Spitzer/IRAC and ISOCAM/CVF insights on the origin of the near to mid-IR Galactic diffuse emission

Abstract

Spitzer/IRAC images of extended emission provide a new insight on the nature of small dust particles in the Galactic diffuse interstellar medium. We measure IRAC colors of extended emission in several fields covering a range of Galactic latitudes and longitudes outside of star forming regions. We determine the nature of the Galactic diffuse emission in Spitzer/IRAC images by combining them with spectroscopic data. We show that PAH features make the emission in the IRAC 5.8 and 8.0 μm channels, whereas the 3.3 μm feature represents only 20 to 50% of the IRAC 3.6 μm channel. A NIR continuum is necessary to account for IRAC 4.5 μm emission and the remaining fraction of the IRAC 3.6 μm emission. This continuum cannot be accounted by scattered light. It represents 9% of the total power absorbed by PAHs and 120% of the interstellar UV photon flux. The 3.3 μm feature is observed to vary from field-to-field with respect to the IRAC 8.0 μm channel. The continuum and 3.3 μm feature intensities are not correlated. We present model calculations which relate our measurements of the PAHs spectral energy distribution to the particles size and ionization state. Cation and neutral PAHs emission properties are inferred empirically from NGC 7023 observations. PAHs caracteristics are best constrained in a line of sight towards the inner Galaxy, dominated by the Cold Neutral Medium phase: we find that the PAH cation fraction is about 50% and that their mean size is about 60 carbon atoms. A significant field-to-field dispersion in the PAH mean size, from 40 to 80 carbon atoms, is necessary to account for the observed variations in the 3.3 μm feature intensity relative to the IRAC 8.0 μm flux. However, one cannot be secure about the feature interpretation as long as the continuum origin remains unclear. The continuum and 3.3 μm feature emission process could be the same even if they do not share carriers.