measurements
One goal of the 
Galactic Center Survey is a
comparison with other isotopomers of CO, especially with 
. For
this purpose it is necessary to determine the comparability of the
data, i.e., any positional or other misalignment of our data compared
to data from other telescope systems. Because no large scale
survey of the Galactic center region was
available up to now, it was decided to observe the Galactic center
region in 
using the same system. The last widely
published large scale data of the Galactic
center (Liszt & Burton 1978; Bania
1977, 1980, 1986) are more than 20 years old and not
very sensitive. However, there exist more recent data, one set observed in
1984 with the 1.2m SMWT (Bitran 1987; Bitran
et al. 1997), and another one which is in observation since 1986
with the 7m Bell-Labs-Antenna (Uchida et al. 1997). In
addition, Jackson et al. (1996) have very recently announced
a survey obtained with QUARRY at the Five College Radio
Astronomy Observatory (FCRAO).
For 12, the AOS frequency resolution per channel of 1.07339 MHz
corresponds to 2.79 which resulted in a total velocity
bandwidth of 715 . The data were taken in March 1994 and
August 1994. The observations were done in the upper sideband of the
receiver with an image frequency of 112.506104 GHz. The observing
procedure was the same as for the observations
(see Appendix A (click here)).
To make comparisons easier, we made the
measurements with the same spacing and coordinate alignment as the
survey. This has the disadvantage that the data are
slightly less than half sampled because the FWHM beam width of the
telescope is 8
8 at the line frequency of the
transition, but with this setup point by point
comparison becomes possible. Our 12 map covers the range from
l=3
6 to -09 and from b=-03 to +03.
From l=30 to 36 the observations could be
extended to b=+075 which includes most of Clump 2.
The positions of the grid were typically observed once, with a single
10-minute-scan. In the rare case of bad weather conditions a position
was measured twice, so that an rms noise temperature of about 0.15
to 0.25 K was achieved. The resulting signal-to-noise ratio of the
data is much better than that of the data because
the emission is easily forty times stronger than the
emission. Thus, this rms is more than sufficient for a
reliable comparison. The OFF positions used were the same as for
the observations (see Appendix B (click here)).
The data reduction was done in the same way as for the data.
The baselines subtracted were typically of the order of 3. Higher order
baselines were not necessary, since due to the more intense line
emission in , the long period sinusoidal waves (from
reflections in the telescope) did not affect the spectra as much as in
the case of .
As in , the calibration stability was monitored by taking a
10-minute-spectrum toward the position l = 0625, b = 00
(SgrB2) once or twice per day. It was found to be as stable as for
.
Also as in the case of , the scaling factor of the AOS
calibration has to be determined. Again, we chose to compare the
emission of SgrB2 observed with the AOS at the
SMWT to observations of the same source with the
0.5 MHz-resolution-filterbank at the 1.2m NMWT (compare
Sect. 3.4 (click here)). To obtain sufficient bandwidth, it was necessary
to merge sets of spectra centered on -200, -100, 0, 100, and
200 because at the bandwidth of the
256-channel-filterbank of 128 MHz corresponding to only 332.9
was not sufficient. Then, the peak intensity of the SMWT and the NMWT
were fitted.
With these fits, the "efficiency'' of the AOS calibration at the
line emission, 
, is:
Using this result, the following conversions for and can
be established:
data
In this section, we present the results based on the
Galactic center data. The intensities of the
data are scaled to , following Eq. (9 (click here)). In
Fig. 14 (click here), the full set of survey spectra on the same
grid used for 18 is given. The mean rms per channel of all 174
spectra is 
.
Figure 14: The full set of spectra of our Galactic
Center Survey. The x-axis of the spectra is and ranges from
-357.3 to +357.3 , the y-axis is and ranges from -1.5 to
15.5 K. The longitude range of the plot is from l = 36 to
-105 and the latitude range is from b = -09 to
+075
of is given in Fig. 15 (click here) as a
contour map. This covers the velocity range from -225.0 to
+275.0 , i.e. the complete emission range of the Galactic center
region in the observed area, as can be seen from the lv-plot in
Fig. 17 (click here). The plot was produced in the same way as
Fig. 5 (click here).
Figure 15: The integrated intensity of the Galactic center region in
. The velocity the intensity is integrated over
ranges from -225.0 to +275.0 . The solid contour levels range
from 150.0 to 1650.0 in steps of 100.0 K. The minimum in the
contour map is 51.4 K which is well above the 
-value
of 18.7 K. The circle in the lower left corner of the plot
represents the beam size of 88
As in , one finds a coincidence of the main emission regions
with the major continuum sources SgrA (with an extension to SgrB1)
and SgrB2, SgrC and SgrD. The emission near SgrA and SgrB1
appears as a ridge from l=-002 to +02 at b=-003.
At the position of SgrB2, peaks at (l=+072,
b=-004). Near SgrC, peaks at (l=-053,
b=-012), and near SgrD at (l=+13, b=-01) with
an extension to (l=+105, b=-015). In addition, Clump 2
appears as a strong and widespread emission feature, peaking at
(l=+315, b=+03). A further comparatively weak maximum
is at (l=+195, b=-015). The global differences to the
data are:
emission is much more widespread than the
emission. In fact, emission is visible over all the
map. However, the smaller latitude extent of the
emission might be influenced by the detection limit of the rather
weak line.
.
, the continuum sources G 0.5 0.0 and G 0.9+0.1
(Downes & Maxwell 1966) do not have counterparts in .
width
of are given in Fig. 16 (click here). The
plots are produced in close analogy to Fig. 6 (click here). In
the following description we mention only the major features. A more
detailed discussion of the channel maps of the 12 data is given by
Dahmen (1995).
Figure 16: a) The integrated intensity of the Galactic center region in
in velocity intervals of 50 width. The
solid contour levels are at 6.0 (which is the -level), 15.0,
25.0, 40.0, 60.0, 90.0, 130.0, 180.0, 240.0, 310.0, 390.0, 480.0,
580.0, and 690.0 K. The dashed contour is at 4.0 K which
is the 
-value. The circle in the lower left corner of the
plots indicates the beam size of 88. a) At the top the
integrated intensity in the velocity range from -225.0 to
-175.0 is plotted, in the middle panel the velocity ranges
from -175.0 to -125.0 , and at the bottom the velocity range
from -125 to -75.0 is shown
Figure 16: b) At the top the integrated intensity of the velocity
range from -75.0 to -25 is plotted, in the middle the
velocity ranges from -25 to +25 , and at the bottom the velocity
range from +25 to +75.0 is displayed
Figure 16: c) At the top the integrated intensity of the velocity
range from +75.0 to +125 is plotted, in the middle the velocity
ranges from +125 to +175 , and at the bottom the velocity range
from +175 to +225.0 is displayed
Figure 16: d) At the top the integrated intensity of the velocity
range from +225.0 to +275 is plotted, in the middle the velocity
ranges from +275 to +325 , and at the bottom the velocity range
from -275 to -225.0 is displayed
At -200 , appears at the negative longitude edge of the
map. A maximum beyond l=-09, the border of the map, is
indicated. In addition, a weak maximum appears near SgrC at
(l=-06, b=-013). At positive longitudes no
emission is visible.
At -150 , the emission has expanded toward
l=+07. The main maximum appears near SgrC at
(l=-065, b=-005).
At -100 , continous emission is extended toward higher
longitudes up to l=+16. Beyond this (between l=+18 and
+31) some isolated weak emission areas are visible. The main peak
appears near SgrC at (l=-05, b=00). A rather strong
ridge starts at this peak and extends along the b=-015-line to
about l=+07.
At -50 , the complete map shows 12 emission at a significant
level, associated with the 3-kpc-arm gas. In general, the emission is
widespread and comparatively weak with hardly any clear emission peaks.
The ridge of maximum emission does not clearly follow the Galactic
plane. The emission seems structureless, compared to the corresponding
emission. The only distinct peak appears at (l=-052,
b=-015) near SgrC.
At 0 , the Galactic center emission is superposed on the
emission of the local gas. However, the local component does not
contribute much to the integrated intensity because of its small
linewidth. Thus, the integrated intensity is dominated by Galactic
center gas. As at -50 , the 12 emission shows little
structure and fills the entire map. It is most widespread and uniformly
distributed at this velocity. However, the ridge of emission follows
the Galactic plane from the negative longitude edge of the map up to
Clump 2 where it turns toward positive latitudes and ends at about
(l=+315, b=+05). Weak maxima appear near SgrA
(l=-015, b=-08), SgrB2 (l=+075,
b=-005), SgrC (l=-055, b=-005), and G 0.5 0.0
(l=+045, b=-002). This is the only correspondence of a
12CO emission maximum with G 0.5 0.0. Clump 2 shows intense
emission peaking at (l=+31, b=+035). A minimum in the
ridge of emission appears at about l=+23, but even there the
emission remains strong. Thus, there is a rather strong connection in
the emission of between the inner Galactic center region and
Clump 2.
At +50 , there is much more structure in the emission
than at 0 and -50 . The Galactic center region appears as
a strong ridge with maxima between SgrA and SgrB1 at
(l=+01, b=-005), near SgrB2 at (l=+072,
b=-01), SgrC at (l=-058, b=-017), and SgrD
at (l=+13, b=-012). The emission from Clump 2 is
stronger at +50 than at 0 and peaks at (l=+315,
b=+035). The emission bridge between Clump 2 and the inner
Galactic center region is weaker than at 0 and has a minimum at
about l=+25. In general, the emission is almost as widespread
as at 0 and -50 . However, the contrast between the maxima
and the minima is much higher. SgrB2 is the strongest maximum of all,
but not as prominent as in .
At +100 , SgrD is, as in , the dominant
emission feature (the l=15-complex of Bally et al.
1988). However, it peaks at a distinctly different position than in
, namely at (l=+135, b=+015). The ridge of
stronger emission starts there and continues toward about (l=+13,
b=-01), turns toward lower longitudes and remains rather strong
toward (l=+06, b=00). Then it follows the b=00-line
toward l=+015 where it turns into the weak maximum near SgrA
at (l=+012, b=+002) which is the end of the ridge. This
ridge is also visible in where it has nearly the same
extent. In addition, Clump 2 is also rather strong at this velocity,
but peaks at a somewhat different position, namely at (l=+315,
b=+02). As at +50 , there exists a bridge of emission
between Clump 2 and the inner Galactic center region.
At +150 , the emission is still widespread and covers
nearly the complete map whereas the emission has completely
disappeared. However, the shape of the emission has changed greatly
compared to lower velocities. In the inner Galactic center region the
emission peaks at (l=-062, b=+005), near SgrC. There
is a clear minimum of emission at (l=+10, b=+005).
Clump 2 has a strength similar to that of SgrC and peaks at
(l=+305, b=+015). The emission bridge between Clump 2
and the inner Galactic center region is also present but weaker than at
lower velocities.
At +200 , the intensity of the emission starts to decline
at negative longitudes and positive latitudes. In general, the emission
is distinctly weaker than at lower velocities. The main maximum is near
SgrD at (l=+135, b=-015). At longitudes higher than
l=+20 an emission ridge at negative latitudes arises. Clump 2
shows widespread but weak emission without any clear peak.
At +250 , the emission from the inner Galactic center
region and in Clump 2 has completely disappeared. But there is emission
at higher positive longitudes and negative latitudes which first
appears at +200 . It covers the range from l=+12 to
+36 and perhaps to higher longitudes beyond the edge of the map
and from b=+01 to -03, probably continuing toward lower
latitudes.
The middle and the lower plot of Fig. 16 (click here)d show the
channel maps at +300 and at -250 . These confirm that
outside the range from -225 to +275 no
emission is visible.
In Fig. 17 (click here), we have plotted the intensity integrated
over the latitude range from -030 to +030, similar to
Fig. 7 (click here). In the following description we mention only
the major features.
Figure 17: Longitude-velocity plot showing the velocity distribution of
the emission in the latitude range from
-030 to +030. The solid contour levels are at
0.084 Karcdeg (which is the 3
-level), 0.2, 0.35, 0.55, 0.8,
1.1 Karcdeg, and then range from 1.5 to 8.0 in steps of
0.5 Karcdeg. The dashed contour is at 0.056 Karcdeg which is the
-value
In the lv-space, the emission appears as a
broad strip with constant velocity width of about 375 , only the
center of which shifts with longitude. At the lower longitude edge of
the map (l=-09) this center velocity is 
, it
shifts to 
at 
0, and remains
constant from there up to the upper longitude edge of the map at
l=+36. In general, the emission is much more
widespread and homogeneous than the emission. The strongest
feature is not SgrB2, as in , but the 1.5-complex near
SgrD peaking at (
3, 
) which
has the same position as the corresponding peak. In addition
to this peak, only two maxima are clearly present: SgrB2 at
(
7, 
) and Clump 2 at (
15, 
). The emission of gas near
0 with respect to the LSR frame appears at about +5 for
longitudes larger than +2
. At lower longitudes a ridge of this gas
is not distinguishable from the overall emission but strong emission is
always present at this velocity. The 3-kpc-arm, at about -50 , is
visible in the complete longitude range of the map. However, it is more
patchy than the gas near 0 . The diagonal chain of emission centers
which is very prominent in is represented in by the
prominent peaks SgrB2 and SgrD. SgrC and SgrA are seen only as
weak maxima at (
5, 
) and
(15, 
), respectively. On the
other hand, the emission tail, which ranges from SgrA to about
(3, 
) in , is also
rather prominent in and seems to connect to the 1.5-complex
and to the Expanding Molecular Ring (EMR, Scoville
1972; Kaifu et al. 1972) at about +170 . From
the inner Galactic center region toward Clump 2, a connecting bridge of
emission exists not only at 0 but also at +120 , connecting the
maxima of the 1.5-complex and of Clump 2. This +120 -bridge has a
maximum at (0, 
). More prominent
minima of emission are visible at (35, 
), at (0, 
), in the range
from l=+20 to 25 and v=+60 to +100 , and in the range
from l=+20 to 28 and v=+165 to +195 .
A more detailed discussion of this latitude integrated plot of the 12 data can be found in Dahmen (1995). There, lv-plots for selected latitudes of the 12 data, analogous to Fig. 8 (click here), are also presented and discussed in detail.
In Fig. 18 (click here), we show the intensity integrated over two
longitude ranges, the inner Galactic center from -090 to
+2025 (a) and Clump2 from +2025 to +36 (b), in
analogy to Fig. 9 (click here).
Figure 18: Latitude-velocity plots showing the velocity distribution of
the emission. a) The longitude range from
-090 to +2025. The solid contour levels are at 0.18
(which is the -level), 0.45, 0.9, and then range from 1.8 to
18.0 in steps of 1.8 Karcdeg. The dashed contour is at 0.12 Karcdeg
which is the -value. b) The longitude range from
+2025 to +360. The solid contour levels are at 0.12 (which
is the -level), 0.3, 0.6, and then range from 1.2 to 4.8 in
steps of 1.2 Karcdeg. The dashed contour is at 0.08 Karcdeg which
is the -value
In general, the emission in bv-space has a limited amount of
structure, compared to the lv-plot. However, some interesting
features can be recognized, in particular in comparison to the
data.
In contrast to , the
emission in plot "a)'' appears
as a bulge-like plateau in the range from b = -02 to +005
and v = +15 to +100 . The maximum of emission appears at (b =
+005, v = +85 ), and the v-extent of the emission is
from -210 to +280 . Rather strong extensions of the emission are
visible along the v = 0 line, toward SgrC (b = -015,
v = -40 ) and toward the 1.5-complex (b = +02, v =
+110 ).
Plot "b)'' in Fig. 18 (click here) shows 3 emission features at
about +5 , +95 and +240 . The first two components are
also (although not as widespread) visible in . These represent
Clump 2 with its rather large extension toward positive b-values. The
high velocity component which is restricted to negative latitudes is
not visible in and represents the elongated ridge visible in
the channel maps of +200 and in particular of +250
(Fig. 16 (click here)).
measurements
If one compares the data presented here with the more than 20
years old data of Bania (1977, 1980, 1986), one
first has to take into account that the data of Bania are by
a factor of 10 undersampled latitude cuts at b = -20
, -10,
0, +10, and +20. Thus, these have to be compared
with latitude cuts of our 12 data. Doing this (see
Dahmen 1995), it is obvious that Bania's survey has a
distinctly lower sensitivity than the observations presented here, even
though our data are just 10-minute-scans. However, with these restrictions
in mind, both data sets show the same features. A comparison of the absolute
intensities is not possible because Bania gives only
for his data. However, as already noted, there exist more recent and
more complete data sets, one observed in 1984 with the 1.2m SMWT
(Bitran 1987; Bitran et al. 1997), another one
which is in observation since 1986 with the 7m Bell-Labs-Antenna
(Uchida et al. 1997), and one observed in 1993 and 1994 with
the FCRAO QUARRY (Jackson et al. 1996). The data of
Bitran (1987) and Uchida et al. (1997) are
available to us in digital format so that a more detailed comparison was
possible.
Previous to the observations presented here the
Galactic center region was surveyed by Bitran in the time from
February to November 1984 and published in his Ph.D.thesis
(Bitran 1987; see also Bitran et al. 1997).
This survey covers a much larger area, is slightly better sampled
(75) and has twice the sensitivity of our 12 observations. We
performed a detailed comparison (see Dahmen 1995) between our
data and Bitran's to make sure that both data sets are
compatible for further analysis.
A comparison of the 12 SgrB2 spectrum obtained at the 1.2m NMWT in February 1995 with Bitran's respective spectrum shows that over a time scale of 10 years and for two different telescopes with different receivers (even though the telescopes are of the same design) the calibration agrees to within 10% which we consider to be excellent. In addition, the shapes of the observed spectra agree very well.
Contour maps of the integrated intensity of our 12 data (Fig. 15 (click here)) and Bitran's survey are in excellent agreement. Both the shape and the location of the visible features are identical which indicates that the system and the pointing of the telescope were very stable over a time scale of 10 years.
Channel maps obtained from Bitran's 12 data are also in
very close agreement with those obtained from our data, as are
lv-plots. The maxima of the contour plots from Bitran's data,
converted to , tend to be 
% higher than the corresponding
values of our data. Thus, the scaling of the integrated intensity is
different from the scaling of a single spectrum by 5%. It is, however,
not clear if this difference is in fact significant.
Summarizing, our data are in excellent agreement with the
data of Bitran (1987). The system and the pointing of the
telescope are found to be stable over a time scale of 10 years. Only the
calibration deviates by 10% to 15%. However, on a time scale of 10 years
and taking into account that the calibration accuracy of the 1.2m NMWT
(which was the standard for the data presented here) has recently been
improved, this deviation seems to be rather small.
A large survey of the Galactic center region is
being performed with the 7m Bell-Labs-Antenna with 15 spacing
and 2.6 velocity resolution (Uchida et al. 1997). The
survey currently covers a region from l=-225 to +375 in
longitude and from b=-06 to +06 in latitude.
Using COMB, the AT&T Bell Laboratories data reduction package, the data cube was resampled to our resolution in l, b and v. All work at the 7m telescope is done on a scale that cannot be readily converted to , preventing the comparison of the absolute calibration of the different data sets. However, the lineshapes of individual spectra toward prominent positions once again agree closely. Contour plots of the total integrated intensity, channel maps and lv-plots are in excellent agreement with our and with Bitran's data. A detailed discussion of this can be found in Dahmen (1995).
All three telescope systems have achieved essentially identical results in 12 when convolved to the same resolution, and we can thus be confident in the accuracy of calibration and pointing of the 18 data.