The central region of our Galaxy may hold the key to many questions related to the structure, kinematics, chemistry, dynamics and energetics of the whole Milky Way. Radio lines are a powerful tool to investigate this inner region since they are not affected by obscuring dust along the line of sight. Because the line profiles provide velocity information which can be used to study the kinematics of the emitting gas, there has been a keen interest in observing the HI line at 21 cm and many molecular transitions.
Studies of the galactic central region can concentrate on the nuclear area (Sgr A complex) or on the large scale aspects of the whole central area. Although the study of the complex and unique galactic nucleus requires high-resolutions observations, the large-scale properties of the central region of the Galaxy can be adequately studied in surveys made with relatively low-resolution instruments within a reasonable amount of time.
There have been several molecular line surveys of the central region of the Galaxy, for example, of the hydroxyl radical (McGee et al. 1970; Cohen & Dent 1983; Boyce & Cohen 1994), formaldehyde (Scoville et al. 1972; Cohen & Few 1981), ammonia (Kaifu et al. 1975), carbon sulfide (Bally et al. 1987).
and
.
Of particular importance are the carbon monoxide surveys
because this molecule is a good tracer of molecular hydrogen, which cannot
be observed directly by ground based telescopes. The
observations of various CO isotopes can yield different
information on the physical state of the emitting gas.
For example, observations of the optically thick CO
emission can be used, in principle, to estimate the excitation temperature
of a molecular cloud; however, due to the large temperature
gradients that almost certainly exist, the CO emission can trace the
excitation temperature
at the 
surface of the cloud.
Alternatively, observations of the optically thinner 13CO can be used to
estimate the column density. A problem with the 13CO and with the
C18O, which is also optically thin, is that their radiation is
several times weaker than that of CO
and thus, their observations are very time consuming. However,
the velocity-integrated CO intensity can be used as a measure of
the column density of molecular clouds, in roughly the same way
that the luminosity of a globular cluster provides a measure of its
mass: by being proportional to the number column density of
unresolved optically thick objects that do not seriously overlap (Bronfman
et al. 1988).
Surveys of the galactic center in several CO isotopes
are presented in Table 1 (click here). This table shows
that most of the CO observations of the galactic
center region have been obtained from the northern hemisphere, where the
galactic center is observable only at low elevations, and with relatively
high angular resolution (
).
As a consequence of these conditions, most of CO northern
hemisphere observations
of the molecular gas within a few kpc from the galactic center are
severely under-sampled.
Well sampled observations have been restricted only to relatively small areas
around the nucleus and near the galactic plane, while surveys with
larger coverage consisted of longitude strips at a few fixed latitudes,
with typical spacings of (
30 beam-widths). These coarse
grids can easily miss distant molecular clouds close to the galactic
center.
Results of the out-of-plane surveys of the inner galactic disk
(Cohen et al. 1980, 1986;
Grabelsky et al. 1987; Bronfman et al. 1988) have
shown that
well sampled observations (spaced at 
beam-width) are necessary in
order to follow the continuity
of large scale galactic features.
The purpose of this work is to present a well sampled and sensitive
large scale CO survey of the central region of the Galaxy, providing
more complete information about the region within 2 kpc from the center of
the Galaxy. The results of this survey are well suited for
direct comparison with HI surveys of our Galaxy made with
the largest single-dish telescopes, and with
high-resolution interferometric CO observations of other
spiral galaxies. For example, the 100-m Effelsberg
Telescope has a beam-width of 
(FWHM) at 21-cm wavelength.