# Giant molecular cloud G18.1-0.3+51 associated with HII regions and supernova
remnant in the 3-kpc expanding ring

Yoshiaki Sofue
Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo 186-0015, Japan

(Accepted; Received YYY; in original form)

###### Abstract

Analyzing the high-resolution CO-line survey of the Galactic plane with the
Nobeyama 45-m telescope (FUGIN), we show that the star-forming complex
G18.15-0.30+51 (G18) at radial velocity of 51 km s-1is a tight triple
association of a giant molecular cloud (GMC), HII regions, and a supernova
remnant (SNR). The radial velocity of G18 allows three possible kinematic
distances of $d=3.9\pm 0.2$ kpc for near solution or $12\pm 0.2$ kpc for far
solution, if we assume circular Galactic rotation, or $d=6.1\pm 0.1$, if it is
moving with the 3-kpc expanding ring at an expanding velocity of 50 km s-1.
The HI -line absorption of radio continuum from the HII region constrains the
distance to $5.6\lesssim d_{\rm SNR}\leq 7.6$ kpc. The $\Sigma-D$ (radio
brightness-diameter) relation yields the distance to the SNR of $d_{\rm
SNR}=10.1^{+11.5}_{-4.7}$ kpc, allowing for a minimum distance of 5.4 kpc.
From these we uniquely determined the distance of G18 to be $6.07\pm 0.13$ kpc
in the 3-kpc expanding ring with the SNR being physically associated. The
molecular mass of the GMC is estimated to be $M_{\rm mol}\sim 3\times
10^{5}M_{\odot}$. The ratio of Virial to luminous molecular masses is greater
than unity in the central region and decreases outward to $\lesssim 0.2$ at
the cloud edge, indicating that the central region is dynamic, while the
entire cloud is stable. We discuss the origin of the G18 triple system and
propose a sustainable GMC model with continuous star formation.

###### keywords:

ISM: bubbles — ISM: clouds — ISM: kinematics and dynamics — ISM: supernova
remnant — ISM: HII regions —stars: formation

††pubyear: 2023

##  1 Introduction

In the survey of molecular-gas shells associated with supernova remnants (SNR)
in the Galactic disc using the FUGIN111Four-receiver-system Unbiased Galactic
plane Imaging survey with the Nobeyama 45-m telescope CO-line data, several
molecular complexes associated with both star-forming region and SNR have been
found (Sofue et al., 2021). The molecular gas complex G18.15-0.30+51
(hereafter G18) is one of such ’triple’ systems, composing a tight association
of a giant molecular cloud (GMC), HII regions, and an SNR (Paron et al., 2013;
Dey et al., 2022). Such a system will provide us with an opportunity to study
the star formation process from the birth till death in a closed system.

However, physical association of the SNR with the system is still
controversial because of the uncertain distances: Radial velocities in the
molecular, HI , and HII recombination lines in emission are measured to be
$V_{\rm lsr}\sim 51-54$ km s-1. The kinematic distance to the HII regions and
molecular cloud assuming the circular rotation has been obtained to be $\sim
4$ kpc for the near-distance solution because of the absence of HI absorption
at the terminal velocity of the Galactic disc (Quireza et al., 2006; Paron et
al., 2013; Dey et al., 2022). On the other hand, the distance to the SNR has
only been constrained to $\lesssim 8$ kpc from the absence of HI line
absorption at the terminal velocity (Leahy et al., 2014). One of the purpose
of the present paper is put a more precise constraint on the distances to the
objects, and examine if the apparently triple system is indeed physically
associated with each other.

Besides the radio shells like the SNR and HII regions, bubble structures of
molecular gas are found, and are interpreted as due to the outflow driven by
star forming activity in the cloud center (Paron et al., 2013), where cloud
collisions are thought to play a role to trigger the massive-star formation
(Dey et al., 2022). We also aim at exploring detailed morphological and
kinematical properties with particular emphasis of the relation of the
molecular bubbles to the complex GMC. We further try to construct an
integrated view of the star formation and feedback mechanism in the G18
system.

In section 2, we investigate the kinematic and dynamic properties of the GMC
in G18 complex based on the detailed analysis of FUGIN (Four-receiver system
Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope) CO
line data (Umemoto et al., 2017). In section 3 we determine the distances of
the vrious components in G18, taking account of the kinematic model of the
Galaxy including the expanding 3-kpc ring. In section 4, based on the new
distance and analysis, we investigate the interstellar physics and dynamics of
the complex. In section 5 we discuss the origin of the G18 system, and propose
a model in which G18 is explained by a gravitationally stable structure with
star-forming activity sustained by efficient feedback of gas. Section 6
presents the conclusion of the work.

##  2 The Data and Maps

###  2.1 Data

In order to obtain the general view of the region around G18, we show in
figure 1 the radio continuum image a wide region around G18 at 90-cm taken
with the VLA (Very Large Array) (Brogan et al., 2006) and a composite map of
far-infrared, 20-cm and 90-cm radio continuum survey maps extracted from the
Multi-Array Galactic Plane Imaging Survey (MAGPIS) (Helfand et al., 2006), and
VLA Galactic Plane Survey (VGPS) (Stil et al., 2006).

Figure 1:  Galactic plane at 90 cm from the SNR survey by VLA (Brogan et al.
2006) reproduced from NRAO News Letter at
https://www.nrao.edu/archives/items/show/33519). The complex G18.1-0.3 is
marked by the ellipse. The right panel shows $30^{\prime}\times 30^{\prime}$
area around G18 at 20 cm (red), 8$\mu$m (green) and 90-cm (blue) extracted
from MAGPIS (Haffner et al. 2006; Stil et al. 2006) HII regions and an SNR are
marked following (Paron et al. 2013) G18.305-0.391 is a background HII region
at a distance 12.6 kpc (see Appendix).

For molecular cloud analysis, we made use of the FUGIN survey data (Umemoto et
al., 2017). The full beam width at half maximum of the telescope was 15″at the
12CO ($J=1-0$)-line frequency (115 GHz), and the velocity resolution was 1.3
km s-1. The effective beam size of the final data cube is 20″, and the rms
noise levels are $\sim 1$ K. The 3D FITS cubes have a pixel size of $(\Delta
l,\Delta b,\Delta V_{\rm lsr})=$ (8”.5, 8”.5, 0.65 km s-1). Fig. 2 shows
examples of the channel maps of 12CO -line brightness temperature ($T_{\rm
B})$ and longitude-velocity ($(l,V_{\rm lsr})$) diagrams at several latitudes
and velocity channels.

Figure 2: [Left] 12CO -line channel maps in grey scale from $T_{\rm B}=0$ to
30 K. [Right] LVD across G18 at various latitudes.  Figure 3:  12CO -line
intensity map (moment 0) in $2^{\circ}\times 2^{\circ}$ around G18.0+0.0
integrated from $V_{\rm lsr}=40$ to 60 km s-1. Note the significant north-
south asymmetry about the Galactic plane in the molecular gas distribution in
this velocity range.

12CO 13CO

C18O Composite

Figure 4: [Top left, right, middle left] Moment 0 maps in the 12CO , 13CO ,
C18O lines, respectively, for $1^{\circ}\times 1^{\circ}$ region. [Middle
right] Composite moment 0 map of the 12CO , 13CO , C18O in RGB colors for
$1^{\circ}\times 1^{\circ}$ around G18.15-0.30. Intensities are relative.
Spokes A to E, composing an ’X’ structure centered on the hub at the cloud
center, molecular bubbles (cavities) 1 to 4, and a big shell P, discussed in
the text, are marked by white lines and circles. Bubble 4 might not be an
independent object, but might trace the outskirts of the inner three bubbles.
[Bottom] Averaged 12CO intensity as a function of the distance from the hub
center along the spokes. Background emission at $b\sim-1^{\circ}$ is
subtracted. Dashed line shows a profile expected for two perpendicularly
overlapped filaments with constant intensity.

###  2.2 Intensity distributions

Fig. 1 illustrates the location of G18 in a 90-cm radio continuum survey map
reproduced from the SNR survey with the VLA (Brogan et al., 2006). G18 is
marked by a circle, which is a compact association of several HII regions and
an SNR in the northern top. The right panel enlarges a $30^{\prime}\times
30^{\prime}$ area around $l=18^{\circ}.15$, $b=-0^{\circ}.5$ in 20 cm radio
continuum by red, 90 cm by blue, and 8 $\mu$m infrared emission by green
color, as extracted from MAGPIS.

Figure 2 shows channel maps of the 12CO line brightness and longitude-velocity
diagrams (LVDs) at various latitudes. Figs. 3 shows a wide area map of the
12CO intensity, showing an extremely lopsided distribution of the CO line
emission in the observed region with respect to the Galactic plane, indicating
that the molecular gas near this velocity, $\sim 50$ km s-1, is almost empty
in the northern side of the Galactic plane.

Figure 4 shows intensity maps (moment 0 maps) around G18 integrated from
$V_{\rm lsr}=40$ to 60 km s-1in the 12CO , 13CO , and C18O line emissions
along with an overlay of these figures in RGB (red, green, blue) color. The
bottom panel shows an intensity profile as a function of the distance from the
hub center averaged among the major spokes.

###  2.3 Hub-filament/spoke-bubble structure of the molecular gas

High concentration of molecular gas toward G18 at
$(l,b)=(18^{\circ}.15,-0^{\circ}.30)$ is seen as an isolated peak in the
field, which we name it here the ”hub” according to the terminology of the
molecular cloud structure (Myers, 2009). Several molecular branches extend
from the hub extending in the directions at $PA\sim 20^{\circ},70^{\circ},\
150^{\circ},\ 220^{\circ}$ and $\sim 300^{\circ}$, which we call ”spokes” A,
B, C, D, and E, respectively, as marked by the white lines in Figs. 3 and 4.
The system composes a structure often called the hub-filament system (HFS) of
interstellar dust lanes (Myers, 2009; Kumar et al., 2020). We here introduce a
term ’spoke’ to represent the constructive role of the giant and straight
structures in conjunction to the ’hub’, while it has the same meaning as
’giant filament’ in the filament paradigm (Hacar et al., 2022; Pineda et al.,
2022).

The spokes compose a large X-shaped structure extending over $\pm
0^{\circ}.6$. Spoke A is densest but shortest, composing multiple cavity and
molecular bubble (shell) structures. Spoke B extends almost straightly for
more than $\sim 0^{\circ}.6$ toward NE, making a counter part to the also
straight spoke D to the SW. Spokes B and E approximately trace the southern
rim of the big molecular shell noticed in the earlier study marked as Shell P
(Paron et al., 2013).

Spoke A toward the NW is connected to several open cavities, as marked by the
white circles, which we name the molecular bubbles 1, 2, 3 and 4. The
molecular bubbles are more clearly seen in the moment 0 map shown in figure
5(a) for the core $30^{\prime}\times 30^{\prime}$ area, where the cut-off
brightness was set to $T_{\rm B}\leq 5$ K.

Molecular bubble 1 (hereafter, ”bubble”) is almost perfect, and the bubble’s
center coincides in position with the strong radio continuum source (spot) of
the HII region G18.146-0.280 as shown in figure 6 (b). The inner edge of the
bubble exactly traces the southern rim of the HII shell surrounding the radio
spot. Their coherent and concentric orientation strongly support the picture
that they are formed by caving compression of the molecular hub by a high
pressure HII sphere produced around the hot spot, possibly nesting O stars
(Paron et al., 2013).

Bubbles 2 and 3 coincide with fainter HII regions, apparently overlapping with
the western rim of SNR G18.1-0.1 on the same sight line. Bubble 4 encircles
the entire HII region and SNR area, which might not be an independent object
from the others.

The multiple bubbles with their intensity getting fainter according to their
increasing sizes and distances from the GMC center (hub) suggests a sporadic
injection of kinetic energy in the core region of the GMC along the scenario
of sequential star formation (Elmegreen & Lada, 1977; Paron et al., 2013).

Figure 5: [Top] 12CO -line moment 0 map integrated from $V_{\rm lsr}=45$ to 55
km s-1(grey bar scale is K m s-1) with the molecular bubbles marked by red
circles. The cut-off level for integration is set to $T_{\rm B}\leq 5$ K.
[Middle] Moment 1 map (velocity field) in m s-1. [Bottom] Moment 2 map
(velocity dispersion) in m s-1. White regions are where no emission brighter
than 5 K (cut off level) is detected.

Figure 6:  [Top] Overlay of 8$\mu$m intensity in red, 12CO in green, and 20-cm
continuum in blue. [Middle] 12CO intensity overlaid by 20-cm contours every 1
mJy beam-1 with the lowest at 2. [Bottom] Same, but molecular bubble 1 is
enlarged superposed with 20-cm contours every 5 mJy beam-1. The HII hot spot
G18.147-0.280 is located at the bubble centre, while the HII shell traces the
inner wall of the molecular bubble.

###  2.4 Kinematics

The moment 1 map (velocity field) in figure 5 shows a velocity gradient from
the south (blue shifted) to north (red shift). This indicates either large-
scale rotation on the SN plane, or an outflow from the center toward the north
and away from the Sun.

The moment 2 map shows increasing velocity dispersion toward the hub center,
attaining the highest value near the center at two positions. The NW peak of
the velocity dispersion coincides with the CO intensity peak and the brightest
spot of the HII region (Paron et al., 2013).

Note that the moment 2 map here is biased to indicate smaller velocity
dispersion due to the cut-off brightness at $T_{\rm B}\leq 5$ K, which
suppressed extended low brightness emission with broader velocity width.

##  3 The Distance

The kinematical distance for circular rotation curve has been derived to be
$d\simeq 4$ for the near-side solution and $\sim 14$ kpc for far-side
corresponding to the Galacto-centric distance of $R_{\rm GC}\simeq 6$ kpc
(Downes et al., 1980). The near/far ambiguity has been resolved using the
absence of HI absorption of the radio continuum emission of the HII region at
the terminal velocity of 130 km s-1, yielding the near distance of $d=4.1$
(Quireza et al., 2006) to $4.2$ kpc (Anderson et al., 2009). In this section,
we newly determine the distances to the GMC, HII region, and SNR applying the
kinematic method to the CO line, radio recombination line (RRL), HI absorption
lines, and $\Sigma-D$ (surface brightness-diameter) relation to the SNR
G18.15-0.17. We particularly consider the non-circular rotation of the Galaxy
in the 3-kpc expanding ring (3-kpc ER).

###  3.1 Distance of G18 in circular rotation

Fig. 8 shows LVD along the Galactic plane from the Columbia 12CO line survey
(Dame et al., 2001) and an LVD at $b=-0^{\circ}.3$ made from the FUGIN 12CO
survey. Two major Galactic structures are indicated by dashed lines: the 4-kpc
molecular ring (4-kpc MR) and 3-kpc ER drawing an inclined ellipse with the
expanding velocity of 50 km s-1at $l=0^{\circ}$. The molecular complex G18 is
located at $(l,v)\sim(18^{\circ},+51)$ km s-1) near the intersection of the
4-kpc MR and the 3-kpc ER. The FUGIN LVD demonstrates how G18 is compact and
particular in the plotted area. There are three possible solutions about the
kinematic location of G18 on the LVDs. One and second are the near- and far-
side arms on the 4-kpc MR in ordinal circular rotation, and the other is the
approaching side of the 3-kpc ER.

Using the CO-line profiles shown in figure 10 we measured the radial velocity
of the molecular cloud to be $V_{\rm lsr}=51\pm 2$ km s-1. This velocity
agrees with that measured for the hydrogen recombination lines H$110\alpha$ of
52 to 53 km s-1(Downes et al., 1980; Quireza et al., 2006).

We here use the most accurate rotation curve (CO) of the first quadrant of the
inner Milky Way as shown in figure 7 (Sofue, 2021). The radial velocity
$v_{\rm r}$ and distance $d$ to the object in circular rotaiton are related by

$v_{\rm r}=\left(\frac{R_{0}}{R}V(R)-V_{0}\right)\sin\ l,$ (1)

and

$d=R_{0}\cos\ l\pm\sqrt{R^{2}-R_{0}^{2}\sin^{2}l},$ (2)

where $V(R)$ is the rotation velocity at Galacto-centric distance $R$,
$V_{0}=238$ km s-1, and $R_{0}=8.2$ kpc is the Galactic constants. By
iteration we obtain $R=4.7\pm 0.2$ kpc, and the near side solution of $d$ is

$d_{\rm near}=3.9\pm 0.2{\rm kpc},$ (3)

and a far side solution

$d_{\rm far}=11.7\pm 0.2{\rm kpc}.$ (4)

Because of the absence of the HI line absorption against continuum of the HII
region, as described in the previous subsection, we may choose here the near
solution of $d=3.9\pm 0.2$ kpc for the circular rotation.

Figure 7: The most accurate rotation curve of the inner Milky Way in the first
quadrant constructed from the FUGIN CO-line terminal velocities (Sofue 2021).
Big circle indicates the Galacto-centric distance and rotation velocity of G18
obtained by iteration. The bar to the circle represents input error of the
rotation velocity, considering the intrinsic fluctuation and errors of RC.

###  3.2 Distance of G18 in the 3-kpc expanding ring

As shown by the LVDs, G18 is located at the intersection of LVD ridges. So,
another possibility is that G18 is located on the 3-kpc ER. If we assume that
the source is moving with the expanding ring at $V_{\rm expa}$ and rotating at
$V_{\rm rot}$ obeying the RC, we have

$v_{\rm r}=\left(\frac{R_{0}}{R}V(R)-V_{0}\right)\sin\ l\pm V_{\rm
expa}\cos(l+\theta_{\pm}),$ (5)

where $\theta_{\pm}$ is the GC angle satisfying

$d_{\pm}\cos\ l\pm R\cos\ \theta_{\pm}=R_{0}.$ (6)

For the approaching motion of G18, we can attribute it in the near side of the
3-kpc ER and uniquely solve the equation to yield

$d=R_{0}\cos\ l-\sqrt{R_{\rm GC}^{2}-R_{0}^{2}\sin^{2}l}.$ (7)

Inserting $R_{\rm GC}=R_{0}\sin\ l_{\rm tan}=3.07\pm 0.14$ pc with $l_{\rm
tan}=22^{\circ}\pm 1^{\circ}$ as read by an ellipse fit to the 3-kpc ER’s LV
ridge in figure 8 , we obtain

$d=6.07\pm 0.13\ {\rm kpc}.$ (8)

###  3.3 Intersection of 4-kpc molecular ring and 3-kpc expanding ring

Fig. 8 shows a longitude-velocity diagram of the 12CO -line emission at
$b=-0^{\circ}.3$ in the central disc at $30\geq l\geq-30^{\circ}$. The two arm
structures are evident: One is the 4-kpc MR composed of dense molecular
complexes, running straightly from $(l,v)\sim(-30^{\circ},-50{\rm km\
s^{-1}})$ to $(+30^{\circ},70{\rm km\ s^{-1}})$, as marked by the red dashed
line. The straight LV behavior indicates a circular rotating ring. The other
is the 3-kpc ER, composing a tilted ellipse on the LV plane, as traced by the
red dashed line, which represents an expanding ring of radius 3 kpc and
expansion velocity 50 km s-1. It is stressed that G18 is located near the
intersection of the two rings on the LV plane.

(a)

(b)

(c)

Figure 8: (a) FUGIN LVD at $b=-0^{\circ}.3$ from $l=14^{\circ}$ to
$22^{\circ}$. Note that G18 is located near the intersection of the 3-kpc ER
and 4-kpc MR. (b) Colombia LVD at $b=-0^{\circ}.375$ in the 12CO -line survey
(Dame et al. 2001). (c) Illustration of the mutual positions of the structures
in the Galactic plane.

###  3.4 Distance of SNR and physical association with G18

The $\Sigma-D$ (surface brightness-diameter) relation at 1 GHz has been
obtained for Galactic SNRs with known distances (Case & Bhattacharya, 1998;
Pavlović et al., 2013). We here adopt the most recent relation by (Pavlović et
al., 2013)

$\Sigma_{\rm 1GHz}=3.89^{+12.81}_{-2.98}\times 10^{-15}(D/1{\rm pc})^{-3.9\pm
0.4}{\rm w~{}m^{-2}Hz^{-1}sr^{-1}},$ (9)

where $\Sigma_{\rm 1GHz}$ and $D$ are the surface radio brightness at 1 GHz
and linear diameter of the SNR, respectively. In order to make the
contamination by the thermal emission from the HII regions overlapping the SNR
eastern shell as small as possible, we use here the lower frequency (90-cm
continuum) map from VGPS survey (Stil et al., 2006). We measure the mean
brightness on the map and converted it to that at 1 GHz assuming a spectral
index of $\alpha=-0.6$. Using the thus obtained $\Sigma_{\rm 1\ GHz}$, we
determined the linear diameter by applying the $\Sigma-D$ relation to obtain
$D=20.1^{+22.9}_{-9.3}$ pc and the distance

$d_{\rm SNR}\sim 10.1^{+11.5}_{-4.7}{\rm kpc}.$ (10)

This distance locates the SNR farther than G18 molecular complex. However, the
large error allows for a possible minimum distance of $d\sim 10.1-4.7=5.4$
kpc.

On the other hand the HI -line absorption measurement toward the SNR has shown
that the continuum radio emission from the SNR G18.15-0.17 exhibits absorption
at $V_{\rm lsr}\simeq 55$ and 100 km s-1, but no absorption is found at the
terminal velocity 130 km s-1(Leahy et al., 2014). The absorption at 55 km
s-1is attributed to the 3-kpc ER at 6.1 kpc. The 100 km s-1absorption is due
to the HI disc arm in circular rotation in front of the 3-kpc ER, and the
kinematic distance is determined to be $5.6\pm 0.4$ kpc as the near solution,
in good agreement with the current determination using HI -line absorption
(Leahy et al., 2014). Another constraint is given by the absence of absorption
at the terminal velocity, indicating that the SNR is located nearer than the
tangent point at $\leq 7.6$ kpc.

From these estimations, we can give a firm constraint on the distance of the
SNR as $4.0\leq d_{\rm SNR}\leq 7.6$ kpc. This means that it is reasonable to
locate the SNR in the 3-kpc ER at a distance of 6.07 kpc. Besides the
distance, the apparently compact location on the sky of the SNR in touch with
the HII regions and encircled by the molecular shells (Figs. 6) as well as the
isolation of the entire G18 system on the sky (Fig. 1) supports the idea that
the SNR is physically associated with G18. In more detail, the sharp HI
absorption at 55 km s-1(Leahy et al., 2014) indicates that the SNR is located
in the far side of G18 as illustrated in figure 8.

###  3.5 The reliable distance

In table 1 we list the possible solutions for the distance to G18, and plot
them in figure 9. We have obtained four possible distances:
(a) If the GMC and HII regions are rotating on a circular orbit, we have
$d_{\rm MC,\ near}=3.9\pm 0.2$ kpc as the near solution.
(b) Or, $d_{\rm MC,\ far}=11.7\pm 0.2$, as the far side solution. However,
this is not allowed, because no HI absorption is observed at the terminal
velocity (Dey et al., 2022).
(c) HI -line absorption of radio continuum emission at $V_{\rm lsr}=55$ km
s-1by the 3-kpc ER (Leahy et al., 2014) indicates a distance of $d\geq 6.1$
kpc.
(d) If MC and HII regions are located on the 3-kpc ER, we uniquely obtain
$d_{\rm MC}=6.07\pm 0.13$ for the near side solution with approaching
(negative) radial velocity.
(e) The SNR’s distance is estimated by the $\Sigma-D$ relation as $d_{\rm
SNR}\sim 10.1^{+11.5}_{-4.7}$ kpc, which allows a minimum distance of 6.1 kpc.

We thus obtain a unique combination of GMC, HII and SNR distances consistent
with each other, (b) + (c) + (d) + (e), but (a) is not favoured. Figure 9
demonstrates the consistency of the solutions, except for those assuming the
circular rotation of G18. The compact and isolated appearance of G18 in the
LVD is also particular, showing an exceptionally high brightness clump among
the others in Fig. 8. Another point is the LV location, where G18 is located
near the intersection of the 3-kpc ER and 4-kpc MR, but displaced from the
latter. From these facts, we conclude that the distance to G18 is $d=6.07$ kpc
at a Galacto-centric distance of $R_{\rm GC}=3.1$ kpc.

Table 1: Distances to G18.15-0.30

Object/method | Quantity | Result | Remark
---|---|---|---
HII region G18 |  |  |
HI-line absorption or not | No HI abs. at term. velo. |  $d\leq 7.6$ kpc | HI abs. (Anderson et al., 2009)
| HI abs. at 100 km s-1 |  $d\geq 5.6\pm 0.4$ kpc | HI abs. by foreground disc
| HI abs. at 55 km s-1 |  $d\geq 6.1$ kpc | HI abs. by 3-kpc ER
GMC & HII regions |  |  |
Kin. dist. by circ. rot. curve | Radial velocity |  $V_{\rm lsr}=51\pm 2$ km s-1 | (1)
| Galactic rot. velo. |  $V_{\rm rot}=230\pm 8$ km s-1 | (2)
| GC distance |  $R=4.7\pm 0.2$ km s-1 |
| Distance, near |  $d_{\rm near}=3.9\pm 0.2$ kpc | Allowed.
| Distance, far |  $d_{\rm far}=11.7\pm 0.2$ kpc | Not allowed by no HI tangent abs.
GMC & HII regions |  |  |
Kin. dist. 3-kpc expa. ring | Radial velocity |  $V_{\rm lsr}=51\pm 2$ km s-1 |
| Galactic rot. velo. |  $V_{\rm rot}=230\pm 8$ km s-1 |
| Expanding motion |  $V_{\rm expa}=50$ km s-1 | at $l=0^{\circ}$
| GC distance |  $R=3.07\pm 0.14$ kpc |
| Distance |  $d_{\rm 3kpc}=6.07\pm 0.13$ kpc | Allowed.
SNR G18.15-0.17 |  |  |
HI-line absorption or not | No HI abs. at term. velo. |  $d_{\rm SNR}\leq 7.6$ kpc |
| HI abs at 100 km s-1 |  $d_{\rm SNR}\geq 5.6\pm 0.4$ kpc | agrees with (Leahy et al., 2014)
| HI abs at 55 km s-1 |  $d_{\rm SNR}\geq 6.1$ kpc | HI abs. by 3 kpc ring
$\Sigma-D$ relation |  |  | (Pavlović et al., 2013)
| Angular diameter |  $\theta=6.85$ arcmin | (3)
| Surface brightness | $\Sigma_{\rm 1GHz}=3.21\times 10^{-20}\ {\small\rm w\ m^{-2}sr^{-1}}$ | (3)
| Diameter by $\Sigma-D$ |  $D=20.1^{+22.9}_{-9.3}$ pc | (4)
| Distance |  $d_{\rm SNR}=10.1^{+11.5}_{-4.7}$ kpc |

(1) Molecular line velocity coincides with the H110$\alpha$ line velocity of
$V_{\rm lsr}=52$ km s-1(Downes et al., 1980; Anderson et al., 2009).
(2) $V_{0}=238$ km s-1, $R_{0}=8.2$ kpc, and the new rotation curve of the 1st
quadrant of MW by FUGIN 12CO data (Sofue, 2021).
(3) Angular diameter and 20-cm brightness were measured on the VLA 20-cm map
using VGPS survey (Stil et al., 2006), and converted to $\Sigma_{\rm 1GHz}$
for $\alpha=-0.6$. (4) $\Sigma-D$ relation for Galactic SNRs (Pavlović et al.,
2013).

Figure 9: Derived distances of the SNR, HII regions and GMC in G18 by various
methods from Table 1. allowed ranges are indicated by the horizontal bars. All
the values, except for circular rotation assumption (red marks), are
consistent within the allowed ranges. The vertical blue line indicates the
distance $d=6.07\pm 0.13$ kpc to G18. TP stands for tangent point, and fore.
arm means a foreground arm in front of the 3-kpc ER.

We thus obtain a unique combination of GMC, HII and SNR distances consistent
with each other is (b)+(c)+(d)+(e), and (a) is not favoured. Figure 9
demonstrates the consistency of the solutions, except for those assuming the
circular rotation of G18. The compact and isolated appearance of G18 in the
LVD is also particular, showing an exceptionally high brightness clump among
the others in figure 8. Another point is the LV location, where G18 is located
near the intersection of the 3-kpc ER and 4-kpc MR, but displaced from the
latter. From these facts, we conclude that the distance to G18 is $d=6.07$ kpc
at a Galacto-centric distance of $R_{\rm GC}=3.1$ kpc.

##  4 Kinematics, Mass and Energy

###  4.1 Molecular mass

Given the distance of $d=6.07$ kpc, we then estimate the molecular mass of the
region enclosed by a circle centered on the peak intensity position at
G18.15-0.30. We measure the full velocity width of half maximum of the 12CO
line emission $\sigma_{v}$, and the peak brightness temperature $T_{\rm p}$
using the line profiles averaged in each circle. Then the averaged integrated
intensity inside the circle is calculated by

$I_{\rm CO}=\sigma_{v}T_{\rm p},$ (11)

which yields the mean column density of $H_{2}$ by

$N_{\rm H_{2}}=X_{\rm CO}I_{\rm CO}.$ (12)

The molecular mass inside radius $r$ is estimated by

$M_{\rm mol}=\mu\ 2m_{\rm H}\ X_{\rm CO}I_{\rm CO}A,$ (13)

where $\mu=1.38$ is the mean molecular weight per hydrogen including metals,
$A=\pi r^{2}$ and $r=d\ {\rm tan}\theta_{\rm dia}/2$. We adopted the
conversion factor with radial gradient given by

${\rm log}(X_{\rm CO}/{X_{\rm CO}}_{0})=0.41(R-R_{0})/r_{e}$ (14)

with $r_{e}=6.7$ kpc (Arimoto et al., 1996), which yields the conversion
factor at $R_{\rm GC}=3$ kpc as $X_{\rm CO}=0.96\times 10^{20}$ for the local
value of ${X_{\rm CO}}^{0}=2\times 10^{20}$ and $1.4\times 10^{20}$ H2 cm -2
[K km s${}^{-1}]^{-1}$for $d=3.9$ kpc.

Table 2 lists the measured parameters and calculated masses for various radii,
and figure 11 plots them. In order to compare the results with those by the
current studies, we also present the values calculated for the near distance
of 3.9 kpc. Note that the velocity width $\sigma_{\rm v}$ in this table varies
rather slowly with the radius compared to the steeper increase of velocity
dispersion toward the center in the moment 2 map in figure 5. The reason for
the difference is because the values here are means within the averaging
circles, while moment 2 map indicates individual local values and is biased to
pick up peaky profile component due to the cut-off brightness at $T_{\rm
B}\leq 5$ K, leading to smaller velocity widths.

Regardless the object’s distance, the calculated mass increases with the
radius, so that the total mass is not uniquely determined from the
observation. We here define the cloud’s size as the region inside a circle of
diameter $0^{\circ}.4$ corresponding to $r_{\rm cloud}=22$ pc, and total mass
to be $M_{\rm G18}\sim 2.8\times 10^{5}M_{\odot}$ for $d=6.07$ kpc.

The mass estimated here may be compared with that obtained in a wider area
from 13CO intensity (Paron et al., 2013) of $5\times 10^{5}M_{\odot}$ for a
distance of 4 kpc and radius 24 pc, which may be converted to $1.2\times
10^{6}M_{\odot}$ and 38 pc at our distance of 6.1 kpc. The difference will be
partially due to the adopted conversion factor, which is about a half in the
present calculation, and to the defined cloud size on the sky, which is also
about a half here.

###  4.2 Virial Mass

The half-width-of-half-maximum velocity related to the observed full velocity
width $\sigma_{v}$

$\sigma_{\rm 3D}=1/2\sigma_{v}$ (15)

and radius $r$ are then used to calculated the Virial mass of the cloud inside
by

$M_{\rm vir}=\sigma_{\rm 3D}^{2}r/2G.$ (16)

We define the specific volume densities in a sphere of radius $r$ by

$\rho_{\rm mol}=M_{\rm mol}/(4\pi/3\ r^{3})$ (17)

and

$\rho_{\rm vir}=M_{\rm vir}/(4\pi/3\ r^{3}).$ (18)

We plotted the calculated values against $r$ in figure 11 .

We introduce a ratio of the Virial to molecular masses $\mathcal{R}=M_{\rm
vir}/M_{\rm mol}$. The ratio is about $\sim 2$ in the centre, and decreases
with the radius at $\mathcal{R}=4(r/1{\rm pc})^{-0.6}$ as indicated by the
dashed line in the figure. The cloud is more unstable near the center, and is
stable, or gravitationally bound, at the edge of the cloud. Such a radial
variation may be attributed to a high rate of injection of kinetic energy in
the central region. The energy source may be the expanding HII shell produced
by the massive star formation in the centre observed as the high-brightness
HII region at G18.15-0.30, coinciding in position with the cloud density
maximum (Fig. 1).

Figure 10:  12CO line profile at the intensity peak
$(l,b)=(18^{\circ}.10,-0^{\circ}.32)$ by thin line and averaged profile within
the big circle of diameter $0^{\circ}.4$ (thick line).

(a) Mass (b) Mass density

(c) Energy (d) Energy density

Figure 11:  (a) Molecular (circles) and Virial (triangles) masses within
radius $r$. Black symbols are for adopted distance of $d=6.07$ kpc. We also
plot the quantities for 3.9 kpc by the grey lines in order for comparison with
the current studies. Small symbols show the ratio of Virial to molecular
masses $\mathcal{R}$, which is approximately fitted by
$\mathcal{R}=1.1(r/1{\rm pc})^{-0.6}$. (b) Same, but densities in $M_{\odot}\
{\rm pc}^{-3}$. (c) Same, but ${\rm log}\ E_{k}$ and ${\rm log}\ E_{g}$ in
erg. (b) Same, but for energy densities $u_{k}$ and $u_{g}$ in erg pc-3.

###  4.3 Energy

We have calculated the kinetic energy of the cloud with the derived molecular
mass

$E_{\rm k}=1/2M_{\rm mol}\sigma_{\rm 3D}^{2}$ (19)

and compared it with the gravitational energy

$E_{\rm g}=GM_{\rm mol}^{2}/r,$ (20)

which are also listed in table 2 along with their ratio, $E_{\rm k}/E_{\rm
g}$. We also calculated their density

$u_{\rm k}=E_{\rm k}/(4\pi/3\ r^{3})$ (21)

and

$u_{\rm g}=E_{\rm g}/(4\pi/3\ r^{3}),$ (22)

as well as their ratio. In figure 11 we plotted their values as a function of
the radius from the cloud center. The energies of the entire cloud to the edge
is on the order of $\sim 10^{50-51}$ erg, We point out that the gravitational
energy increases with the radius more steeply than the kinetic energy. The
estimated values depend on the molecular mass, and hence on the conversion
factor $X_{\rm CO}$, while the general trend of mutual radial variations do
not change by the $X_{\rm CO}$ variation within a factor of $\sim 2$.

In the entire cloud, $E_{\rm g}$ significantly exceeds $E_{\rm k}$, or the
total energy $U=E_{\rm k}-E_{\rm g}$ is negative. On the contrary, the core
region inside $\sim 2$ pc has positive $U$, so that the gas is unstable or
expanding. From these energetics, we may consider that the GMC is
gravitationally bound and contracting, whereas the central region is expanding
due to some injection of kinetic energy from inside. This property does not
depend on the cloud’s distance.

Table 2: G18.15-0.30 molecular cloud mass and energy.

Ang. radius | Radius $r$ | Mean $T_{\rm B}$ | $\sigma_{\rm FWHM}$ | $M_{\rm vir}$ | $M_{\rm mol}$ | log $E_{k}$ | log $E_{g}$ | $M_{\rm vir}/M_{\rm mol}$
---|---|---|---|---|---|---|---|---
$(^{\circ})$ | (pc) | (K) | (km s-1) | $(M_{\odot})$ | $(M_{\odot})$ | (erg) | (erg) | $=E_{k}/E_{g}$
$d=6.07$ kpc† |  |  |  |  |  |  |  |
0.005 | 0.265 | 45.0 | 6.0 | 0.28E+03 | 0.13E+03 | 46.05 | 45.71 | 2.20
0.010 | 0.530 | 41.0 | 6.0 | 0.55E+03 | 0.46E+03 | 46.61 | 46.53 | 1.21
0.025 | 1.324 | 32.5 | 10.0 | 0.38E+04 | 0.38E+04 | 47.97 | 47.97 | 1.02
0.050 | 2.649 | 26.0 | 10.5 | 0.84E+04 | 0.13E+05 | 48.54 | 48.72 | 0.67
0.100 | 5.297 | 22.0 | 10.5 | 0.17E+05 | 0.43E+05 | 49.07 | 49.48 | 0.39
0.150 | 7.946 | 17.5 | 10.0 | 0.23E+05 | 0.73E+05 | 49.26 | 49.76 | 0.31
0.200 | 10.594 | 15.2 | 10.5 | 0.34E+05 | 0.12E+06 | 49.51 | 50.06 | 0.28
0.300 | 15.891 | 12.4 | 9.0 | 0.37E+05 | 0.19E+06 | 49.58 | 50.28 | 0.20
0.400 | 21.188 | 10.3 | 9.0 | 0.50E+05 | 0.28E+06 | 49.75 | 50.49 | 0.18
$d=3.9$ kpc‡ |  |  |  |  |  |  |  |
0.005 | 0.170 | 45.0 | 6.0 | 0.18E+03 | 0.52E+02 | 45.67 | 45.13 | 3.42
0.010 | 0.340 | 41.0 | 6.0 | 0.35E+03 | 0.19E+03 | 46.23 | 45.96 | 1.88
0.025 | 0.851 | 32.5 | 10.0 | 0.25E+04 | 0.16E+04 | 47.59 | 47.39 | 1.58
0.050 | 1.702 | 26.0 | 10.5 | 0.54E+04 | 0.52E+04 | 48.16 | 48.14 | 1.04
0.100 | 3.403 | 22.0 | 10.5 | 0.11E+05 | 0.18E+05 | 48.69 | 48.90 | 0.61
0.150 | 5.105 | 17.5 | 10.0 | 0.15E+05 | 0.30E+05 | 48.88 | 49.19 | 0.49
0.200 | 6.807 | 15.2 | 10.5 | 0.22E+05 | 0.49E+05 | 49.13 | 49.48 | 0.44
0.300 | 10.210 | 12.4 | 9.0 | 0.24E+05 | 0.77E+05 | 49.19 | 49.70 | 0.31
0.400 | 13.614 | 10.3 | 9.0 | 0.32E+05 | 0.11E+06 | 49.36 | 49.91 | 0.28

† G18 MC and HII are on the 3-kpc ER. Plotted in figure 11 by black symbols
for $X_{\rm CO}=0.96\times 10^{20}$ H2 cm -2 [K km s${}^{-1}]^{-1}$.
‡ M18 belongs to the 4-kpc MR. Plotted in figure 11 by grey symbols for
$X_{\rm CO}=1.4\times 10^{20}$ H2 cm -2 [K km s${}^{-1}]^{-1}$.

The increase of kinetic energy toward the center is related to the increase of
velocity dispersion. Although the averaged values over the cloud at different
radii are not so variable, the moment 2 map in figure 5 shows an increase in
the specific velocity dispersion toward the center.

###  4.4 Position-velocity diagrams

The kinematic behaviour and gaseous motion can be examined using longitude-
velocity diagrams. The right panel of figure 2 shows 12CO -line LVD from FUGIN
at various latitudes, and figure 12 enlarges the LVD across the peak intensity
position of the cloud along longitude, latitude, and the major axis of the
molecular hub at $PA=45^{\circ}$. Impressive in these figures is the oval
feature symmetrically elongated in the direction of the velocity, composing an
ellipse in the LVD.

Figure 12: [Top] Longitude-velocity diagram across $b=0^{\circ}.3$. [Middle]
Latitude-velocity diagram at $l=18^{\circ}.15$. [Bottom] Position-velocity
diagram at $PA=45^{\circ}$ across G18.15-0.3, showing rotation and/or
expansion of the cloud.

The elongated feature in the LVD may be explained by various mechanisms such
as expanding shells due to the pressure of HII regions (Paron et al., 2013),
cloud collision (Dey et al., 2022). interaction of two filaments (Kumar et
al., 2020), or by the gravitational contraction.

We here notice that the LVD oval in Fig. 12 is extending for $\sim\pm 5{\rm
km\ s^{-1}}$ symmetric with respect to the systemic velocity $V_{\rm lsr}=51$
km s-1. Such symmetry of velocity may be naturally attributed to an expansion
of a shell rather than to a cloud collision, which is characterized by a
lopsided bridge feature extending from the targeted GMC to the colliding body
(Haworth et al., 2015).

So, we next examine if the observed position-velocity diagrams and morphology
of the molecular bubbles can be explained by expanding shells. The top panels
show expanding-shell model by star-forming activity at the edge of a molecular
cloud represented by the white ellipse. The expansion of a gaseous shell was
traced by the adiabatic shock wave approximation in order to examine the
qualitative behavior of the expanding bubble (Sakashita, 1971; Sofue, 2022).
The left panel shows the shock fronts at two epochs at $t\sim 0.5$ and 1 My as
projected on the $(x,z)$ plane, and the right shows corresponding position-
velocity diagram seen from the $z$ direction of the shell. Here, the kinetic
energy is injected continuously at the coordinate origin by an injection rate
of $dE/dt\sim 2\times 10^{49}$ erg My-1. The parent cloud is assumed to have
an off-center Gaussian density profile as indicated by the white ellipse with
the center density of $10^{4}$ H2cm-3. The parameter combination corresponds
to a linear scale of the figure frame of $\sim 20$ pc and velocity frame $\sim
10$ km s-1.

Shock wave simulation: Projected on the sky (left), and position-velocity
diagram (right)

$I_{\rm CO}$: On the sky (37 pc $\times$ 37 pc) and LVD (37 pc $\times V_{\rm
lsr}$)

$T_{\rm B}$ at 50 km s-1channel map: On the sky (19 pc $\times$ 19 pc), and
LVD (19 pc $\times\pm 17$ km s-1

Figure 13: [Top] Left: Star-forming bubble model, showing an expanding shock
front at two epochs. White ellipse illustrates the background cloud. Right:
Corresponding position-velocity diagram seen from the bubble top. Oval as well
as bridge features are reproduced. (Scale is arbitrary.) [Middle] Left:
Integrated 12CO intensity of G18 in K m s-1. Right: LVD of $T_{\rm B}$ (K)
averaged between $b=-0^{\circ}.31$ and $-0^{\circ}.275$. [Bottom] Left:
Channel $T_{\rm B}$ map around bubble 1 at $V_{\rm lsr}=50$ km s-1, showing
the cavity around the strong continuum source of the HII region shown by the
contours. Scale is enlarged by three times than the middle panels. Right: LVD
across the central HII region of the left panel, showing a spherical LV
feature indicative of a symmetric expansion of a bubble at $\sim\pm 8$ km s-1.

The middle panels show the integrated 12CO intensity (K m s-1) of G18
molecular bubble1 1 to 3 and the averaged LV diagram near bubble 1 from Fig.
12. The bottom panels show an enlarged (three times than the middle panel)
channel map $T_{\rm B}$ (K) at $V_{\rm lsr}=50$ km s-1around bubble 1 and LVD
across the strong continuum source of the HII region shown by the contours in
the left panel. The LVD shows an expanding oval feature indicative of a
spherical expansion of a shell (bubble) at $\sim\pm 8$ km s-1. Such a
symmetric and high velocity expansion cannot be explained by a cloud
collision, which postulates a lopsided oval with a bridge feature Haworth et
al. (2015).

The cavity structure opening from the cloud’s edge toward the north and the
position-velocity oval elongated in the velocity direction are well simulated
by this simple model. The bridge feature, often raised as the evidence for the
collision model (Fukui et al., 2021), is also produced in the expanding bubble
model. This model is also consistent with the location of the brightest HII
region with a hot spot G18.144-0.282 as the energy injection source inside the
cavity as observed in figure 6, where a couple of O stars are reported to be
located (Paron et al., 2013). It is also pointed out that molecular bubbles 2
and 3 are approximately centred by HII region G18.19-0.18 overlapping with the
SNR. These multi-bubble structure is recognized as corresponding multi-LVD
ovals in figure 12. We also stress that the outer shell-shaped HII regions
G18.18-0.40 (N21) and G18.26-0.31 (N22) are located surrounded by CO shell or
arcs.

##  5 Discussion

We here try to explain the origin of the G18 system according to a classical
scenario of star formation in a GMC based on the facts presented in this
paper. We consider a core-haloed GMC associated with expanding molecular
bubbles driven by an OB star cluster born by the sequential star formation in
the hub centre and their feedback.

Due to the gravitational contraction of the spokes toward the gravity center,
the density in the hub increases in the free-fall time of $t_{\rm ff}\sim 2$
My for $n_{\rm H_{2}}\sim 10^{3}$ H2 cm-3in the spokes. Star formation will
take place in the compressed hub center by the gravitational (Jeans)
instability (Paron et al., 2013), with the Jeans mass of $M_{\rm J}\sim
9M_{\odot}$ and growth time of $t_{\rm J}\sim 0.7$ My for the observed density
of $n_{\rm H_{2}}\sim 10^{4}$ H2cm-3(Fig. 11) and assumed temperature of
$T\sim 20$ K. Namely, massive (OB) stars greater than $9M_{\odot}$ are born in
a relatively short time scale of $t_{\rm J}\sim 1$ My. Although unresolved in
the present observations, the hub center would be turbulent, full of higher
density clumps, which would in parallel leads to the formation of lower Jeans
mass stars in shorter time scales. Thus, once the hub is compressed to $\sim
10^{4}$ H2 cm-3, a star cluster including massive to low mass stars are born
within a time scale of $\sim 1$ My.

Energy injection by the OB stars produces an HII region and compresses the
surrounding molecular gas to induce the next-generation stars following the
sequential star formation scenario (Elmegreen & Lada, 1977; Paron et al.,
2013), as indeed observed in the bottom panel of Fig. 6. The expanding
velocity of the innermost bubble 1 surrounding the HII region is about $v_{\rm
exp}\sim\pm 7$ km s-1according to the LVD in Fig. 8. Corresponding compression
time of the high-density molecular hub is, therefore, $t_{\rm comp}\sim r_{\rm
bub}/v_{\rm exp}\sim 0.4$ My for $r_{\rm bub}\sim 2.6$ pc.

At the same time the extruded molecular gas expands and blows out to the lower
density side of the hub between the spokes, and creates the observed bubbles 1
to 4. Fig. 13 shows a simple simulation of such expanding shell producing an
elongated LVD. The expanding bubbles, then compress the neighboring spokes
(Inutsuka et al., 2015), making the spokes denser and tighter.

A part of the blown off molecular gas further expands outward, but it is
stopped by the surrounding ISM and fed back to the spoke and hub by the
gravity. The fed-back gas is gathered through funnels along the spokes and
returns to the hub (Peretto et al., 2013; Williams et al., 2018; Treviño-
Morales et al., 2019). Such a funnel flow will cause velocity gradient as
observed in the velocity field in Fig. 4. The outermost regions of the spokes
with large cross sections increase the chance of capture of surrounding ISM,
supplying more gas to the system.

The hub, spokes (filaments) and bubbles therefore constitute an internal
combustion engine (ICE) as illustrated in Fig. 14. Gas in-taken from the ISM
through the spoke manifold is compressed into the hub by the gravitational
force as the piston, explodes by the nuclear energy released by the newly born
massive stars, expands and is exhausted through the low-density region between
the spokes. G18 thus constitutes a standalone sustainable system around the
GMC with hub-spoke (filament)-bubble structure with continuous SF activity.
The SF is continuous but can be sporadic on a time scale of one cycle of about
the free-fall time in the spokes of $\sim 1$ My. This time scale is on the
same order as that required for the triggered formation of massive stars by
the cloud-collision scenario (Fukui et al., 2021), by gravitational
construction of the hub from multiple filaments (Myers, 2009), or the
sequential star formation (Elmegreen & Lada, 1977).

Figure 14: ICE (internal combustion engine) model for sustainable star
formation in a GMC with molecular hub-spoke (filament)-bubble structure.

##  6 Conclusions

We have obtained detailed analysis of the morphology and kinematics of the GMC
G18.15-0.30 associated with multiple HII regions and an SNR. The cloud
comprises a dense central cloud making a hub, several bubbles, and X-shaped
spokes extending radially for more than $\sim 0^{\circ}.6$ ($\sim 50$ pc).

G18 is located on the LVD at $V_{\rm lsr}=51$ km s-1close to the intersection
of the 4-kpc molecular ring and 3-kpc expanding ring. Carefully determining
the distance ranges for the GMC, HII regions and SNR using the velocities of
CO and HII recombination lines, HI absorption line, and $\Sigma-D$ relation
for SNR, we concluded that the distance to G18 is $6.1$ kpc in the 3-kpc
expanding ring. We argue that the SNR G18.15-0.17 is physically associated
with G18 system, composing a triple association of GMC, HII region, and SNR.

Based on the derived distance We determined the physical parameters such as
the mass and energy of the GMC, and considered their behaviours inside the
cloud (table 2 and figure 11). We comment that the result is dependent on the
CO-to-H2 conversion factor, which include uncertainty of a factor of two.
However, the relative variation between the quantities led to the following
physical conditions in the cloud.

In the cloud center within a few pc, the kinetic energy exceeds over the
gravitational energy, indicating that the core is dynamic and expanding. In
the outer region from the outer hub and spokes, it is vise versa, indicating
that the system is stable and gravitationally bound to the system.

The dynamic state of the central region is explained by the energy injection
expanding motion due to star forming activity. The entire cloud system is
gravitationally stable, and the star forming activity in the hub is sustained
by the feedback of the exhausted gas as well as by capture of interstellar gas
through the longitudinal flow along the spokes (filaments), and composes an
internal combustion engine (Fig. 14).

Finally, we comment that, if the location in the 3-kpc expanding ring is the
case, G18 would be the first example of a star forming activity in a strongly
shocked arm induced by the Galactic Centre explosion or by a deep bar
potential. In this context we stress the absence of the molecular disc in the
upper half of figure 3 at $V_{\rm lsr}\sim 50$ km s-1, which might be related
to the sweeping of the upper disc by the expanding shock wave from the GC
producing the North Polar Spur (Sofue & Kataoka, 2021).

## Acknowledgments

The author is indebted to the FUGIN team for the CO data observed with the
Nobeyama 45-m telescope operated by the NAOJ (National Astronomical
Observatory of Japan).

## Data availability

The FUGIN data were retrieved from the JVO portal at
http://jvo.nao.ac.jp/portal, and MAGPIS data from
https://third.ucllnl.org/cgi-bin/colorcutout.

## Conflict of interest

The author declares that there is no conflict of interest.

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##  Appendix A A distant molecular cloud with compact HII region
G18.305-0.391+32

The compact HII region at G18.305-0.391 with the RRL velocity of 32 km
s-1(Downes et al., 1980) is located in the center of a core-haloed molecular
cloud at radial velocity $V_{\rm lsr}=34$ km s-1in the FUGIN CO data cube.
Fig. 15 shows close up maps of this source in 20-cm continuum and 12CO
brightness at some different velocities. The kinematic distance to the
molecular cloud is determined to be $5.4\pm 0.4$ and $d=12.6\pm 0.5$ kpc for
near- and far-side solutions, respectively. Therefore, the source is not
related to the G18 system discussed in this paper. The peak $T_{\rm B}$ in the
12CO line is 30 K at the core, and the averaged temperature within
$0^{\circ}.1$ diameter circle is 11 K. If we adopt the far distance, the
molecular mass is estimated to be $\sim 7\times 10^{4}M_{\odot}$.

$V_{\rm lsr}=29.7$ km s-1 $32.3$ km s-1

$34.2$ km s-1 $34.8$ km s-1

Figure 15: G18.305-0.391+34 in 20-cm continuum (red: 0 to 0.2 Jy beam-1) and
12CO brightness (green: 0 to 30 K). The source is not related to G18, and the
kinematic distance is 12.6 kpc.