The Astrophysical Journal, 773:31 (10pp), 2013 August 10
C 2013.
doi:10.1088/0004-637X/773/1/31
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
GREEN BANK TELESCOPE OBSERVATIONS OF THE NH3 (3, 3) AND (6, 6)
TRANSITIONS TOWARD SAGITTARIUS A MOLECULAR CLOUDS
Young Chol Minh1 , Hauyu Baobab Liu2 , Paul T. P. Ho2,3 , Pei-Ying Hsieh2 ,
Yu-Nung Su2 , Sungsoo S. Kim4 , and Melvyn Wright5
1 Korea Astronomy and Space Science Institute, Daeduk-daero 776, Yuseong, Daejeon 305-348, Korea
2 Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 10617, Taiwan
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
5
4 Department of Astronomy and Space Science, Kyung Hee University, Yongin, Kyungki-do 446-701, Korea
Radio Astronomy Laboratory, University of California, Berkeley, 601 Campbell Hall, Berkeley, CA 94720, USA
Received 2013 April 21; accepted 2013 June 1; published 2013 July 22
ABSTRACT
Ammonia (3, 3) and (6, 6) transitions have been observed using the Green Bank Telescope toward the Sgr A region.
The gas is mainly concentrated in 50 km s−1 and 20 km s−1 clouds located in a plane inclined to the galactic plane.
These “main” clouds appear to be virialized and influenced by the expansion of the supernova remnant Sgr A
East. The observed emission shows very complicated features in the morphology and velocity structure. Gaussian
multi-component fittings of the observed spectra revealed that various “streaming” gas components exist all over
the observed region. These components include those previously known as “streamers” and “ridges,” but most of
these components appear not to be directly connected to the major gas condensations (the 50 km s−1 and 20 km s−1
clouds). They are apparently located out of the galactic plane, and they may have a different origin than the major
gas condensations. Some of the streaming components are expected to be sources that feed the circumnuclear disk
of our Galactic center directly and episodically. They may also evolve differently than major gas condensations
under the influence of the activities of the Galactic center.
Key words: Galaxy: center – ISM: individual objects (Sgr A) – ISM: molecules – radio lines: ISM
Online-only material: color figures
the gas away from its nucleus, forming ridges of material on
all sides; however, some material, such as the NH3 filamentary
streamers, may also move toward the nucleus and possibly feed
the nucleus (Pedlar et al. 1989; Okumura et al. 1989; Ho et al.
1991; Zylka et al. 1995; Coil & Ho 2000; McGary et al. 2001).
To study the molecular clouds associated with the Sgr A
region, we observed the NH3 (3, 3) and (6, 6) transitions
using the Green Bank Telescope (GBT) over roughly the
10 × 16 region centered at Sgr A*. To date, these observations
have the greatest sensitivity, the widest velocity range, and
the highest velocity resolution, which will provide a more
complete picture of NH3 in the central region of our Galaxy.
The rotation inversion transitions of ammonia have been proven
to be effective tracers of dense (104 –105 cm−3 ) cores of
giant molecular clouds (GMCs; Ho & Townes 1983). The
line ratios between different inversion transitions can be used
to calculate the rotational temperature of the gas, and the
opacity of the observed transitions can be estimated directly
using their hyperfine lines (e.g., Ho & Townes 1983). We
investigated the structure of the Sgr A molecular clouds in detail,
in particular the nature of the “streamers” associated with the
Galactic center components. In Section 2, we summarize our
observations including the data acquisition, characteristics of
the observed lines, and their features. Section 3 introduces the
observed results, measured parameters, and related discussions
on the major gas condensations and streaming gas components.
Conclusions are provided in Section 4.
1. INTRODUCTION
Neutral gas in the Galactic center is predominantly molecular.
Most of the gas within ∼20 pc (in projection) from Sgr A∗
is concentrated into two massive clouds, called M-0.02-0.07
(also referred to as the “50 km s−1 cloud”) and M-0.13-0.8
(the “20 km s−1 cloud”) according to their galactic coordinates
(Güsten et al. 1981). These two clouds have comparable masses
(∼5 × 105 M ) and linear dimensions (∼10–15 pc), and they
have a complicated morphological structure (e.g., Güsten et al.
1981; Armstrong & Barrett 1985; Herrnstein & Ho 2002).
The large line widths of the observed transitions indicate the
existence of a high degree of turbulence. Subsequent to early
absorption line observations of OH and H2 CO, spectral lines and
dust emissions from these clouds have been studied extensively
(e.g., Snyder et al. 1969; Güsten & Downes 1980; Zylka et al.
1990; Lis & Carlstrom 1994; Minh et al. 1992, 2005), and it has
been found that the physical and chemical properties of these
molecular clouds differ substantially from those in the galactic
disk.
This molecular gas has been thought to interact with the
central components in the inner 10 pc of our Galactic center,
such as the circumnuclear disk (CND) or the supernova remnant
(SNR) Sgr A East (e.g., Ho et al. 1991; Coil & Ho 1999, 2000).
The CND is a ring-like, dense (105 cm−3 ), highly turbulent
(Δv 40 km s−1 ), and clumpy molecular gas, which surrounds
Sgr A∗ with an inner edge at ∼1.5 pc and an outer edge at
∼3–4 pc (Güsten et al. 1987; Wright et al. 2001; Christopher
et al. 2005; Montero-Castaño et al. 2009; Liu et al. 2012, 2013).
Sgr A East is an expanding shell of a synchrotron emission that
lies behind Sgr A∗ ; it is thought to interact with the CND and also
with the ambient 50 km s−1 cloud (Ho et al. 1991; Yusef-Zadeh
et al. 2000; Coil & Ho 2000). Sgr A East appears to be pushing
2. OBSERVATIONS
2.1. GBT Observations
We performed the dual polarization on-the-fly mapping observations of the NH3 (3, 3) and (6, 6) rotational inversion
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Minh et al.
transitions using the NRAO6 GBT K-band Focal Plane Array
(KFPA; Langston et al. 2009) on 2011 November 7. We used the
observing mode with four beams and two spectral bands. The
spectral bands were configured to have a 50 MHz total bandwidth, centered at the rest frequencies of 23.870129 GHz and
25.056025 GHz. The spectrum had 4096 spectral channels with
a 12.207 kHz (∼0.15 km s−1 ) channel width. The beamwidth is
about 31 at our observing frequencies.
The mapping observations were divided into three consecutive sessions. In the first session, the KFPA beam-1 scanned once
through a 10 (in R.A.) × 12 (in decl.) field of view, centered at
(R.A., decl.)J2000 = (17h 45m 45.s 250, −29◦ 50 54. 56). The scans
were made in the declination direction with a 30 s scan duration,
a 2 Hz sampling rate, and the 0. 65 strip spacing. Observations in
the second and the third sessions had the same map dimension
and scan parameters, but they were offset toward the east by 13
and 26 , respectively. The effective integration time per beam
area per polarization is therefore ∼12 s. A line-free reference position (R.A., decl.)J2000 = (17h 43m 43.s 344, −29◦ 59 32. 27) was
integrated for 30 s before and after the target observations in each
session for off-source calibration data. The quasar 1733−130
was observed at the beginning of each session for calibrating
the reference pointing. Throughout our observations, the system
noise temperatures were about 60 K for the KFPA beam-1, 2,
and 3, but ∼1.4 times higher for the fourth beam. We used the
GBTIDL software package to calibrate the GBT data, and the
Astronomical Image Processing System package of NRAO to
produce images. Hanning smoothing for four channels has been
applied to the spectra for final figures.
profiles (one main and four satellite lines) to derive the physical
parameters for one velocity component, as shown in Figure 1. In
the fitting for each position, we assumed the same line widths for
five Gaussian profiles for one component; up to three different
velocity components (three sets of five Gaussian profiles) have
been applied when necessary. In contrast, the main line of the
(6, 6) transition has about 97% of the total intensity, and the
satellite line intensities are less than 1% for each group of
hyperfine lines. Therefore, we ignored the satellite lines of the
(6, 6) transition in fitting the spectra.
In the Gaussian fitting, we have assumed that observed
transitions toward one clump have same half-power (HP) widths,
same line intensities for all (3, 3) hyperfine lines, and the same
lsr velocity of the (3, 3) and (6, 6) transitions. Other parameters
except these values have been derived from the fit. The overall
Gaussian fitting was conducted several ways: (1) by fitting the
(6, 6) line first and then applying the derived values of center
velocity and line width to the (3, 3) line fittings; or (2) by fitting
the (3, 3) line first, changing the fitting order if there are several
components in a spectrum, and finally selecting the best fitting
by eye. The derived values in selected positions are included
in Table 1. Many of the obtained spectra were decomposed
into several different velocity components in the line of sight,
some of which have been previously known to have either very
wide line widths or asymmetric line profiles. We believe that
even the spectra which fit very well with a single Gaussian
could consist of an overlap of multiple components, and our
decomposition results must be the lower limit of the number of
clouds identifiable in that line of sight.
2.2. Observed Ammonia Lines
3. RESULTS AND DISCUSSIONS
The ammonia (J, K) = (3, 3) and (6, 6) transitions result
from the tunneling of the nitrogen atom through the plane of
three hydrogen atoms, where J is the total angular momentum
and K is its projection along the molecular axis. The three
hydrogen atoms of ammonia provide the ortho- and paraspecies depending on the hydrogen spins. Both the (3, 3)
and (6, 6) lines from the ortho-NH3 are brighter by a factor
of two than those from the para-NH3 because of a quantum
mechanical degeneracy of the states (Townes & Schalow 1975;
Ho & Townes 1983). The tunneling phenomenon results in
a splitting of the ground vibrational state of the nitrogen
atom into two levels with different energies. The inversion
doublets are further split by hyperfine interactions caused by
the interaction between the electric quadrupole moment of the
nitrogen nucleus and the electric field of the electrons. This
results in 18 hyperfine components which may be grouped into
five distinct components by blending the transition frequencies:
the five groups consist of a main line and two pairs of satellite
lines with roughly equal intensities (Townes & Schalow 1975;
Ho & Townes 1983).
The main line of the (3, 3) transition has about 90% of the total
intensity, and the remainder is approximately equally divided
between the two pairs of the satellite lines. These lines are
separated from the main line by about ±1.7 and ±2.3 MHz.
The large line width spectral lines of the Galactic center and the
complicated hyperfine structures make it difficult to distinguish
the different velocity components in the line of sight. We fitted
each spectrum obtained in 30 spacings with five Gaussian
3.1. Overall Emission Distribution
Figure 2 shows the velocity-integrated emission maps for
the observed NH3 (3, 3) and (6, 6) lines. Both emissions are
mainly concentrated in the GMCs known as the “50 km s−1
cloud” (M-0.02-0.07) and the “20 km s−1 cloud” (M-0.130.08; Güsten et al. 1981) which extend along the galactic plane
direction. The morphology of these major gas condensations
appears very similar to previous results (e.g., Güsten et al.
1981; Minh et al. 1991). The spectra obtained, however, show
evidence of multiple components in many lines of sight. Figure 1
shows the spectra obtained toward the position indicated in
Figure 2 (left), which are the peak emission positions of the
identified cloud components in Figure 3. As shown in Figure 1,
we decomposed the spectra into several Gaussian profiles for
the cases showing clear multiple features using the method
explained in Section 2.2. The results of the Gaussian fittings
are shown with red lines in Figure 1, and the identified cloud
components are indicated in Figure 3. Figures 4 and 5 are
examples of the derived parameters by Gaussian fittings.
The first panel of Figure 3 shows the main gas condensations,
identified as Cloud-A, B, C, and D. These extend continuously
along the galactic plane direction. Cloud-A corresponds to the
50 km s−1 cloud (M-0.02-0.07) and Cloud-B and C correspond
to the 20 km s−1 cloud (M-0.13-0.08; Güsten et al. 1981).
Cloud-B and C seem to be separate clouds that are probably
associated with each other, as previously argued by Minh et al.
(2005). Cloud-D roughly coincides with M-0.07-0.08 (Güsten
et al. 1981), which seems to connect Cloud-A and the disturbed
gas associated with the shock features in the northern area. We
may consider these Cloud-A, B, C, and probably D as the “major
gas condensations” in the Sgr A region.
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The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc.
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Minh et al.
Figure 1. Spectra observed toward the positions in Figure 2 (right), which are peak emission positions of the identified clouds indicated in Figure 3. The positions
in the parentheses have either a similar profile or contain its component in the spectrum. The Gaussian fit results with five components for the (3, 3) line and one
component for the (6, 6) line are shown in red.
(A color version of this figure is available in the online journal.)
Several different velocity components have been identified in
the multiple Gaussian fittings in many lines of sight, and the
results are shown in the second and third panels of Figure 3.
There also exist, however, relatively weak emissions which can
be clearly decomposed in the Gaussian fitting but cannot be
identified as a separate cloud components. In Figure 1, for
example, the emission at about −30 km s−1 in panel G and
40 km s−1 in panel P has been well fitted with another Gaussian,
but not identified as separate clouds. We believe that the number
of identified clouds in Figure 3 is the lower limit, and even a case
fitted well with a single Gaussian could be an overlap of several
different components, especially in this Galactic center region.
The clouds shown in the second and third panels of Figure 3 are
relatively weak in emission and scattered in their locations, and
they may be distinguished as the “streaming gas components.”
Compared with the previous identification by Güsten et al.
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Minh et al.
Figure 1. (Continued)
Table 1
Parameters of the Identified NH3 Clouds
NH3
Cloud
Vlsr a
(km s−1 )
X × DY b
DHP
HP
(pc×pc) (pa)
Ag
Bh
Ch
D
E
F
G
H
I
J
K
L
Mi
N
O
P
44.1
15.1
6.8
43.7
27.1
93.1
71.6
66.7
75.6
−79.7
−132.5
20.9
−12.6
−10.2
−17.7
−15.9
3.5 × 4.9 (−14.6)
3.6 × 6.8 (−21.2)
4.8 × 3.9 (−21.2)
3.4 × 5.3 (−18.6)
2.7 × 2.4 (2.3)
1.6 × 3.6 (8.2)
2.8 × 4.2 (−2.2)
2.3 × 4.6 (−34.1)
1.8 × 3.5 (5.4)
1.8 × 2.3 (5.4)
5.3 × 5.1 (3.1)
4.9 × 3.4 (−8.9)
2.2 × 4.3 (−16.3)
3.0 × 4.5 (11.5)
2.5 × 4.2 (−11.9)
3.3 × 2.6 (2.7)
(3,3)
(6,6) c,d
/Tp
(K)
FWHMd
(km s−1 )
Trot d
(K)
N (NH3 )d,e
(cm−2 )
Masse,f
(M )
11.7 ± 0.3/1.8 ± 0.2
15.2 ± 0.4/2.0 ± 0.1
11.2 ± 0.4/1.5 ± 0.1
4.3 ± 0.2/0.8 ± 0.1
4.2 ± 0.3/0.8 ± 0.1
2.1 ± 0.1/0.5 ± 0.05
0.9 ± 0.09/0.1 ± 0.03
0.8 ± 0.06/0.3 ± 0.07
0.5 ± 0.06/0.1 ± 0.04
0.3 ± 0.04/0.1 ± 0.04
0.4 ± 0.04/0.1 ± 0.04
0.5 ± 0.05/0.2 ± 0.06
3.4 ± 0.1/0.5 ± 0.04
1.7 ± 0.1/0.3 ± 0.05
0.5 ± 0.09/0.1 ± 0.03
1.4 ± 0.2/0.3 ± 0.05
21.0 ± 0.1
16.7 ± 0.1
15.5 ± 0.1
12.5 ± 0.2
18.6 ± 0.1
32.0 ± 0.2
14.1 ± 0.1
25.2 ± 0.3
28.7 ± 0.4
54.2 ± 1.3
16.5 ± 1.2
32.0 ± 0.4
12.5 ± 0.2
10.8 ± 0.4
36.5 ± 7.5
13.9 ± 0.3
92 ± 4
84 ± 1
75 ± 1
87 ± 4
81 ± 3
94 ± 4
76 ± 7
156 ± 26
108 ± 19
134 ± 32
96 ± 16
139 ± 27
79 ± 2
86 ± 5
93 ± 11
82 ± 5
2.3 ± 0.1(16)
2.8 ± 0.1(16)
3.3 ± 0.1(16)
7.4 ± 0.3(15)
1.4 ± 0.1(16)
8.9 ± 0.4(15)
1.8 ± 0.2(15)
1.0 ± 0.1(15)
1.0 ± 0.1(15)
1.0 ± 0.2(15)
8.5 ± 1.5(14)
1.1 ± 0.2(15)
6.8 ± 0.2(15)
2.5 ± 0.1(15)
2.1 ± 0.3(15)
4.0 ± 0.3(15)
3.6 ± 2.9(5)
6.3 ± 5.1(5)
5.5 ± 4.5(5)
1.2 ± 1.0(5)
8.4 ± 6.9(4)
4.7 ± 3.8(4)
2.0 ± 1.6(4)
9.8 ± 8.0(3)
5.7 ± 4.7(3)
3.9 ± 3.2(3)
2.1 ± 1.7(4)
1.7 ± 1.4(4)
5.9 ± 4.8(4)
3.1 ± 2.5(4)
2.0 ± 1.6(4)
3.1 ± 2.5(4)
Tp
Notes.
a Mean velocity averaged within the HP area.
b Half-power diameters of the cloud. The values in the parentheses are position angles.
c NH (3, 3) peak intensity from Gaussian fitting.
3
d Errors are from the rms values of the fit residuals.
e a(b) means a × 10b .
f Errors are from the applied f
NH3 range (Section 3.4).
g M-0.02-0.07 (50 km s−1 cloud).
h M-0.13-0.08 (20 km s−1 cloud).
i M-0.02-0.05 (Güsten et al. 1981).
change and its morphology may suggest that Cloud-A, B, and C
are connected with each other along the galactic plane direction
and probably rotate around Sgr A∗ . However, the planes containing these clouds appear to be inclined against the galactic
plane, and their origin and evolution need to be studied further.
The velocity structures of other clouds besides Cloud-A, B,
and C do not show specific patterns with each other or largescale velocity gradients. Figure 6 shows position–velocity maps
made along the cuts indicated in the right panel of Figure 2
for the (6, 6) line which has negligible hyperfine emissions.
If we count on the different velocity components decomposed
by the Gaussian fitting (Figure 3), we do not see any apparent
velocity gradients along the cuts except for a couple of weak
extended emissions. The ammonia emission features appear
(1981), Cloud-M roughly coincides with M-0.02-0.05. Most
of these streaming gas components appear to exist in the region
off the galactic plane and may be the main source feeding the
CND. Table 1 lists the cloud parameters derived toward the peak
position of the identified clouds in Figure 3.
3.2. Velocity Structure
Figure 3 and Table 1 include the averaged velocities derived
within the HP boundaries of the identified clouds. As shown
in Figure 4, the velocity clearly increases continuously along
Cloud-A, B, and C from the south to the north, approximately
with a radial form centered at Sgr A∗ . This was also found
previously (e.g., Güsten et al. 1981). The continuous velocity
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The Astrophysical Journal, 773:31 (10pp), 2013 August 10
Minh et al.
Figure 2. Total velocity-integrated intensity maps of the NH3 (3, 3) (left) and (6, 6) (right) lines. The unit of the false color image is K km s−1 and is included as a bar
on the right side of each map. The letters in the left panel are the peak emission positions for the identified clouds of Figure 3, and the lines in the right panel are cuts
for the position–velocity map in Figure 6. Sgr A* is shown by the magenta plus sign.
(A color version of this figure is available in the online journal.)
Figure 3. Clouds identified by Gaussian decomposition as shown in Figure 1. Boundaries of the half-power (HP) emission are shown as black contours for each cloud
component. Their parameters are listed in Table 1. The numbers above the cloud identification are the averaged velocities within the HP contours and are also listed
in Table 1. The small dots in the background are the positions where the map was made. The magenta contour line is for the 10 K km s−1 level of the NH3 (6, 6) total
integrated intensity map of Figure 2 (right). Sgr A* is shown with a plus sign.
(A color version of this figure is available in the online journal.)
from the cloud to the right boundary of the map. These velocity
gradients may result from the orbital motion of the falling gas
to the CND. Especially, Cloud-O and its extended emission,
probably together with Cloud-J, may correspond to the W-1 to
W-4 arms associated with CND, which was identified with HCN
(4–3) by Liu et al. (2012). The measured velocities of Cloud-O
and J are also similar to the CND velocities in that position.
to be somewhat localized as separate components. They are
probably scattered by the interaction with the Galactic center
activities while they are moving inward to the Galactic center.
Cloud-N and O are, however, connected with weak extended
emissions toward the southwest direction. These extended emissions show velocity gradients changing roughly from −10 to
−40 km s−1 (Cloud-N) and from −18 to −5 km s−1 (Cloud-O),
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Minh et al.
Figure 4. Velocity distribution along the emission distribution for the cloud
components in the first panel of Figure 3. Contour levels are inserted which
increase by 10 km s−1 . Others are the same as in Figure 3.
(A color version of this figure is available in the online journal.)
Figure 5. Rotation temperature distribution map. Contour levels are 70, 100,
and 130 K. Tex is 70–100 K in the region shaded with gray and >100 K in the
region with darker gray.
(A color version of this figure is available in the online journal.)
3.3. Rotational Temperatures and Line Widths
fittings. Generally speaking, the identified cloud components
seem to have line widths of about 15 km s−1 as a representative
value, which is in agreement with previous results (e.g., Güsten
et al. 1981; Armstrong & Barrett 1985; Herrnstein & Ho 2002).
Rotational temperatures were derived using the relation given
by Townes & Schalow (1975; Equation (1) of Herrnstein & Ho
2002), assuming an LTE and the same beam filling factors for
both emissions. We used the optical depth of the NH3 (3, 3)
line and the (3, 3) and (6, 6) line intensity ratios estimated from
Gaussian multi-component fittings. The calculated rotational
temperatures provide the lower limits for the true kinetic
temperature as discussed by Martin et al. (1982), Hüttemeister
et al. (1993), and Herrnstein & Ho (2005). Relatively high
rotational temperatures of 100 K have been found for CloudH, J, and L, which have relatively weak emissions and may
still be under the influence of shocks. With the exception of
these clouds, however, we measure pretty consistent rotational
temperatures of about 70–100 K for most identified clouds in
Table 1. The rotational temperature distribution is shown in
Figure 5. This suggests that the ammonia traces the warm dense
gas, localize as separate clouds, and quickly relax after being
disturbed since the thermal equilibrium timescale is only about
∼103 s (Herrnstein & Ho 2002).
Measuring the line width of the observed spectra largely
depends on the number of multiple components applied to
the fitting. Even in cases fitted to multiple components, some
of them, such as Cloud-J, appear to be an overlap of several
different components in the same lines of sight, as revealed by
the large line widths of the NH3 (3, 3) line and/or the line
profiles of the (6, 6) line. Therefore, it is difficult to discuss the
line width distribution in this region. However, in many cases we
derive line widths of about 10–30 km s−1 from good Gaussian
3.4. Column Density and Mass
We derived the optical depths of the NH3 (3, 3) line by comparing the intensities of the main and hyperfine lines estimated
from the Gaussian fittings. However, many ambiguities occur
in the derivation of opacities, resulting mainly from the large
line widths and the overlap of multiple components in the line
of sight. Therefore, we calculated the NH3 column density of
the clouds from the (6, 6) line intensity by assuming an LTE
and optically thin emission. Since the energy of the (6, 6) line is
about 412 K above the ground state (Ho & Townes 1983), this
emission traces a relatively warm gas and we may underestimate
the total NH3 column density if a lower temperature component
coexists in the same cloud. In deriving the column densities, we
applied the rotational temperatures listed in Table 1 that are well
determined in the cloud centers. The results are included in the
seventh column of Table 1.
The mass in the last column of Table 1 was derived using the
−1
equation Mass = 2 × N(NH3 ) × fNH
× μmH × Area, where
3
N(NH3 ) is the total column density of NH3 at the peak emission
position, fNH3 is the fractional abundance of NH3 relative to H2 ,
μ = 1.4 amu per H nuclei accounts for the mass of He and other
elements, mH is the hydrogen mass, and Area is the area within
the HP boundary listed in the table. The largest error source
in deriving the total column density is fNH3 , which is probably
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Minh et al.
Figure 6. Position–velocity map along the cuts in Figure 2 (right) for the NH3 (6, 6) line. Contour levels are 0.2 K (starting level) and 0.3 K (increment) for PV-1,
0.2 K and 0.4 K for PV-3, 0.1 K and 0.2 K for PV-2, 0.2 K and 0.4 K for PV-4, 0.1 K and 0.2 K for PV-5, and 0.1 K and 0.1 K for PV-6. Letters inserted in the figure
are the identified cloud components in Figure 3. The Vlsr = 0 km s−1 is indicated with a dashed line and the Sgr A* position is indicated with a solid line for PV-2 and
PV-6.
(A color version of this figure is available in the online journal.)
in the range of 10−7 –10−8 in the Galactic center regions (see
the discussion in Herrnstein & Ho 2005). We applied the same
range for fNH3 and results are included in the last column of
Table 1. Considering the various uncertainties in the derivation
of the mass, we believe that there could be an error up to an
order of magnitude in the final values. The masses derived for
Cloud-A, B, and C are similar to previous estimates by, for
example, Güsten et al. (1981), Armstrong & Barrett (1985),
Zylka et al. (1990), and Minh et al. (2005) within a factor of
a few. However, an appreciable amount of gas also exists as
streaming components in off-plane positions as listed in Table 1.
The virial masses for Cloud-A, B, and C are estimated to be
much less than the derived mass in Table 1, which means that
these clouds are gravitationally unstable and can be sites for
star formation, as observed by masers and compact H ii regions
(e.g., Yusef-Zadeh & Morris 1987; Yusef-Zadeh et al. 1999).
Cloud-D, E, M, N, and P appear to be barely virialized or close
to being virialized, and other clouds have not been observed to be
gravitationally bound. Especially the clouds near the Galactic
center, Cloud-I, J, and O are far from being virialized. The
clouds associated with the major gas condensations seem to
gravitationally bound and may remain stable for a while before
being significantly disturbed by new star formations. On the
other hand, the streaming gas components appear to be unstable
and may be in the process of “falling” to the CND or the Galactic
center.
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Minh et al.
Figure 7. Color-coded velocity channel maps of the NH3 (6, 6) line for velocity ranges of roughly 0 ∼ +55 (left), and −33 ∼ 0 and +55 ∼ +120 km s−1 (right). The
velocity scale bar is inserted in the top of the panel. The velocity width of each channel is about 4.6 km s−1 , and the boundary of the colored feature is for the contour
level of 0.5 K km s−1 for all channels. The 90 cm continuum feature (Pedlar et al. 1989) is inserted in the left panel. The boundaries of Sgr A West and Sgr A∗ are
shown in green and magenta. Contours of the right panel are for the velocity-integrated intensity of the left panel and their levels are 7, 17, and 27 K km s−1 .
(A color version of this figure is available in the online journal.)
Cloud-A and affects the gas between Cloud-A and B, which
corresponds to the component partly known as the “molecular
ridge” (cf. Coil & Ho 2000). There is another SNR G359.920.09 (cf. Coil & Ho 2000) which may be impacting Sgr A East
from the southern edge, but we do not find any clear evidence
of interaction between this SNR and the observed gas near the
northern part of Cloud-B.
The component known as the “northern ridge” coincides
with Cloud-M (Figure 3) in its location and velocity. This
is also shown in Figure 7 (right) with the blue color in the
back of the extended red component to the north of Cloud-A.
Its morphology and large velocity difference suggest that
Cloud-M (the northern ridge) is not associated with Cloud-A
(the 50 km s−1 cloud). Cloud-N also has a similar velocity and
morphology with Cloud-M. These clouds are among the many
“streaming components” found in the area, which may have different origins from Cloud-A and may be falling directly to the
CND. We do not believe that a significant part of these streaming
components have been affected by the expansion of Sgr A East.
The “western streamer” located in the west side of the CND
with an elongated filamentary shape along the north–south
direction is thought to be affected by Sgr A East (McGary & Ho
2002). Compared with our cloud identification (Figure 3), parts
of Cloud-I, J, and O may correspond to the western streamer,
and also share similar velocities with each other. Since these
components locate at the boundary of Sgr A East, a part of
3.5. Interaction with Sgr A East
The concave morphology of the 50 km s−1 cloud in the
boundary region meeting with Sgr A East has been interpreted
as evidence of the interaction (Mezger et al. 1989). However,
debate exists regarding the evidence of interaction between
Sgr A East and its surrounding gas. Some properties related
to “the northern ridge” and “the western ridge,” located around
Sgr A East, have also been controversial regarding their relation
to Sgr A East (McGary et al. 2001; Herrnstein & Ho 2005).
Figure 7 (left) shows a color-coded velocity channel map of
the observed NH3 (6, 6) line, where the 90 cm continuum
map for Sgr A East (Pedlar et al. 1989) is overlapped in the
figure for comparison. The outer boundary of the continuum
emission coincides very well with the west side of Cloud-A (the
50 km s−1 cloud) and the north side of Cloud-B (the 20 km s−1
cloud). In addition, there exists a sharp boundary of Cloud-A
in the northeast direction of Sgr A East, and no significant
emission from ammonia was found in the cavity of Sgr A East,
which could have resulted from the supernova explosion event of
Sgr A East. Except for the morphological coincidence, however,
the measured cloud parameters along this boundary do not show
specific signs of shocks or interactions; they may have faded out
because of the short relaxation timescale of the shock (103 yr).
We believe that this apparent morphological agreement cannot
result by accident, and the expansion of Sgr A East pushes away
8
The Astrophysical Journal, 773:31 (10pp), 2013 August 10
Minh et al.
the gas must be affected by the expansion of Sgr A East.
However, Cloud-I and J, for example, have a large velocity
difference compared to each other, and this cannot be explained
by the velocity gradient along one gas component. Rather, as we
have identified, they may be simply different cloud components
associated with the central region. For this reason, and with the
extended emission of Cloud-O toward the southwest direction
and its velocity gradient, these components may be related to
the infalling of gas as explained in Section 3.2. Therefore, we
believe that the expanding Sgr A East has mainly influenced
and shaped the major gas condensations, Cloud-A and B, but
the northern and western ridges are components approaching
the CND without much interaction with Sgr A East.
4. CONCLUSIONS
The molecular gas components of the Sgr A region have
been observed with the ammonia (3, 3) and (6, 6) transitions.
These observations used the GBT with the highest sensitivity
ever achieved. The gas is mainly concentrated in the wellstudied 50 km s−1 and 20 km s−1 clouds distributed along the
direction of and inclined to the galactic plane. These clouds
appear to be virialized and influenced by the expansion of
SNR Sgr A East. The observed emission, however, shows
very complicated morphological features and also velocity
structure. In addition to the major gas condensations in this
region, Gaussian multi-component fittings revealed that various
different velocity components exist all over the observed region.
Many of the “streaming” gas components have been identified
for the first time in this work, and we have derived their
physical parameters. These streaming components include those
previously known as “streamers” and “ridges,” which could be
sources interacting with the CND around Sgr A∗ . We found
that most of these components are not directly connected to
the major gas condensations (the 50 km s−1 and 20 km s−1
clouds), and locate apparently out of the galactic plane. They
may have a different origin than the major gas condensations.
Some of the streaming components might be the sources feeding
the CND directly and episodically. They may have not only
a different origin but they may also evolve differently with
major gas condensations under the influence of the activities
and potential structure of the Galactic center.
3.6. Other Streaming Gas Components
A component known as the “southern streamer” has been
thought to connect the 20 km s−1 cloud to the CND (Okumura
et al. 1989; Ho et al. 1991; Coil & Ho 1999, 2000). The
morphological connection between these two sources has also
been observed with other tracers (e.g., Dent et al. 1993; Marshall
et al. 1995; Zylka 1998). Additional filamentary features, SE1
and SE2, have also been found in the east of the southern
streamer as a part of the connection between the 20 km s−1 cloud
and the CND (Ho et al. 1991; McGary et al. 2001). However, the
southern streamer does not show a significant velocity gradient
and it is not clear whether it is directly associated with either
the CND or the nucleus (Herrnstein & Ho 2002, 2005). These
filamentary features have not been clearly identified in our ∼30
beam, probably due to a beam size that was larger than those used
in previous interferometric observations. The southern streamer
seems to be an extended filament all the way to near the CND of
a different cloud. This cloud is identified as Cloud-P and shown
in Figure 7 (right) as a bluish component in the southwest of
Cloud-C with a velocity gradient of about 5 km s−1 pc−1 ; it is
probably not a part of Cloud-B and C (the 20 km s−1 cloud). We
also found a negative velocity component near the “converging”
point of the CND (Liu et al. 2012), which could be a part
of Cloud-P falling to and interacting with the CND material.
This gas component may be the major component feeding the
southern part of the CND.
Figure 7 (right) also shows other various streaming components in the northern part of the CND. Some of them have been
discussed in Section 3.5. Two very different velocity components, Cloud-F and N, appear to overlap in the line of sight.
Cloud-F and some streaming components in the northern area
have velocities of about 100 km s−1 shown in red in Figure 7
(right). This highly positive velocity component may be the
source feeding the northern part of the CND. We found another
component, Cloud-E in the east side of Cloud-A, with a velocity of about 30 km s−1 ; this can be decomposed clearly in
multi-component Gaussian fitting. This cloud is shown in light
pinkish color in Figure 7 (left) together with the major gas condensations. We believe that this is also one of the streaming
components associated with the Galactic center. Most of the
streaming components have different velocities, shapes, and locations compared with the major gas condensations, and show
elongated filamentary features roughly to the direction of the
Galactic center. Some of these components must be the sources
feeding the CND directly. Various gas clouds identified in the
Sgr A region must have different origins from each other and
evolutionary phases under the influence of the activities of the
Galactic center.
We thank the GBT staff for their capable assistance in
making these observations. S.S.K.’s work was supported by
the Mid-career Research Program (No. 2011-0016898) through
the National Research Foundation (NRF) grant funded by the
Ministry of Education, Science, and Technology (MEST) of
Korea.
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