4.3.1. The Global Infrared Spectrum
Turning now to the long wavelength end of the spectrum, photometry at 120-200 µm using ISO-PHOT is starting to constrain the distribution of dust temperatures at low heating levels, especially in nearby well resolved galaxies such as M31 (Haas et al. 1998, 1999), M51 or M 101 (Hippelein et al. 1996), where cold dust dominates the luminosity. Similar analysis on more active galaxies is also under way (e.g. Klaas et al. 1997; Klaas, Haas & Schulz 1999) to obtain the best possible estimates of the total infrared emission and therefore of the dust mass. Alton et al. (1998) have reported that the emission is more extended at 200 µm than at shorter wavelengths in several galaxies. This result however hinges closely on a very precise knowledge of the beam shapes at various wavelengths, which was not yet achieved at the time of that publication.
|
Figure 7. The synthesized model spectra of Dale et al. (2000a), showing the progression of shape from quiescent to active star-forming galaxies. The same set is plotted here as summarized in Table 1. The spectra are plotted so the Aromatic Features are scaled to the same amplitude just for ease of graph-reading, these features being the most stable part of the spectra. Note the march of the peak towards shorter wavelengths as activity increases. These synthesized spectra do not account for the general correlation between R(60, 100) and the infrared-to-visible light ratio. |
A proper determination of the total infrared luminosity and the
long-wavelength spectral shape in normal galaxies is critical to
estimating the contribution of galaxies to the infrared and submm
extragalactic background light, and thereby deriving the infrared term
in the star formation history of the Universe. While work
on the ISO-PHOT calibration continues, one could estimate an improved
infrared spectral energy distribution by combining the mid-infrared results
described above with existing IRAS data (Figure 7).
Such a rough estimation is presented
in Table 1 for several different levels of
activity in galaxies, parametrized
by the 60-to-100 µm ratio. It should be noted that all information
at 
> 100 µm in
Table 1 is based on modeling IRAS data using a
power-law distribution of dust mass as a function of heating intensity,
and does not use any empirical constraint
(Dale et al. 2000a;
Helou et al. 2000).
Table 1 illustrates the general trend, but also
ignores variations in spectral shape at constant 60-to-100
µm ratio,
including intrinsic scatter in the ratio of mid-infrared to far-infrared
(Lu et al. 1999),
and dispersion in the 25-to-60 µm ratio.
The 20 to 40 µm range appears to show the most significant
growth in fractional terms at the expense of the submillimeter
as the activity level increases, suggesting that the 20-40
µm continuum
may be the best dust emission tracer of current star formation in galaxies.
Even after ISO, our knowledge of the detailed
shape of the spectrum at >
100 µm remains model-dependent, and may not
improve significantly until the launch of SIRTF, and then of FIRST (Far
infraRed and Submillimeter Telescope).
f (60 µm)/f(100 µm)
| F(3 - 5 µm) | F(5 - 13 µm) | F(13 - 20 µm) | F(20 - 42 µm) | F(42 - 122 µm) | F(122 - 1100 µm) |
| 0.40 | 0.024 | 0.122 | 0.033 | 0.10 | 0.41 | 0.32 |
| 0.63 | 0.017 | 0.086 | 0.037 | 0.18 | 0.50 | 0.19 |
| 1.00 | 0.008 | 0.048 | 0.043 | 0.28 | 0.54 | 0.09 |