared luminosity L-TIR using L[8(dust)] and L[24] are re-scaled and we find that the L[8(dust)] (L[24]) formula likely underestimates (overestimates) L-TIR for galaxies with unusualcurrent star formation activity.