The survey was carried out with the Southern Millimeter-wave
Telescope (Cohen 1983; Bronfman et al. 1988, 1989) located at Cerro Tololo
Interamerican Observatory, Chile. The telescope is a 1.2-m Cassegrain with
a beam-width of 8.8
(FWHM) at 115 GHz, the frequency of the
CO(
) transition. It was equipped with a
super-heterodyne receiver with a SSB noise temperature of 380 K.
The first stage of the receiver consisted of a Schottky barrier
diode mixer and a GaAs field-effect transistor amplifier cooled to
77 K by liquid nitrogen.
The spectrometer was a 256-channel filter bank of standard NRAO
design. Each filter, 0.5 MHz wide, provided a velocity resolution
of 1.3 km s-1 at 115 GHz and a coverage of 333
km s-1. This is adequate for most of the galactic CO emission, except
within the central few degrees (
) where the
velocity range of the emission
reaches 500 km s-1. In such region two spectra centered at
different VLSRs were taken at each position and were subsequently
combined (Table 2 (click here)).
|
Galactic longitude |  to  | |
| Galactic latitude |  to  | |
| Sampling interval |  (0.85 beam-widths) for  | |
 (1.7 beam-widths) for  | ||
| Telescope beam |  at 115 GHz | |
| (FWHM) | ||
| Velocity range | -244, +88 km s ) | |
-179, +153 km s ) | ||
-319, +319 km s ) | ||
-88, +244 km s ) | ||
| Velocity resolution | 1.3 km s-1 | |
| RMS sensitivity |  K ( in
 at 1.3 km s-1) | |
|
|
The system was calibrated daily and checked against reference
sources before starting the observations. A standard blackbody
chopper-wheel technique (Kutner & Ulich 1981) was used to
produce antenna temperatures, , corrected for atmospheric
attenuation, resistive losses, and rearward spillover and scattering. To
correct for forward spillover and scattering, is divided by
the main beam efficiency, 
, the fraction of the forward power that
enters the main beam. This yields the radiation temperature 
, the physical temperature of a blackbody that just
fills the main beam. The main beam efficiency of the telescope, 
, has been derived by Bronfman et al. (1988).
The atmospheric conditions at Cerro Tololo are extremely favorable
for mm-wave observations, the water vapor opacity per air mass being
usually less than 0.1. Since the galactic center transits there almost
at the zenith, most of the spectra were taken through an air mass of
about 1. These circumstances made possible, even with a Schottky
receiver, to reach rms antenna noise
temperatures of K in 10 minutes or less.
The pointing of the telescope was checked twice a day by
observing the Sun. Right ascension and declination scans across
the Sun's limb were used to locate the center of the solar disk in 115
GHz continuum emission to within 
. The long term pointing
accuracy was checked every few months by observing stars with a
small optical telescope collimated with the radio
telescope; errors smaller than 
were obtained.
Computer control of the dome and of the altitude-azimuth mount of the telescope allowed fast pointing changes which made practical to have reference positions several degrees away from the galactic equator.
Figure 1: Representative survey spectra along two adjacent
longitude strips, illustrating their high signal-to-noise ratios
and flat baselines. The baselines are spaced by 6 K.
Notice that even in this case where two
overlapping spectra, 333 km s-1 wide each, were combined to
achieve
the necessary
velocity range, the baselines are still perfectly flat. Also the
difference
in sampling for
can be clearly seen
|
l | b | l | b | l | b | |||||
( ) | () | () | () | () | () | |||||
|
347.00 | +3.00 | 357.00 | -5.00 | 5.00 | -4.00 | |||||
| 348.00 | -5.00 | 357.00 | +3.00 | 7.00 | +5.00 | |||||
| 348.00 | -4.00 | 359.80 | -5.75 | 8.00 | -5.00 | |||||
| 352.00 | -5.00 | 0.00 | -4.00 | 8.50 | +2.75 | |||||
| 353.00 | -5.00 | 0.00 | -3.00 | 9.00 | +5.00 | |||||
| 353.00 | +4.00 | 0.70 | -3.30 | 10.00 | +5.00 | |||||
| 354.00 | -5.00 | 1.00 | -3.90 | 11.00 | -4.00 | |||||
| 355.00 | -5.00 | 1.90 | -5.35 | 12.00 | +5.00 | |||||
| 355.00 | +5.00 | 3.80 | -4.80 | 14.00 | +2.50 | |||||
| 356.04 | -5.23 | 4.75 | +2.50 | 15.00 | -5.00 | |||||
|
|