The continuum radio emission of the sky background is a mixture mainly
of thermal (free-free) and non-thermal (synchrotron) components, in a
proportion that depends on the observing frequency. At
frequencies higher than about 1 GHz the largest fraction of the radiation
is thermal, and this
fraction decreases with frequency. At intermediate frequencies, roughly a few
hundreds of MHz, the nonthermal radiation begins to dominate and continues
to do
so with decreasing frequency until the absorption by ionized hydrogen
sets in at
about a few tens of MHz. Studies of many
astrophysical processes in our Galaxy rely on the separation of the two
components. For example, since the
synchrotron emission results from the interaction of the galactic
cosmic ray electrons with the galactic magnetic fields, its study is
important to learn about the distribution and energy spectrum of the
energetic electrons, as well as about the distribution of the magnetic
fields. The identification of the components has recently received
special attention in the study of anisotropies of the cosmic microwave
background (Kogut et al. 1996;
Davies et al. 1996). The separation of the
thermal and nonthermal components has
been a problem not satisfactorily solved so far.
In spite of the
advantage that low-frequency observations ( MHz) offer in
this respect,
very
few surveys have been made in this frequency range, probably because
of the many inherent difficulties. For
example, it is necessary to build
very large arrays in order to
achieve good angular resolution, this usually limits the
telescopes to be only transit instruments; the interference (man-made
and natural) is a severe problem; the ionosphere can produce
considerable changes in the incomig signal; depending on the design of
the array
the observations may be strongly affected by meteorological conditions
(fog, rain, etc.). The design of the array imposes also limitations on
the amount of sky that the instrument can cover. Ideally a survey
should cover the whole sky, the observations should be made with
instruments of similar characteristics and observing at the same
frequency.
Most of the available galactic continuum surveys cover the galactic plane only. The
few surveys covering the whole southern sky below 408 MHz and with single
frequency observations are listed in
Table 1 (click here). It is seen that half of the surveys have a very low
angular resolution (). The fourth column lists the temperature
step between adjacent contours lines near a minimum at
,
the fifth column shows the percentage of the background
temperature corresponding to that step. This number intends to estimate
the sensitivity of the system.
| Coverage (![]() | Resolution (![]() | ![]() | ![]() | Instrument | Reference | ||||||
(MHz) | (![]() | (![]() ![]() | (K) | (%) | ||||||||
200 | -85 - +45 | ![]() | 16 | 100 | array | Allen & Gum(1950) | ||||||
153 | -90 - +10 | ![]() | 20 | 13 | dish | Hamilton & Haynes (1969) | ||||||
100 | -90 - +30 | ![]() | 50 | 9 | array | Bolton & Westfold (1950) | ||||||
85 | -90 - ![]() | ![]() | 100 | 10 | dish | Yates et al. (1967) | ||||||
45 | -86.1 - +19.1 | ![]() | 250 | 6 | array | This survey | ||||||
30 | -90 - 0 | ![]() | 1800 | 25 | dish | Mathewson et al. (1965) | ||||||
16.5 | -90 - 0 | ![]() | ![]() | 25 | array | Ellis (1982) | ||||||
to | to | |||||||||||
2.1 | ![]() | ![]() | 25 | |||||||||
|
Realizing the need of low-frequency observations the University of Chile begun in 1970 the construction of a large array tuned at 45 MHz (Reyes 1977), with the purpose, among others, of making a sky survey. Preliminary results were reported by Bitran (1981), and Bitran et al. (1981). The first results of the survey were presented by Alvarez et al. (1988).