4.2.2. The Mid-Infrared Continuum
In addition to the AFE, there is clear emission bridging between them, even in the 10 µm, trough. This emission may well be related to the AFE carriers, since its shape is constant, scaling with the AFE strength. Boulanger et al. (1998) have discussed this emission in terms of Lorentzian wings to the AFE.
Less energetic but more surprising is the continuum detected in ISO-PHT-S data (Helou et al. 2000) shortward of 5 µm, (see for instance the model spectra of Désert et al. 1990). This unexpectedly strong 4 µm, continuum flux density is positively correlated with the AFE flux, strong evidence linking it to dust rather than stellar photospheres. It appears to follow a power law f +0.65 between 3 and 5 µm, with an uncertainty of 0.15 on the power-law index. However, the flux density f around 10 µm is three times higher than the continuum level extrapolated to 10 µm from the spectral shape between 3 and 5 µm, leaving open the nature of the connection between the 4 µm, continuum and the carriers of the AFE. Bernard et al. (1994) have reported evidence for continuum emission from the Milky Way ISM in COBE-DIRBE broad-band data at these wavelengths, with comparable amplitude; ISO however provides the first clear detection of the spectral shape of the emission.
Extrapolating the continuum from the 3 to 5 µm, range out to longer wavelengths, and assuming the AFE are superposed on it, one finds that the continuum contributes about a third of the luminosity between 3 and 13 µm, and 10% between 6 and 13 µm, the balance being due to AFE and associated bridge emission. Against this extrapolated continuum, the AFE, defined again as the emission from 5.8 to 6.6 µm, 7.2 to 8.2 µm, and 8.2 to 9.3 µm , would have equivalent widths of about 4 µm or 3.4 x 1013 Hz, 18 µm or 9.2 x 1013 Hz, and 13 µm or 4.9 x 1013 Hz, respectively (Helou et al. 2000).
The natural explanation for this continuum is a population of small grains transiently heated by single photons to apparent temperatures near 1000 K. Such a population was invoked by Sellgren et al. (1984) to explain the 3 µm emission in reflection nebulae, and similar populations by other authors to explain the IRAS 12 µm emission in the diffuse medium (e.g. Boulanger et al. 1988). Small particles with ten to a hundred atoms have sufficiently small heat capacities that a single UV photon can easily propel them to 1000 K equivalent temperature (Draine & Anderson 1985). Such a population is a natural extension of the AFE carriers, though it is not clear from these data whether it is truly distinct, or whether the smooth continuum is simply the non-resonant emission from the AFE carriers. While the current data cannot rule out other contributions to this continuum component, the shape does rule out a simple extension of the photospheric emission from main sequence stars. Red supergiants and Asymptotic Giant Branch stars may contribute to the continuum in this region but cannot dominate it, because their contribution would not correlate with AFE from the ISM, and cannot match the shape of a spectrum with f +0.65.