postscript
Abstract...
Abstract...
2 Using to Trace ...
Low frequency measurements of the cosmic microwave background anisotropy are significantly contaminated by the thermal bremsstrahlung and synchrotron radiations emitted by ionised gas in our galaxy. However, we have only limited information on either the frequency spectrum or the spatial distribution of this emission, and nowadays the sensitivity of CMB experiments is high enough that this lack of knowledge of the galactic signal is potentially a serious hindrance. On larger angular scales, greater than a few degrees, the low frequency radio surveys provide some information, as do the COBE DMR skymaps, but on subdegree scales there is indeed almost no information as to what to expect. This problem will become more acute when subdegree scale experiments such as the VSA, MAP, and Planck Surveyor come online. These experiments plan to measure the CMB angular power spectrum, and determine cosmological parameters, with a few percent precision, but of course, this is only possible if the galactic contamination can be modelled with similar precision.
Here we present some results on the spatial distribution of the galactic
free-free emission at intermediate and high galactic latitudes. Free-free
emission is the dominant foreground at 25--75 GHz, and its frequency spectrum
is very well known. On large scales, Bennett et al. (1992) and Bennett et al. (1994) have
constructed a map of the free-free emission by linearly combining the four-year
COBE DMR skymaps at different frequencies to isolate emission with antenna
temperature 
. Kogut et al. (1996a) and Kogut et al. (1996b)
compare this emission with dust emission as mapped by the far infrared channels
on COBE DIRBE and find that the free-free and dust distributions are
correlated. This correlated distribution has an angular power spectrum of the
form 
for angular scales 
. The power spectrum scales
in the same way as the 
power spectrum measured for IRAS 100 
fields on scales 
to 
(Gautier et al. (1992), Gautier & Stewart (1993)) and for
neutral hydrogen by Crovisier & Dickey (1983).
Diffuse galactic 
emission is considered to be a good tracer of
the diffuse free-free emission because both are emitted by the same ionised
hydrogen (Reynolds (1980)). Both scale with electron density in the same
way, 
. An 
intensity of 1 Rayleigh (1
Rayleigh
at the 
line
at 6563Å) implies a free-free antenna temperature of 
at 53
GHz, and 
at 5 GHz. We have used publicly available 
maps of the north celestial pole to determine the spatial
distribution of galactic free-free emission on subdegree scales. Our main
conclusion is very pleasing -- galactic bremsstrahlung is not going to be a
serious contaminant for subdegree scale CMB measurements.