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 micron 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 micron 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 unusual current star
formation activity.
