A model is proposed in which most of the molecular clouds near the galactic center are situated in a radially moving ring of radius 250 pc

Scoville, N. Z.

Caltech Authors - Main

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The Astrophysical Journal, 175:L127-L132, 1972 August 1 
(c) 1972. The American Astronomical Society. All rights reserved. Printed in U.S.A. 
KINEMATICS OF MOLECULAR CLOUDS 
NEAR THE GALACTIC CENTER 
N. Z. ScOVILLE* 
Department of Astronomy, Columbia University 
Received 1972 April 27 
ABSTRACT 
A model is proposed in which most of the molecular clouds near the galactic center are situated in a 
radially moving ring of radius 250 pc. 
I. INTRODUCTION 
Surveys of molecular lines in the direction of the galactic center have shown a pre- 
ponderance of broad lines (AF >20 km s-1) at high radial velocities (F > 40 km s“1). 
Most of the molecules producing these lines appear to be in discrete clouds having large 
diameters (^30 pc, McGee 1970) and masses (lO^lO6 Lf0> Scoville, Solomon, and 
Thaddeus 1972). In this Letter we show that these clouds may delineate a single, large- 
scale structure within 300 pc of the galactic center. The kinematics of the structure 
resemble those of a rotating, expanding ring concentric with the galactic center and lying 
in the galactic plane. 
n. H2CO AND CO OBSERVATIONS 
The most complete surveys of molecules near the galactic center have been made in 
18-cm OH absorption (Robinson and McGee 1970; McGee et al. 1970), 6-cm H2CO 
absorption (Scoville et al. 1972; Scoville and Solomon 1972), and 2.6-mm CO emission 
(Solomon et al. 1972). Our discussion is based primarily on observations of the last two 
lines because of their higher angular resolution (6' and 1', respectively); the OH observa- 
tions are, however, most extensive in angular coverage. 
The longitudes and velocities of clouds producing strong molecular absorption lines 
in the direction of the galactic center are shown in figure 1. Here we have plotted the 
average 6-cm H2CO optical depth at latitudes of —2' and —12'; the distribution of OH 
clouds is very similar to that shown in figure 1, although a large range in relative opacity 
of the two lines is apparent (cf. fig. Id of McGee 1970). 
At present CO emission has been surveyed only at positive velocities near the strong- 
est sources of centimeter-wavelength continuum radiation in the galactic center (cor- 
responding to the right-hand side of fig. 1). It is in the direction of these sources that we 
might expect the greatest misrepresentation from absorption lines, which are biased 
toward gas lying in front of the sources. Thus it is noteworthy that in addition to CO 
emission from virtually all the positive velocity H2CO clouds in figure 1, Solomon et al. 
(1972) also detect a CO feature at ~90 km s-1 between the two molecular clouds at 
l = 0?0 and / = 0?7. This “emission bridge,” which has only a very weak counterpart 
in H2CO (not apparent in fig. 1; but see fig. 2b of Scoville et al. 1972), provides the first 
evidence of a physical link between the well-studied molecular clouds in the directions 
of Sgr A and Sgr B2. The emission can in fact be continuously graced out to l = 1?7, 
where it either shifts out of the observed CO velocity range at +25 km s-1 or fades out 
entirely. 
* Present address: School of Physics and Astronomy, University of Minnesota, Minneapolis. 
L127 
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Fig. 1.—Contour diagram in the longitude-velocity plane of the average 6-cm H2CO optical depth 
measured at = —2' (Scoville et cd. 1972) and at 6 = —12' (Scoville and Solomon 1972). Optical-depth 
contour unit is 0.025. As discussed in text, the OH and CO distributions are similar to that of H2CO. 
The dotted curves (A and B) show the minimum and maximum radial velocities expected from gas in 
the disk obeying the rotation law of Sanders and Lowinger (1972). The solid (I) and dashed (II) curves 
show the longitude-velocity shape of two ring models in table 1. 
2V 
Fig. 2.—Sketch of rotating molecular ring near the galactic center as seen from above the galactic 
plane. Here the ring is taken to be expanding, although as discussed in the text, contraction cannot be 
ruled out. The suggested locations of continuum sources relative to ring are included; the label Sgr A 
refers only to the non thermal component at ¿ = 0?0, 6 = — 0?0. 
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KINEMATICS OF MOLECULAR CLOUDS L129 
III. DISCUSSION 
The molecular clouds of figure 1 may be separated into three distinct groups: “local” 
clouds, 3-kpc arm clouds, and galactic-center clouds. The local and 3-kpc arm clouds 
have characteristic velocities of 0 and —50 km s“1 and show very narrow lines (^2 
km s-1). 
The broad lines and high velocities of most other clouds in figure 1 indicate that they 
are very different from the 3-kpc arm and local clouds. We shall limit our discussion to 
these clouds, first introducing evidence that they are in the nuclear disk, then showing 
that they may be joined into one continuous feature, and finally presenting a kinematical 
model for this feature. 
a) Distance 
That the broad-line clouds are in the galactic nucleus is most clearly manifest by their 
high velocities; over half of them are observed at velocities greater than 40 km s-1. If 
they were between 3 and 10 kpc from the galactic center, their kinematics would prob- 
ably be similar to those of hydrogen gas in the same region, that is, | F | <40 km s-1 for 
this range of longitudes (Kerr and Vallak 1967 ; Mezger et al. 1970). The general velocity 
gradient of the clouds (negative velocities at negative longitudes and positive velocities 
at positive longitudes) is also much steeper than that of gas between the 3-kpc arm and 
the Sun, although the sense of the gradient is the same. 
Both the latitude and longitude distributions of the broad-line clouds are suggestive 
of their location in the nuclear disk. The OH observations cover the greatest latitude 
range and clearly show most of these clouds in a thin layer (A£ ~ 20') aligned with the 
galactic plane and centered at — —10'. From the lack of dense H2CO clouds at longi- 
tudes between 2° and 4?5 (at b = —2') and the small number of OH clouds between 
1 = 2° and / = 4?25 we infer that the molecular clouds are concentrated about 1 = 0° 
with only a few beyond / = 2°. At present there are no systematic molecular observa- 
tions for / < — 2° (mainly because of the lack of dense clouds between l = —1?5 and 
l = —2°). Finally, we make use of the physical association between the cloud appearing 
in figure 1 at / = 0?7 with F = 62 km s_1 and the H n region Sgr B2. The radiation 
from this H n region is also absorbed by H2CO in the 3-kpc arm, and thus both the 
cloud and H u region must lie behind the 3-kpc arm. 
Although direct evidence for one cloud and the longitude, latitude, and velocity 
distributions of these clouds, taken as a group, suggest that they are probably at dis- 
tances of several hundred parsecs from the galactic center, their velocities show little 
correspondence to pure rotation in the H i nuclear disk. Because H i lines near the center 
of the disk are severely blended with low-velocity galactic background emission, the 
rotation curve there is probably best determined indirectly from the gravitational 
potential of stars in this region. Becklin and Neugebauer (1968) have estimated the 
stellar density in the galactic nucleus from 2.2-¡jl observations, and Sanders and Lowinger 
(1972) have used these data to derive circular velocities (0C) in the disk given by 
dc = \/(7 X 104ó0-2 + 104æ — 41æ3) km s-1, (1) 
where æ is the distance in kiloparsecs from the center of the Galaxy. 
A majority of the clouds in figure 1 have velocities not “permitted” by this rotation 
law (allowed motion corresponds to the region between curves A and B in fig. 1) ; indeed, 
much gas is seen at positive longitudes with negative velocities and must therefore have 
large radial motions either toward or away from the galactic center. 
b) A Ring with Circular and Radial Motions 
Inasmuch as the kinematical behavior of the clouds does not correspond to either 
spiral-arm or nuclear-disk circular motion, we consider the two alternatives of the 
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L130 N. Z. SCOVILLE Vol. 175 
clouds being in unrelated elliptical orbits or in a single coherent structure possessing 
both circular and radial motions. The first alternative is not easily ruled out on the 
basis of the observed velocities but does appear unsatisfactory in light of the obvious 
links between many of the clouds. 
The continuity of the negative-velocity gas producing broad absorption lines is evi- 
dent in both 6-cm H2CO and 18-cm OH (see fig. \d in McGee 1970). The absorption 
appears much less continuous between the more centrally condensed positive-velocity 
clouds; indeed, no absorption is apparent in figure 1 between those clouds at l = 0?0 
and Z = 0?7. However, very definite CO emission does bridge this gap at 90 km s_1 
(§ II), and the low, nearly constant intensity of the corresponding H2CO absorption 
could be understood if G0.2—0.0, the dominant continuum source in this region, were 
in front of the 90 km s-1 gas. 
We therefore suggest that most of the galactic-center molecular clouds at both posi- 
tive and negative velocities are situated in a single structure. There is great nonuni- 
formity in this structure, especially at positive velocities, and a real gap may exist at 
slightly negative longitudes and low positive velocities. Moreover, at / = 1?5 the situa- 
tion becomes confused, there being two possible connections between the negative- and 
positive-velocity clouds. Such confusion could, however, result from “local” and 3-kpc 
arm absorption lines. 
The elliptical shape of this sequence of clouds in the longitude-velocity diagram 
might be described kinematically as a ring that is rotating and either expanding or con- 
tracting. The ring would have its center at l = 0?0 and lie in the galactic plane. Rota- 
tional motion could account for the displacement of the ends of the sequence from zero 
velocity while the expansion or contraction would cause the velocity separation of the 
two sides. Table 1 lists parameters obtained by fitting model rings separately to the 
negative- and positive-velocity H2CO and CO clouds (curves I and II of fig. 1). Most of 
the broad H2CO lines near the top and bottom of figure 1, not described by either model 
I or model II, can be encompassed by a series of models which continuously change 
parameters from I to II in going from negative to positive velocities. Since a single model 
clearly does not fit all clouds, the real structural form is probably not a ring at all but 
more like one or two tightly wound arms, but the ring description has the obvious 
advantages of simplicity and definiteness. 
Whether the ring is expanding or contracting is a difficult question whose answer will 
require higher-resolution absorption measurements. Analysis of variations in the 6-cm 
absorption intensity near the sources G0.2—0.0 and Sgr B2 indicates that the ring is 
expanding (Scoville 1971), and a suggested picture including the possible locations of 
continuum sources is shown in figure 2. However, if the ring is expanding, it is difficult 
to understand the high intensity of the OH and H2CO absorption at +40 km s_1 in the 
direction of Sgr A since that gas should lie behind the galactic center (Kerr and Sand- 
TABLE 1 
Model Rings of Figure 1 
Parameter Model I* Model Ilf 
Radiusf (pc)  218 305 
Rotational velocity (km s“1)  50 50 
Expansion or contraction velocity (km s-1)  145 70 
* Fit to negative velocity clouds in figure 1. 
t Fit to positive velocity clouds in figure 1 and figure 1 of Solomon et al. 
1972. 
J The assumed ring center is the galactic center (/ = 0?0 at 10 kpc). 
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No. 3, 1972 KINEMATICS OF MOLECULAR CLOUDS L131 
quist 1968; Fomalont and Weliachew 1971). If one makes the usual assumption that 
the nonthermal component in Sgr A is at the galactic center, then in order to form this 
absorption line, most of the other components in Sgr A must be beyond the galactic 
center and unrelated to the nonthermal component. The direction of the ring’s radial 
motion is therefore uncertain, and we must await better knowledge of the structure in 
both the molecular clouds at +40 and —140 km s-1 and the Sgr A sources. Eventually, 
the apparent inconsistencies in the position of the 40 km s-1 cloud may force a revision 
of the model proposed here—for example, possibly there are two separate arms (nega- 
tive- and positive-velocity gas), both of which are in front of the galactic center, or 
possibly the 40 km s-1 cloud is not part of the ring at all—however, we do not feel that 
such complexities are warranted on the basis of the present observations. 
The existence of large expansion motions in the gas near the center of our Galaxy has 
long been known from 21-cm observations. Rougoor and Oort (1960) and Rougoor 
(1964) made detailed studies of the 3-kpc and the +135 km s-1 expanding arms, and 
more recent observations have revealed additional features above and below the galactic 
plane which are presumably also moving outward from the galactic center. (Shane 1971 ; 
van der Kruit 1970; Simonson and Sancisi 1972; Sanders and Wrixon 1972). Including 
the molecular ring discussed here, there would appear to be evidence for at least three 
distinct, radially moving gas structures near the galactic center, and it becomes increas- 
ingly difficult to understand their formation from resonances in the galactic gravitational 
potential (Lin and Shu 1971) or from a continual gentle outflow (Moore and Spiegel 
1968). If instead, these radial motions are the result of explosive events in the galactic 
nucleus, such events must have occurred often in the recent past of our galaxy (see 
Sanders, Scoville, and Spiegel 1972 and references cited there). 
The total mass of the molecular ring, estimated from H2CO observations (Scoville and 
Solomon 1972), is 3 X 106 and the kinetic energy of its radial motion is 6 X 1053 
ergs. [These estimates are probably only lower limits since they are based solely on mo- 
lecular observations close to the galactic plane. Van der Kruit (1970) notes an H i 
feature (F in his notation) at ô = —-Io with longitude-velocity dependence similar to 
the negative velocity side of the ring. Also, the existence of CO emission at 2.6 mm 
requires a minimum density of hydrogen molecules A7^ > 103 cm-3, to excite the rota- 
tional transition above the background. This emission is observed over A¿> > 20' and 
at all longitudes in the ring where observations have been made, which gives a total 
mass ^108 Mq and a total radial kinetic energy of 2 X 1055 ergs.] The low angular 
momentum of the ring, corresponding to, circular motion at ö = 50 pc, suggests that 
much of this gas may have come from close to the center of the nuclear disk. An energy 
in excess of 4 X 1064 ergs would be required to move 3 X 106 Af0 from 50 pc out to 250 pc 
from the galactic center; and if this was supplied by a spherically symmetric explosion 
about 106 years ago, the total energy would be several times the above number. 
Our conclusions follow. 
1. Most of the molecular clouds producing broad lines in the direction of the galactic 
center are probably within 300 pc of the galactic center. 
2. The observed velocities of these clouds show little correspondence to pure rotation 
in the nuclear disk, and many clouds clearly have large radial motions either toward or 
away from the galactic center. 
3. The apparent physical links between clouds suggest that they are contained in a 
coherent structure resembling a ring, probably expanding outward at ^100 km s-1. 
I would particularly like to thank R. H. Sanders for many stimulating discussions 
on this work. 
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L132 N. Z. SCOVILLE 
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Becklin, E. E., and Neugebauer, G. 1968, Ap. 151, 145. 
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Kinematics of Molecular Clouds Near the Galactic Center

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Kinematics of Molecular Clouds Near the Galactic Center

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