Annu. Rev. Astron. Astrophys. 1992. 30: 653-703 Copyright © 1993 by Annual Reviews. All rights reserved

4.3 Confusion by Galactic Free-Free Emission

Far less is known, away from the Galactic plane, about the Galactic free free emission than about the Galactic synchrotron emission. The combination of observations of pulsar dispersion measures and recombination lines, particularly H 6563 Å observations along the line of sight to pulsars (Reynolds 1984), make it possible to determine the average electron density, < n_e >, in the ionized regions and the fraction, f, of the line of sight occupied by these regions. At high Galactic latitudes the dominant component of the interstellar medium contributing to free free emission is the ``warm ionized medium'' (e.g. Kulkarni & Heiles 1988), which has an electron temperature T_e = 5400-8000 K (Reynolds 1989). Reynolds (1984) finds that the H intensity in Rayleighs, as a function of Galactic latitude, b, is well represented by I(b) = (0.5-1.7) cosec (b) for |b| 10°. The emission measure is EM(cm^-6 pc) = 2.75 T_{e, 4}^0.9 I(b) (Reynolds 1977), so that for T_{e, 4} = 0.7 we have EM = (1.0-3.4) cosec (b) cm^-6 pc. The brightness temperature is given by T_b ~ 0.6 T_e^{-1/2 ^-2.1} EM K, where is in GHz. Thus T_b ~ (7-25) ^-2.1 cosec (b) mK. At a frequency of 30 GHz, for example, T_b ~ (6-20) cosec (b) µK. Thus for 10% fluctuations the free free emission will only begin to pose serious problems as sensitivities approach 10^-6 in T/T, but they could be lurking just beneath the present sensitivity levels if the fractional fluctuations in free free emission are large. The ratio of the specific intensity of Galactic free free emission to that of the microwave background radiation is given by:

(11)

Thus, as in the case of the Galactic synchrotron emission (see Equation 10), it should be possible to discriminate between the free free foreground emission and the microwave background radiation by means of multi-frequency observations. The simulations mentioned in the previous section would very likely have worked equally well for subtracting out a contribution of Galactic free free emission, or, indeed, for a combination of synchrotron and free free emission. It will be far more difficult to discriminate between these two foregrounds because their spectral indices are more similar than they are, in either case, to the microwave background radiation. However, for the purposes of microwave background radiation observations we do not need to discriminate between these foregrounds, but simply need to subtract out their combined effect.