#  The Distant Outer Gas Arm Between $35^{\circ}$ and $45^{\circ}$

Yang Su11affiliationmark:  , Yan Sun11affiliationmark:  , Chong
Li11affiliationmark:  22affiliationmark:  , Shaobo Zhang11affiliationmark:  ,
Xin Zhou11affiliationmark:  , Min Fang11affiliationmark:  , Ji
Yang11affiliationmark:  and Xuepeng Chen11affiliationmark:  1 Purple Mountain
Observatory and Key Laboratory of Radio Astronomy, Chinese Academy of
Sciences, Nanjing 210008, China
2 Graduate University of the Chinese Academy of Sciences, 19A Yuquan Road,
Shijingshan District, Beijing 100049, China

###### Abstract

The Galactic plane has been mapped from $l=$34$\fdg$75 to 45$\fdg$25 and
$b=-$5$\fdg$25 to 5$\fdg$25 in the CO ($J$=1–0) emission with the 13.7 m
telescope of the Purple Mountain Observatory. The unbiased survey covers a
large area of 110 square degrees sampled every 30′′ with a velocity resolution
of $\sim 0.2\,{\rm km}\,{\rm s}^{-1}$. In this paper, we present the result of
an unbiased CO survey of this longitude and latitude range in the velocity
range from $-60$ to $-10\,{\rm km}\,{\rm s}^{-1}$. Over 500 molecular clouds
(MCs) are picked out from the 12CO ($J$=1–0) emission, and 131 of these MCs
are associated with 13CO emission. The distant MCs, which lie beyond the solar
circle and are mostly concentrated in the Galactic plane, trace the large-
scale molecular gas structure over 10 degrees of Galactic azimuth. We find
that the distribution of the distant MCs can be well fitted by a Gaussian
function with a full width at half maximum (FWHM) of 0$\fdg$7 with the
Galactic latitude. We suggest that the CO emission of the segment is from the
Outer Arm. The physical mid-plane traced by the Outer Arm seems to be slightly
displaced from the IAU-defined plane on a large scale, which could be
explained by the warped plane at large Galactocentric distances of  $>$$\sim$
10 kpc and the apparent tilted mid-plane to the projected IAU-defined plane
caused by the Sun’s $z$-height above the disk for distances near and within
the Solar circle. After removing the effect of the warp and tilted structure,
the scale height of the MCs in the Outer Arm is about 0$\fdg$6 or 160 pc at a
heliocentric distance of 15 kpc. If the inner plane of our Galaxy is flat, we
can derive an upper limit of the Sun’s offset of $\sim$17.1 pc above the
physical mid-plane of the Milky Way. We also discuss the correlations between
the physical parameters of the distant MCs, which is quite consistent with the
result of other studies of this parameter.

###### Subject headings:

Galaxy: structure – ISM: molecules – radio lines: ISM

##  1\. INTRODUCTION

Molecular clouds (MCs), as observed in CO surveys of the Galactic plane, play
an important role in studying star formation and the structure of the Galaxy
(e.g., Clemens, 1985; Dame et al., 1987, 2001; Heyer & Terebey, 1998; Jackson
et al., 2006; Burton et al., 2013). Several works were concentrated on the
large-scale structure traced by MCs in the first Galactic quadrant (e.g., Dame
& Thaddeus, 1985; Clemens et al., 1986, 1988; Dame et al., 1986; Scoville et
al., 1987; Jacq et al., 1988; Solomon & Rivolo, 1989; Roman-Duval et al.,
2010). However, the CO emission lying beyond the solar circle in the first
quadrant of the Galaxy is less studied (e.g., Section 3.1.5 and Figure 3 in
Dame et al., 2001). The only cases are the study of the MCs in the Outer Arm
of the Galaxy for the region of $l>55^{\circ}$ in the first quadrant (using
the NRAO 11 m telescope, Kutner & Mead, 1981; Mead & Kutner, 1988) and for the
region between $l=65^{\circ}$ and $71^{\circ}$ (using the CfA 1.2 m and the
Bell 7 m telescopes, Digel et al., 1990).

The Milky Way Imaging Scroll Painting (MWISP) project
111http://www.radioast.nsdc.cn/mwisp.php is a high resolution
($50^{\prime\prime}$) 12CO ($J$=1–0), 13CO ($J$=1–0), and C18O ($J$=1–0)
survey of the northern Galactic plane, performed with the Purple Mountain
Observatory Delingha 13.7 m telescope. The survey started in 2011, and will
cover Galactic longitudes from $l=-10^{\circ}$ to $250^{\circ}$ and latitudes
from $b=-5^{\circ}$ to $5^{\circ}$ over a period of $\sim$10 years. The
Galactic plane will be covered by full-sampling observations with the spectral
line On-The-Fly (OTF) mode. The survey has equal sensitivity over all regions
of the sky mapped and thus it is unbiased. One of the goals of this project is
to study the physical properties of MCs along the northern Galactic plane. As
of this writing the survey has completed about half of its planned area of
coverage.

Benefiting from the large unbiased survey of the MWISP, we can systematically
investigate the characteristics of the structure of the Galaxy using the
molecular gas. Recently, a CO spiral arm lying beyond the Outer Arm in the
first Galactic quadrant was identified by Dame & Thaddeus (2011) using the CfA
1.2 m telescope. Based on our new CO data of the MWISP, the extension of the
above arm, which is probably the Scutum-Centaurus Arm into the outer second
quadrant, has been revealed very recently by Sun et al. (2015).

In this paper, we investigate the Outer Arm (the Norma-Cygnus Arm, e.g., see
the recent review in Vallée, 2014) in the first quadrant of the Galaxy
according to the distribution of the CO gas between $l=35^{\circ}$ to
$45^{\circ}$. The distant MCs beyond the solar circle traced by CO emission
have a negative velocity in the direction that we are interested in. The
velocity range of the CO gas within the arm is about $-60$ to $-10\,{\rm
km}\,{\rm s}^{-1}$ in the direction (Section 3.3). The Outer Arm lies beyond
the solar circle in the Galactic longitude range of $l\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$>$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}30^{\circ}$ in the first quadrant of the Milky Way, thus the
molecular gas traced by CO emission does not suffer the kinematic distance
ambiguity encountered in the region within the Solar circle. Generally
speaking, in this quadrant, the more negative the value of the velocity is,
the larger the distance of the MC. Moreover, the large sample of the distant
MCs provides a good opportunity to study the global cloud parameters (Section
3.6) since the data have a high signal-to-noise ratio and there is little
overlap between the clouds (Section 3.1).

We present observations and data reductions of the survey in Section 2. In
Section 3.1 and 3.2, we describe the MCs identification and the distribution
of the distant MCs, respectively. We find that the physical mid-plane traced
by the distant CO gas is slightly displaced from the IAU-defined plane of
$b=0^{\circ}$ (Section 3.3). We also discuss the relationship between the two
planes in Section 3.4. In Section 3.5, we derive the Sun’s offset above the
physical mid-plane because of the apparent displacement between the two planes
for regions near and within the solar circle. The statistical properties of
the distant MCs are discussed in Section 3.6. Finally, the summary is given in
Section 4.

##  2\. CO OBSERVATIONS AND DATA REDUCTIONS

We mapped $\sim$110 square degrees (439 cells) in the region between Galactic
longitudes 34$\fdg$75–45$\fdg$25 and latitudes $-$5$\fdg$25 to 5$\fdg$25
during 2011 November to 2015 March using the 13.7 m millimeter-wavelength
telescope located at Delingha in China. The nine-beam Superconducting
Spectroscopic Array Receiver (SSAR) system was used at the front end and each
Fast Fourier transform spectrometer (FFTS) with a bandwidth of 1 GHz provides
16,384 channels and a spectral resolution of 61 kHz (see the details in Shan
et al., 2012). The molecular lines of 12CO ($J$=1–0), 13CO ($J$=1–0) and C18O
($J$=1–0) were observed simultaneously with the OTF method. Each cell with
dimension 30${}^{\prime}\times 30^{\prime}$ was scanned at least in two
orthogonal directions, along the Galactic longitude and the Galactic latitude,
in order to reduce the fluctuation of noise. The half-power beamwidth (HPBW)
of the telescope was about $50^{\prime\prime}$ at a local oscillating
frequency of 112.6 GHz and the pointing accuracy was better than
$5^{\prime\prime}$ in all observing epochs.

All cells were reduced using the GILDAS/CLASS
package.222http://www.iram.fr/IRAMFR/GILDAS After removing the bad channels in
the spectra and correcting the first order (linear) baseline fitting, the
antenna temperature ($T_{\rm A}^{\star}$) was converted to the main beam
temperature ($T_{\rm mb}$) with the relation $T_{\rm mb}=T_{\rm
A}^{\star}/(f_{\rm b}\times\eta_{\rm mb})$. In the above conversion, the beam-
filling factor of $f_{\rm b}$ is assumed to be one and the beam efficiency
$\eta_{\rm mb}$ is from the status
report333http://www.radioast.nsdc.cn/mwisp.php of the 13.7 m telescope of the
Purple Mountain Observatory. The typical sensitivity (rms) is about 0.5 K for
12CO ($J$=1–0) at the channel width of 0.16$\,{\rm km}\,{\rm s}^{-1}$ and 0.3
K for 13CO ($J$=1–0) and C18O ($J$=1–0) at 0.17$\,{\rm km}\,{\rm s}^{-1}$ with
a spatial resolution of $<1^{\prime}$.

Finally, all spectra of CO in each cell were converted to the three-
dimensional (3D) cube mosaic data together with a grid spacing of
$30^{\prime\prime}$ and the channel width of 0.16$\,{\rm km}\,{\rm s}^{-1}$
for 12CO ($J$=1–0) and 0.17$\,{\rm km}\,{\rm s}^{-1}$ for 13CO ($J$=1–0) and
C18O ($J$=1–0) for subsequent analysis.

##  3\. RESULTS AND DISCUSSION

###  3.1. MCs Identification

We carefully checked the 3D cube data channel by channel before investigating
the characters of the MCs that we are interested in. We find that the distant
MCs in the velocity interval of $-60$ to $-10\,{\rm km}\,{\rm s}^{-1}$ are
isolated in the $l$–$b$–$v$ space. That is, most of them show the distinct
structure in the space of $l$–$b$ and there is, therefore, little overlapping
in the velocity axis over the large survey’s area. Moreover, it is not
surprising that their 12CO ($J$=1–0) emission is stronger than that of the
13CO ($J$=1–0). No $>3\times$rms C18O ($J$=1–0) emission ($T_{\rm peak18}$) is
detected in the whole region in the velocity range. Only insignificant C18O
emission ($T_{\rm peak18}\sim 2\times$rms) can be found toward the MCs’ region
with some detectable 13CO emission (e.g., $T_{\rm peak13}>5\times$rms). We do
not discuss these weak C18O emission features in this paper.

According to the discrete feature of the distant MCs, we used the FINDCLUMPS
tool in the CUPID package (part of the STARLINK package) to identify MCs in
the 12CO and 13CO FITS cube. The CLUMPFIND algorithm (Williams et al., 1994)
is applied in the process of identification. We set the parameter
Tlow=3$\times$rms for 12CO and Tlow=2.7$\times$rms for 13CO, which determines
the lowest level to contour an MC, in order to obtain as much of the emission
as possible and to avoid the contamination of the noise based on our large
survey data. The parameter of DeltaT=9$\times$rms, which shows the contour
increment, is large since we do not care about the properties of the structure
of the bright parts embedded in the extended MCs. Other input parameters of
the CLUMPFIND algorithm are typical to identify those distant MCs for the 3D
cube data. Considering the irregular shape of the MCs, we use a polygon to
describe the MC’s boundary in the $l$–$b$ space. Some objects are rejected if
their sizes are less than the criteria: FwhmBeam=1.5 pixel, VeloRes=2 channel,
and MinPix=16.

It is worthwhile to note that sometimes an individual MC is decomposed to
several MCs because of the small velocity separation of $\sim 1-2\,{\rm
km}\,{\rm s}^{-1}$ within roughly the same $l$–$b$ region. However, the above
case is a rare population compared to that of all detected MCs (Figures 1 and
2). On the other hand, some point-like sources ( $<$$\sim$ 3–4 pixels) cannot
be picked out because of their smaller sizes. Some faint diffuse structures
where 12CO ($J$=1–0) emission is near the noise level of the survey cannot be
identified due to their poor signal-to-noise ratio. Further observations are
needed to confirm these MCs with small size or faint emission.

Finally, we identified 575 and 131 MCs from the 12CO and 13CO emission based
on the CLUMPFIND algorithm, respectively. The parameters of each MC, such as
position, LSR velocity, one-dimensional velocity dispersion, the peak value,
the size, and the luminosity, were obtained directly from the automated
detection routine. The parameters of the resolved MCs are summarized in Tables
1 and 2: (1) the ID of the identified MCs, arranged from the low Galactic
longitude; (2) and (3) the MCs’ Galactic coordinates ($l$ and $b$); (4) the
MCs’ LSR velocity ($V_{\rm LSR}$); (5) the MCs’ full width at half maximum
(FWHM, $\Delta V_{\rm FWHM}$), defined as 2.355 times of the velocity
dispersion $\sigma_{v}$ for a Gaussian line; (6) the MCs’ peak emission
($T_{\rm peak}$); (7) the MCs’ area; (8) the MCs’ integrated intensity
($W_{\rm CO}$) within a defined area; (9) the MCs’ total luminosity ($L_{\rm
CO}=W_{\rm CO}\times\rm{Area}$). Finally, we use MWISP Glll.lll$\pm$bb.bbb to
name an MC detected from the survey data.

We made the integrated intensity map of the 12CO ($J$=1–0) emission in the
velocity interval of $-1.6\times l$+$13.2\,{\rm km}\,{\rm s}^{-1}$ to
$-10\,{\rm km}\,{\rm s}^{-1}$ (Section 3.3) in Figure 1. After overlapping the
575 resolved MCs on the map, we find that the distribution of the CO gas is
well traced by the result from the automated cloud-finding routine. It shows
that the CLUMPFIND algorithm is thus reliable to pick out the distant MCs. In
addition, the sample of the MCs can be used to investigate the large
structures of the spiral arm (Section 3.3) and the properties of the distant
MCs (Section 3.6).

Figure 2 displays the intensity-weighted velocity (the first moment) map of
the 12CO ($J$=1–0) emission. The distribution of the 131 13CO MCs is also
consistent with that of the bright 12CO ($J$=1–0) emission. The blue stripes
at $l=$41$\fdg$5 and $b=-$1$\fdg$5 are from the bad channels, which show
regular oscillation along the velocity axis with intensity of $-2$ to 3 K. The
abnormal feature can be easily distinguished from the CO emission of MCs.

We also present the typical spectra of four resolved MCs (12CO with the black
line and 13CO$\times 2$ with blue) in Figure 3. All spectra in Figure 3 show
good baselines. The CO emission of the MCs can be easily discerned from the
spectra and the peak of 13CO ($J$=1–0) emission is roughly corresponding to
that of the 12CO ($J$=1–0) line (Figure 3).

###  3.2. Spatial Distribution of the Distant Molecular Gas

We show the 575 MCs in the Galactic coordinate system in Figure 4, in which
the filled circle indicates the position and the size (scaled with CO
luminosity) of MCs and the color indicates the LSR velocity of MCs. All
identified MCs are in the region of the Galactic latitude $b=-$1$\fdg$8 to
$b=$2$\fdg$2, which indicates that the molecular gas is roughly concentrated
in and around the Galactic plane (mainly in the region of $b\sim-$0$\fdg$5 to
1$\fdg$5). The map also shows that the distribution of the MCs is not uniform
with Galactic longitude.

Figure 5 is the normalized histogram of the MCs with the Galactic longitude
(left panel) and the Galactic latitude (right panel). According to the left
panel of Figure 5, we find that the spatial distribution of the MCs is not
uniform again. Several distribution peaks of the MCs are clearly discerned at
$l=$35$\fdg$25, 39∘, 42∘, and 44∘, respectively. The concentrated groups of
these MCs can also be seen in the same region of Figure 4. The map in the
right panel of Figure 5 shows that the MCs are mainly located within a limited
range of the Galactic latitude, which indicates that the molecular gas in the
velocity interval is actually from the Galactic plane with a distribution peak
of $b=$0$\fdg$42.

We also made the position$-$velocity (PV) map of the MCs along the Galactic
longitude (Figure 6), in which the distribution of the MCs is similar to that
in Figure 5. The velocity gradient of the MCs can be seen on a large scale
(see green dashed line in Figure 6), which shows the velocity trend of the
Outer Arm along the Galactic longitude (Section 3.3). The distribution of the
MCs in the longitude$-$velocity diagram seems to display some asymmetrical
ripples, which is a feature that is probably from the substructures of the
Outer Arm in this direction.

We note that some MCs are concentrated within groups. These MC groups seem to
display partial shell structures, which probably relate to the star forming
activity within the Outer Arm. We searched for the MCs associated with the
massive star formation regions based on our unbiased CO survey. The
information of the associations is listed in Table 3. We find that the
distribution of the massive star-forming regions in the Outer Arm, which are
mainly located at $l=39^{\circ}$, $42^{\circ}$, and $44^{\circ}$, is
associated with that of the CO peaks in the $l$–$b$ space (see the 21 H ii
regions marked with black circles and five 6.7 GHz masers marked with black
triangles in Figure 2). This result shows that star formation activity is very
common in the distant MCs of the Outer Arm. We will study the interesting
structures and the massive star-forming activities in the future.

###  3.3. Outer Gas Arm Traced by CO emission

In the direction of $l=35^{\circ}$ to $45^{\circ}$, we note that the Outer Arm
LSR velocity locates between the Perseus Arm and the Scutum-Centaurus Arm.
That is, the Perseus Arm, the Outer Arm, and the Scutum-Centaurus Arm are
encountered in this direction, each with different and decreased LSR velocity
(see Figures 9 and 10 in Reid et al., 2016). Part of the molecular gas’
emission in the velocity interval of 0 to $-10\,{\rm km}\,{\rm s}^{-1}$ is
from the Perseus Arm. The molecular gas of the Local Arm (Xu et al., 2013)
probably also contributes to some emission in the velocity range of 0 to
$-10\,{\rm km}\,{\rm s}^{-1}$. And the molecular gas with the LSR velocity
$<-1.6\times l$+$13.2\,{\rm km}\,{\rm s}^{-1}$ is probably from a segment of a
spiral arm at Galactocentric radii of R${}_{\rm GC}\sim$13–14 kpc (the value
of -1.6 $\,{\rm km}\,{\rm s}^{-1}$Degree-1 is from Dame & Thaddeus, 2011). In
order to isolate the emission from the three spiral arms, we use the molecular
gas in the velocity cutoff of $-1.6\times l$+$13.2\,{\rm km}\,{\rm s}^{-1}$ to
$-10\,{\rm km}\,{\rm s}^{-1}$ to trace the Outer Arm (see the yellow dashed
lines in Figure 6). Actually, the large-scale segment of the Outer Arm can be
discerned from the longitude$-$velocity diagram of CO emission, in which the
CO emission of the Outer Arm’s MCs is mainly in the above velocity range and
their distribution can be described as $V_{\rm LSR}=-2.78\times l+75.9\,{\rm
km}\,{\rm s}^{-1}$ (see the green dashed line in Figure 6) based on 13CO
emission (excluding MCs near $l\sim 44^{\circ}$ and $V_{\rm LSR}\sim-20\,{\rm
km}\,{\rm s}^{-1}$).

We use a Gaussian function, $f(b)\sim e^{-\frac{(b-b_{0})^{2}}{2\sigma^{2}}}$,
to fit the distribution of the MCs with the Galactic latitude (the right panel
in Figure 5). The dashed blue line shows the best fit: $b_{0}$=0$\fdg$42 and
$\sigma$=0$\fdg$29\. It indicates that the distant MCs in the velocity
interval are mainly distributed within a narrow range of the Galactic
latitudes of FWHM=2.355$\sigma\sim$0$\fdg$7\. We thus suggest that the large
amount of the CO emission actually traces the Outer Arm of the Milky Way.

The best Gaussian fit also shows that the distribution peak of the MCs in the
segment is not around $b=0^{\circ}$ but $b$=0$\fdg$42, which probably
indicates that the Outer Arm traced by the molecular gas is slightly displaced
with respect to the IAU-defined plane (see the details in Section 3.4). We
also use a linear relation, $b$=0$\fdg$0377$\times l-$1$\fdg$0893 (the thick
blue line in Figure 4), to describe the distribution of the MCs in the $l$–$b$
space. The fitted blue line represents the physical mid-plane with respect to
the IAU-defined plane of $b=0^{\circ}$. It indicates that the Outer Arm traced
by the CO gas passes through the IAU-defined plane of $b=0^{\circ}$ at
$l=$28$\fdg$9\. The molecular gas of the Outer Arm in the range of
$l\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$<$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}$28$\fdg$9 is mostly within the solar circle and the distribution of
the MCs with latitude is mainly $b<0^{\circ}$ (see the Norma-Cygnus arm in
Figure 3 of Vallée, 2014). The longitude$-$velocity relation from Figure 6
also indicates that the LSR velocity of the Outer Arm is roughly $\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$>$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}0\,{\rm km}\,{\rm s}^{-1}$ at $l\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$<$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}$28$\fdg$9\. The feature is probably related to the Sun’s offset
from the physical mid-plane (see Sections 3.4 and 3.5).

Using the 21 cm H i data from the VLA Galactic plane survey (VGPS; Stil et
al., 2006), Dame & Thaddeus (2011) found that the Outer Arm traced by the H i
gas is mainly below the plane of $b=0^{\circ}$ in the region of
$l=25^{\circ}-30^{\circ}$ (see Figure 2 in their paper). In other words, at
$l<30^{\circ}$ the Outer Arm seems to be below the IAU-defined plane in the
first Galactic quadrant. According to our linear fit of the CO emission, in
the region of $l=25^{\circ}-30^{\circ}$ the Galactic latitude distribution of
the Outer Arm should peak at $b\sim-$0$\fdg$1, which is roughly consistent
with the result of H i gas (see Figure 2 in Dame & Thaddeus, 2011).

Based on the Galactic parameters of Model A5 of Reid et al. (2014), we suggest
that the heliocentric distance of the 10∘ segment of the Outer Arm is about
15.1–15.6 kpc, which is a value that is quite consistent with the result of
other models (e.g., McClure-Griffiths & Dickey, 2007; Foster & Cooper, 2010).
The Galactocentric radii of the arm, RGC, in the direction is about 9.5–11.4
kpc accordingly. After subtracting the effect of the tilted structure with the
Galactic longitude (the thick blue line in Figure 4), we derived that the
scale height of the MCs in the Outer Arm is about 0$\fdg$6 (or $\sim$160 pc at
a heliocentric distance of 15 kpc). It is roughly consistent with the FWHM of
the molecular disk of the Milky Way in such Galactocentric radii (see Table 1
and Figure 3 in Nakanishi & Sofue, 2006).

The total 12CO luminosity of the segment of the spiral arm is about 2.1$\times
10^{5}$ K$\,{\rm km}\,{\rm s}^{-1}$pc2 at a heliocentric distance of 15 kpc.
Therefore, the total molecular gas within the segment is about 9$\times
10^{5}M_{\odot}$ by adopting the mean CO-to-H2 mass conversion factor $X_{\rm
CO}$=$2\times 10^{20}$ cm-2K-1km-1s (Bolatto et al., 2013) and a mean
molecular weight per H2 molecule of 2.76. We mention that the total mass of
the segment estimated above is probably the lower limit because some weak 12CO
emission is not accounted for in Table 1 (Section 3.1) and the value of
$X_{\rm CO}$ is probably underestimated for the outer Milky Way (Bolatto et
al., 2013).

Based on Table 1, the 12CO luminosity of the segment of the spiral arm is
about 2.1$\times 10^{5}$ K$\,{\rm km}\,{\rm s}^{-1}$pc2 at a heliocentric
distance of 15 kpc. The molecular gas within the segment is about 0.9$\times
10^{6}M_{\odot}$ by adopting the mean CO-to-H2 mass conversion factor $X_{\rm
CO}$=$2\times 10^{20}$ cm-2K-1km-1s (Bolatto et al., 2013) and a mean
molecular weight per H2 molecule of 2.76. For comparison, the total 12CO
luminosity of the segment of $\sim 2.6\times 10^{4}$ K$\,{\rm km}\,{\rm
s}^{-1}$arcmin2 can be obtained by integrating velocity channels from
$-1.6\times l$+$13.2\,{\rm km}\,{\rm s}^{-1}$ to $-10\,{\rm km}\,{\rm s}^{-1}$
for the covered map. Therefore, our estimate for the total molecular mass of
the segment of the Outer Arm is 2.2$\times 10^{6}M_{\odot}$. The result
indicates that over half of the total mass appears to be missed due to the
relatively high criterion of Tlow=3$\times$rms in the CLUMPFIND method. The
difference between the above estimations probably indicates that about 60% of
the molecular gas probably resides within small, cold, and$\//$or faint,
diffuse clouds that cannot be included in our catalog (see also, e.g., Section
5.1.3 of Heyer & Dame, 2015). We mention that the total mass of the segment
estimated above is probably the lower limit because some weak 12CO emission is
not accounted for and the value of $X_{\rm CO}$ is probably underestimated for
the outer Milky Way (Bolatto et al., 2013). Assuming a constant value of
$X_{\rm CO}$= $2\times 10^{20}$ cm-2K-1km-1s and a heliocentric distance of 15
kpc, the mass surface density of the distant MCs is about $22M_{\odot}$ pc-2,
which is roughly consistent with the character of MCs in the outer Galaxy
(see, e.g., Figure 8 in Heyer & Dame, 2015).

###  3.4. Physical Mid-plane Traced by the Outer Arm

In this section, we investigate the relationship between the physical mid-
plane and the IAU-defined plane of $b=0^{\circ}$ in the view of the molecular
gas on a large scale based on the new CO survey.

Generally, warps are common in spiral disks of galaxies (e.g. Reshetnikov &
Combes, 1998; Schwarzkopf & Dettmar, 2001; Reshetnikov et al., 2002). For
example, NGC 4013 shows a prodigious warp in its outer parts, in which the gas
layer curves away from the plane of the inner disk (Bottema, 1996). In the
Milky Way, the Hi layer was also found to be warped from the Solar radius
outwards (e.g., Burke, 1957; Oort et al., 1958; Henderson et al., 1982).
Furthermore, the stellar warp of the Milky Way is similar to the gaseous warp,
but smaller in amplitude (e.g., Freudenreich et al., 1994; Freudenreich, 1998;
Drimmel & Spergel, 2001).

On the other hand, the inner disk of the Milky Way is probably observed to be
tilting because of the offset of the Sun (Hammersley et al., 1995). That is,
the physical mid-plane projected on the sky displays an apparent change in
latitude with respect to longitude across some regions of the sky. Previously,
astronomers often used the star-counts method to derive the position of the
Sun (e.g., Hammersley et al., 1995; Humphreys & Larsen, 1995; Chen et al.,
2001; Maíz-Apellániz, 2001; Joshi, 2007; Majaess et al., 2009). These studies
found that the Sun’s vertical displacement from the physical mid-plane is
$\sim$10–30 pc (refer to Table 1 in Reed, 2006). By studying the infrared dark
cloud (IRDC) “Nessie,” Goodman et al. (2014) recently suggested that the very
long and dense filamentary IRDC represents a spine-like bone of the major
spiral arm, the Scutum-Centaurus Arm, in the fourth quadrant of the Milky Way.
They also suggested that the latitude of the true Galactic mid-plane traced by
the spiral arm is $b\sim-$0$\fdg$4 (but not $b=0^{\circ}$) because of the
Sun’s height off of the physical mid-plane of the Milky Way (see Figure 2 and
Section 3.1 in Goodman et al., 2014).

Here we discuss the Galactic structure traced by CO emission of the Outer Arm.
Both of the effects of the warp and the tilted plane (because of the Sun’s
offset from the physical mid-plane) were considered. We explored three typical
models for comparison with our observations: the gaseous warp model from the
Galactic Hi data ($m=1$ mode for a cutoff at RGC=10 kpc, Levine et al., 2006);
the stellar warp model from 2MASS data (a $\gamma_{\rm warp}$ of 0.09 and a
cutoff at RGC=8.4 kpc, Reylé et al., 2009); and the tilted-plane model for the
inner Galaxy (R${}_{\rm GC}<7$ kpc, and see Equation 5 in Hammersley et al.,
1995). The $\gamma_{\rm warp}$ in the stellar warp model is defined as the
ratio between the displacement (zmid-plane) of the physical mid-plane from the
IAU-defined plane of $b=0^{\circ}$ and the Galactocentric radii of RGC (refer
to Section 2.3 and Figure 9 in Reylé et al., 2009). For two warp models, we
assume that the Sun roughly lies on the line of nodes of the warp (i.e.,
$\phi_{\rm warp}=0^{\circ}$, Burton, 1988; Binney & Merrifield, 1998). Based
on the best fit of the longitude$-$velocity diagram ($V_{\rm LSR}=-2.78\times
l+75.9\,{\rm km}\,{\rm s}^{-1}$; Figure 6), we can kinematically calculate the
Galactocentric radii and the height of the physical mid-plane traced by the
Outer Arm using the A5 rotation curve model of Reid et al. (2014). Thus, when
the RGC–zmid-plane relation is obtained, we can directly compare the slope and
the amplitude of the different models (the dotted lines) with those of the
observations (the blue line) in the $l-b$ map (Figure 4). Here, the slope is
defined as the ratio between zmid-plane and $\Delta l$.

We find that the slopes of the two warp models are larger than that of the
observations. The amplitude of the stellar warp model is larger than the
observed displacement across the present longitude interval. Furthermore, the
amplitude of the gaseous warp model is also larger than the observations in
the region of $l\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$>$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}$40$\fdg$7 or R${}_{\rm GC}\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$>$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}10.6$ kpc. It indicates that the effect of the warp traced by the CO
gas of the Outer Arm is $not$ so large in this segment with R${}_{\rm GC}\sim
9.5-11.4$ kpc. This result is roughly consistent with the interpretation that
the amplitude of the physical mid-plane displacement is small inside a radius
of 10 kpc, and it steeply increases beyond the radius (Nakanishi & Sofue,
2006). Moreover, López-Corredoira et al. (2002) also suggested that the effect
of the stellar warp is large at the place of the larger RGC, but is small in
the vicinity of the solar circle (R${}_{\rm GC}\sim$ 8–10 kpc, see Figure 18
in their paper). Our CO survey shows that the slope of the observations is
intermediate between the warp models and the tilted flat plane model across
the region. Therefore, the warp probably affects the observed slope of the MCs
in such a region, but not to the extent predicted by the current models.

On the contrary, the slope of the tilted-plane model seems very close to that
of the observations in the region of $l\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$<$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}$37$\fdg$5 (or R${}_{\rm GC}\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$<$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}10$ kpc). The displacement between the physical mid-plane and the
$b=0^{\circ}$ plane is also comparable to that of the tilted-plane model in
the above region, in which the offset between the two planes is about
0$\fdg$2–0$\fdg$3 or 50–80 pc at a heliocentric distance of 15 kpc (Figure 4).
Furthermore, the amplitude of the tilted-plane model will be closing to the
displacement between the two planes with decreased RGC (or $l$ for the mid-
plane traced by the Outer Arm). Actually, the inner Hi disk was found to be
tilted against the $b=0^{\circ}$ plane (Nakanishi & Sofue, 2003). Also, the
amplitude of the displacement traced by our CO gas seems comparable to that of
Nakanishi & Sofue (2006) (see Figure 12 in their paper) in the region with
R${}_{\rm GC}\sim$9–11 kpc.

Based on the above analysis, we suggest that the Galactic warp plays a role in
the region with large Galactocentric radii (e.g., R${}_{\rm GC}\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$>$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}10$ kpc) while it could be neglected in regions near and within the
solar circle. By contrast, the tilted-plane model suggests that the Sun’s
offset from the physical mid-plane seems to become the dominant for regions
near and within the solar circle. Roughly speaking, to first order
approximation, the observed tilt of the MCs is due to the Sun’s offset from
the physical mid-plane for regions with R${}_{\rm GC}\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$<$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}10$ kpc.

###  3.5. Position of the Sun

In Section 3.4, we show that the Galactic warp is not strong and the apparent
displacement between the two planes is probably dominated by the tilted-plane
effect in the region of R${}_{\rm GC}\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$<$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}10$ kpc. We thus do not take into account the effect of the gas warp
in the region near and within the solar circle. Indeed, the Galactic warp is
very important for the distant gas arm with large RGC, (e.g., see that the
structure of the extension of the Scutum-Centaurus Arm with R${}_{\rm GC}>14$
kpc exhibits warps along the Galactic longitude, Figure 2b in Sun et al.,
2015). The warp of the Outer arm is also obvious between $l=100^{\circ}$ and
$150^{\circ}$ (Figure 7 in Du et al., 2016), in which the Galactocentric radii
($\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$>$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}13$ kpc) of the Outer Arm is larger than that of the MCs discussed
by us here.

According to Section 3.4, to first order, the displacement between the
physical mid-plane and the $b=0^{\circ}$ plane is dominated by the tilted
effect because of the Sun’s height off of the physical mid-plane for regions
of R${}_{\rm GC}\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$<$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}10$ kpc. If the tilt of the Galactic plane is the cause of the
observations here, then we can derive the offset of the Sun by extrapolating
the fitted Outer Arm into the lower longitude (e.g., $l\sim
25^{\circ}-35^{\circ}$). In Figure 7, we construct a schematic diagram to show
the presumable relationship between the physical mid-plane and the IAU-defined
plane within the solar circle.

In Figure 7, $\alpha$=$-$0$\fdg$046 is the Galactic latitude of Sgr A⋆
($({17}^{{\rm h}}{45}^{{\rm m}}{40}^{{\rm
s}}.0409,{-29}^{\circ}{00}^{\prime}{28}^{\prime\prime}.118)$ in J2000 for Sgr
A⋆, Reid & Brunthaler, 2004) and $\beta$=28$\fdg$9 is the Galactic longitude
of the Outer Arm at $b=0^{\circ}$ (Section 3.3). The tilted angle, $\theta$,
is to be determined from the geometric relation shown in the schematic
diagram. That is, the Outer Arm passes through the IAU-defined plane at the
direction of $l=\beta$ and $b=0^{\circ}$. The value for the fitted parameter
of $\beta=l=$28$\fdg$9 at $b=0^{\circ}$ can be derived from the linear fit
(the blue solid line in Figure 4). Assuming the distance of $d_{\rm
SgrA^{\star}}=$8.34 kpc (Reid et al., 2014), the value of $d_{\rm Crossing}$
is about 13.69 kpc (see the lower left corner of Figure 7). Accordingly, the
tilted angle $\theta$ and the Sun’s offset above the physical mid-plane
$Z_{\rm Sun}$ is about 0$\fdg$072 and 17.1 pc, respectively.

In our geometrical model (Figure 7), the values of $\theta$ and $Z_{\rm Sun}$
that we are interested in are dependent on $\beta$ and the distance to the
Galactic center, $d_{\rm SgrA^{\star}}$. We note that the tilted angle
($\theta$) may slightly vary from $\sim$0.067 to $\sim$0.078 when $\beta$ is
from 27∘ to 31∘. The offset of the Sun ($Z_{\rm Sun}$) may also vary from 15.8
pc to 18.4 pc when $d_{\rm SgrA^{\star}}$ is changed from 8.0 kpc to 8.5 kpc
and the $\beta$ range of 27∘–31∘. Comparing the value of $Z_{\rm Sun}$ with
other investigations (e.g., Table 1 in Reed, 2006), our estimation of $Z_{\rm
Sun}$=15.8–18.4 pc based on the distant large-scale molecular gas agrees well
with those from the method of star counts. Moreover, the above estimation from
the 12CO emission (Table 1) is similar to that from the 13CO data (Table 2;
$b$=0$\fdg$0335$\times l-$0$\fdg$9640 and $l=\beta$=28$\fdg$8 at
$b=0^{\circ}$). Finally, since the warp was neglected in regions near and
within the Solar circle (R${}_{\rm GC}\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$<$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}10$ kpc), the Sun’s $z$-height based on the tilted appearance of the
distant MCs represents an upper limit.

It should be noted that our result is slightly larger than that of Brand &
Blitz (1993), in which the authors found $Z_{\rm Sun}$= $13\pm 7$ pc according
to the MCs in the range of heliocentric distances 0.7–2 kpc. It is probably
due to the uncertainty of the MC samples located nearby the Sun in their
study. On the other hand, our independent estimate is in good agreement with
the recent result of Bobylev & Bajkova (2016), in which they found that the
mean value of $Z_{\rm Sun}$ is $16\pm 2$ pc based on samples of various
objects.

In the paper, we only use the limited survey data ($\sim$ 110 square degrees)
to fit the Outer Arm in CO emission. The MWISP project will cover the
remaining region of the inner Galaxy in the four or five coming years. We hope
that the accumulated data of the new CO survey will be more helpful in the
studying of the Outer Arm’s structure in the first Galactic quadrant.

###  3.6. Correlations of the Distant MCs

In Section 3.3, we show that the distant MCs in the velocity range of
$-1.6\times l$+$13.2\,{\rm km}\,{\rm s}^{-1}$ to $-10\,{\rm km}\,{\rm s}^{-1}$
trace the Outer Arm in the Galactic longitude $l=35^{\circ}$ to
$l=45^{\circ}$. In this section, we investigate correlations of the distant
MCs between the observed parameters, which are directly obtained from the
CLUMPFIND algorithm (Table 1) to avoid the uncertainty from other factors
(e.g., the kinematic distance from the rotation curve and the estimated mass
from the CO-to-H2 mass conversion factor $X_{\rm CO}$). We only consider the
correlation of the physical parameters from 12CO because the sample from 12CO
($J$=1–0) emission of the distant MCs is at least four times larger than that
from 13CO ($J$=1–0). Moreover, the signal-to-noise ratio of the 12CO ($J$=1–0)
emission is better than that of 13CO ($J$=1–0).

The thick blue line in Figure 8 indicates the relationship between the size
(2R) and the velocity dispersion ($\sigma_{v}$) of the MCs (the so-called
size-linewidth relation): $\sigma_{v}(\,{\rm km}\,{\rm
s}^{-1})\sim\rm{size}^{0.42}(\rm{arcmin})$. We find that the correlation is
not strong. The scattering of the velocity dispersion is large
($\sim$0.2$\,{\rm km}\,{\rm s}^{-1}$–2$\,{\rm km}\,{\rm s}^{-1}$) in the small
size range ($\sim$1–8 arcmin). The small dynamical range, the limited
observation sensitivity, and the non-thermal motions are probably responsible
for the poor fitting (Section 4.3 in Zhang et al., 2014). Although the
scattering is large, we also note that the power-law index of 0.42 is between
other works (Larson, 1981, e.g., 0.33 from; and Solomon et al., 1987, 0.5
from). On the other hand, it probably indicates that the velocity dispersion
of MCs is not a simple power-law function of its size (Heyer et al., 2001,
2009).

The left panel of Figure 9 shows a tight correlation between the CO luminosity
($L_{\rm CO}$) and the virial mass ($M_{\rm virial}$): $M_{\rm virial}\sim
L_{\rm CO}^{0.87}$. The power-law index of 0.87 is in good agreement with that
of 0.81 from Solomon et al. (1987). On the other hand, a tight relation
between the CO integrated intensity ($W_{\rm CO}$) and the virial mass is also
found: $M_{\rm virial}\sim W_{\rm CO}^{2.11}$ with a good correlation
coefficient of 0.98. The good correlation between the virial mass and the
column density (right panel in Figure 9) is probably related to the dependence
of $\sigma_{v}/R^{0.5}$ on the column density of MCs (see Figures 7 and 8 in
Heyer et al., 2009).

The thick blue line in Figure 10 displays the power-law relation between the
size ($2R$) and the CO luminosity ($L_{\rm CO}$) of the MCs: $L_{\rm
CO}\sim\rm{size}^{2.74}$. We find that the fit of this power-law relation is
excellent with a correlation coefficient of 0.92. If we use the weighted CO
luminosity to fit it, a power-law relation of $L_{\rm CO}\sim\rm{size}^{2.41}$
also describes the data. We emphasize that the above power-law index 2.41 is
consistent well with the result from the optically thin GRS 13CO ($J$=1–0)
data (Roman-Duval et al., 2010, a power-law index of 2.36 in , also see Figure
1 in their paper). It probably indicates that the distant MCs are also the
unresolved parts of a pervasive fractal structure, which is similar to the
result of Elmegreen & Falgarone (1996). The result also shows that the mass of
the $large$ MCs traced by the 12CO ($J$=1–0) luminosity is similar to that
traced by the optically thin 13CO ($J$=1–0) emission as analyzed by Roman-
Duval et al. (2010). Furthermore, the CO luminosity from the best fit is
overestimated for the true $L_{\rm CO}$ of MCs with smaller size or lower
luminosity.

##  4\. SUMMARY

The MWISP project is a new large-scale survey of molecular gas in 12CO
($J$=1–0), 13CO ($J$=1–0), and C18O ($J$=1–0) emission. Comparing the data
with those of other CO surveys (Jackson et al., 2006, e.g., GRS 13CO ($J$=1–0)
survey in and Dempsey et al., 2013, COHRS 12CO ($J$=3–2) survey in), our new
CO survey has larger spatial and velocity coverage. In the paper, we have
presented the result of 110 square degree CO emission between $l=$ 34$\fdg$75
to $l=$ 45$\fdg$25 and $b=-$5$\fdg$25 to $b=$5$\fdg$25 in the velocity
interval of $-1.6\times l$+$13.2\,{\rm km}\,{\rm s}^{-1}$ to $-10\,{\rm
km}\,{\rm s}^{-1}$ to study the distant MCs of the Milky Way. Based on the new
unbiased CO survey, the main results are summarized as follows.

1\. We have identified over five hundred distant MCs according to the 12CO
($J$=1–0) emission in the velocity range in the 110 square degree region. 131
MCs in the 13CO ($J$=1–0) emission are also identified among these 12CO
($J$=1–0) MCs. The parameters (e.g., position, LSR velocity, peak temperature,
size, and luminosity; see Tables 1 and 2) of the distant MCs are presented for
the first time.

2, The distribution of the distant MCs in the velocity range is not uniform
with the Galactic longitude. Four distribution peaks of the MCs can be seen at
$l=$35$\fdg$25, 39∘, 42∘, and 44∘, respectively. The interesting implication
is that the massive star formation regions seem to be concentrated in the last
three peaks. The associations between the MCs and the massive star formation
regions can be seen in Table 3. It also shows that the star formation activity
is very common in the distant MCs.

3\. The distant MCs seem to be concentrated within a narrow Galactic latitude
range along the Galactic longitude. Most of them are within the region of
$b=-$0$\fdg$5 and $b=$1$\fdg$5\. We find that the distribution of the distant
MCs can be described by a Gaussian model with Galactic latitude. Based on the
PV map of the MCs along the Galactic longitude, we thus suggest that the CO
emission of the MCs is from the Outer Arm in the first Galactic quadrant.

4\. According to the unbiased CO survey and the Galactic rotation curve of
Reid et al. (2014), the heliocentric distance, the Galactocentric radii, the
scale height, and the lower limit to the total mass of the Outer Arm in the
segment are about 15.1–15.6 kpc, 9.5–11.4 kpc, 0$\fdg$6 (or 160 pc at a
heliocentric distance of 15 kpc), and 2.2$\times 10^{6}M_{\odot}$,
respectively. Assuming a constant value of $X_{\rm CO}$= $2\times 10^{20}$
cm-2K-1km-1s and a heliocentric distance of 15 kpc, the mass surface density
of the distant MCs is about $22M_{\odot}$ pc-2.

5\. We note that the physical mid-plane traced by the distant CO arm is
slightly displaced with respect to the IAU-defined plane, $b=0^{\circ}$. We
find that the Galactic warp plays a role in the region of RGC ($\mbox{\hskip
1.99997pt\raisebox{2.15277pt}{$>$}\raisebox{-2.15277pt}{$\sim$}\hskip
1.99997pt}10$ kpc) and the tilted-plane model is probably a good approximation
in the region near and within the Solar circle.

6\. If the mid-plane within the Solar circle is flat, the tilted angle between
the two planes is about 0$\fdg$072\. And the distance from the Sun to where
the two planes cross is $d_{\rm Crossing}\sim$ 13.69 kpc assuming $d_{\rm
SgrA^{\star}}=$8.34 kpc. The location of the Sun, as an upper limit, is thus
about 17.1 pc above the physical mid-plane from the estimate of the tilted
angle. The offset of the Sun, which is determined independently from the view
of the large-scale structure of the distant molecular gas, is also in
agreement with results via the star-counts method.

7\. The $L_{\rm CO}$–$M_{\rm virial}$ and the $W_{\rm CO}$–$M_{\rm virial}$
relations of the distant MCs, as well as the size–$L_{\rm CO}$ relation,
display a convincing power-law relationship, which is consistent with the
results of other studies (e.g., Solomon et al., 1987; Roman-Duval et al.,
2010).

The authors acknowledge the staff members of the Qinghai Radio Observing
Station at Delingha for their support of the observations. We thank the
anonymous referee for critical comments and suggestions that helped to improve
the paper. The TOPCAT tool (Taylor, 2005) was used while preparing the paper.
This work is supported by NSFC grant 11233007. The work is a part of the
Multi-Line Galactic Plane Survey in CO and its Isotopic Transitions, also
called the Milky Way Imaging Scroll Painting, which is supported by the
Strategic Priority Research Program, the Emergence of Cosmological Structures
of the Chinese Academy of Sciences, grant No. XDB09000000.

## References

  * Anderson et al. (2015) Anderson, L. D., Armentrout, W. P., Johnstone, B. M., et al. 2015, ApJS, 221, 26
  * Anderson et al. (2011) Anderson, L. D., Bania, T. M., Balser, D. S., & Rood, R. T. 2011, ApJS, 194, 32
  * Balser et al. (2011) Balser, D. S., Rood, R. T., Bania, T. M., & Anderson, L. D. 2011, ApJ, 738, 27
  * Bania et al. (2012) Bania, T. M., Anderson, L. D., & Balser, D. S. 2012, ApJ, 759, 96
  * Binney & Merrifield (1998) Binney, J., & Merrifield, M. 1998, Galactic Astronomy (Princeton, NJ: Princeton Univ. Press)
  * Bobylev & Bajkova (2016) Bobylev, V. V., & Bajkova, A. T. 2016, Astronomy Letters, 42, 1
  * Bolatto et al. (2013) Bolatto, A. D., Wolfire, M., & Leroy, A. K. 2013, ARA&A, 51, 207
  * Bottema (1996) Bottema, R. 1996, A&A, 306, 345
  * Brand & Blitz (1993) Brand, J., & Blitz, L. 1993, A&A, 275, 67
  * Breen et al. (2015) Breen, S. L., Fuller, G. A., Caswell, J. L., et al. 2015, MNRAS, 450, 4109
  * Bronfman et al. (1996) Bronfman, L., Nyman, L.-A., & May, J. 1996, A&AS, 115, 81
  * Burke (1957) Burke, B. F. 1957, AJ, 62, 90
  * Burton et al. (2013) Burton, M. G., Braiding, C., Glueck, C., et al. 2013, PASA, 30, 44
  * Burton (1988) Burton, W. B. 1988, Galactic and Extragalactic Radio Astronomy (Berlin and New York, Springer-Verlag), ed. K. I. Kellermann & G. L. Verschuur, 295–358
  * Chen et al. (2001) Chen, B., Stoughton, C., Smith, J. A., et al. 2001, ApJ, 553, 184
  * Clemens (1985) Clemens, D. P. 1985, ApJ, 295, 422
  * Clemens et al. (1988) Clemens, D. P., Sanders, D. B., & Scoville, N. Z. 1988, ApJ, 327, 139
  * Clemens et al. (1986) Clemens, D. P., Sanders, D. B., Scoville, N. Z., & Solomon, P. M. 1986, ApJS, 60, 297
  * Dame et al. (1986) Dame, T. M., Elmegreen, B. G., Cohen, R. S., & Thaddeus, P. 1986, ApJ, 305, 892
  * Dame et al. (2001) Dame, T. M., Hartmann, D., & Thaddeus, P. 2001, ApJ, 547, 792
  * Dame & Thaddeus (1985) Dame, T. M., & Thaddeus, P. 1985, ApJ, 297, 751
  * Dame & Thaddeus (2011) —. 2011, ApJ, 734, L24
  * Dame et al. (1987) Dame, T. M., Ungerechts, H., Cohen, R. S., et al. 1987, ApJ, 322, 706
  * Dempsey et al. (2013) Dempsey, J. T., Thomas, H. S., & Currie, M. J. 2013, ApJS, 209, 8
  * Digel et al. (1990) Digel, S., Thaddeus, P., & Bally, J. 1990, ApJ, 357, L29
  * Drimmel & Spergel (2001) Drimmel, R., & Spergel, D. N. 2001, ApJ, 556, 181
  * Du et al. (2016) Du, X., Xu, Y., Yang, J., et al. 2016, ApJS, 224, 7
  * Elmegreen & Falgarone (1996) Elmegreen, B. G., & Falgarone, E. 1996, ApJ, 471, 816
  * Foster & Cooper (2010) Foster, T., & Cooper, B. 2010, in Astronomical Society of the Pacific Conference Series, Vol. 438, The Dynamic Interstellar Medium: A Celebration of the Canadian Galactic Plane Survey, ed. R. Kothes, T. L. Landecker, & A. G. Willis, 16
  * Freudenreich (1998) Freudenreich, H. T. 1998, ApJ, 492, 495
  * Freudenreich et al. (1994) Freudenreich, H. T., Berriman, G. B., Dwek, E., et al. 1994, ApJ, 429, L69
  * Goodman et al. (2014) Goodman, A. A., Alves, J., Beaumont, C. N., et al. 2014, ApJ, 797, 53
  * Green & McClure-Griffiths (2011) Green, J. A., & McClure-Griffiths, N. M. 2011, MNRAS, 417, 2500
  * Gregory et al. (1996) Gregory, P. C., Scott, W. K., Douglas, K., & Condon, J. J. 1996, ApJS, 103, 427
  * Hammersley et al. (1995) Hammersley, P. L., Garzon, F., Mahoney, T., & Calbet, X. 1995, MNRAS, 273, 206
  * Henderson et al. (1982) Henderson, A. P., Jackson, P. D., & Kerr, F. J. 1982, ApJ, 263, 116
  * Heyer & Dame (2015) Heyer, M., & Dame, T. M. 2015, ARA&A, 53, 583
  * Heyer et al. (2009) Heyer, M., Krawczyk, C., Duval, J., & Jackson, J. M. 2009, ApJ, 699, 1092
  * Heyer et al. (2001) Heyer, M. H., Carpenter, J. M., & Snell, R. L. 2001, ApJ, 551, 852
  * Heyer & Terebey (1998) Heyer, M. H., & Terebey, S. 1998, ApJ, 502, 265
  * Humphreys & Larsen (1995) Humphreys, R. M., & Larsen, J. A. 1995, AJ, 110, 2183
  * Jackson et al. (2006) Jackson, J. M., Rathborne, J. M., Shah, R. Y., et al. 2006, ApJS, 163, 145
  * Jacq et al. (1988) Jacq, T., Despois, D., & Baudry, A. 1988, A&A, 195, 93
  * Joshi (2007) Joshi, Y. C. 2007, MNRAS, 378, 768
  * Kutner & Mead (1981) Kutner, M. L., & Mead, K. N. 1981, ApJ, 249, L15
  * Larson (1981) Larson, R. B. 1981, MNRAS, 194, 809
  * Levine et al. (2006) Levine, E. S., Blitz, L., & Heiles, C. 2006, ApJ, 643, 881
  * Lockman (1989) Lockman, F. J. 1989, ApJS, 71, 469
  * López-Corredoira et al. (2002) López-Corredoira, M., Cabrera-Lavers, A., Garzón, F., & Hammersley, P. L. 2002, A&A, 394, 883
  * Maíz-Apellániz (2001) Maíz-Apellániz, J. 2001, AJ, 121, 2737
  * Majaess et al. (2009) Majaess, D. J., Turner, D. G., & Lane, D. J. 2009, MNRAS, 398, 263
  * McClure-Griffiths & Dickey (2007) McClure-Griffiths, N. M., & Dickey, J. M. 2007, ApJ, 671, 427
  * Mead & Kutner (1988) Mead, K. N., & Kutner, M. L. 1988, ApJ, 330, 399
  * Nakanishi & Sofue (2003) Nakanishi, H., & Sofue, Y. 2003, PASJ, 55, 191
  * Nakanishi & Sofue (2006) —. 2006, PASJ, 58, 847
  * Oort et al. (1958) Oort, J. H., Kerr, F. J., & Westerhout, G. 1958, MNRAS, 118, 379
  * Reed (2006) Reed, B. C. 2006, JRASC, 100, 146
  * Reid & Brunthaler (2004) Reid, M. J., & Brunthaler, A. 2004, ApJ, 616, 872
  * Reid et al. (2016) Reid, M. J., Dame, T. M., Menten, K. M., & Brunthaler, A. 2016, ApJ, 823, 77
  * Reid et al. (2014) Reid, M. J., Menten, K. M., Brunthaler, A., et al. 2014, ApJ, 783, 130
  * Reshetnikov et al. (2002) Reshetnikov, V., Battaner, E., Combes, F., & Jiménez-Vicente, J. 2002, A&A, 382, 513
  * Reshetnikov & Combes (1998) Reshetnikov, V., & Combes, F. 1998, A&A, 337, 9
  * Reylé et al. (2009) Reylé, C., Marshall, D. J., Robin, A. C., & Schultheis, M. 2009, A&A, 495, 819
  * Roman-Duval et al. (2010) Roman-Duval, J., Jackson, J. M., Heyer, M., Rathborne, J., & Simon, R. 2010, ApJ, 723, 492
  * Schwarzkopf & Dettmar (2001) Schwarzkopf, U., & Dettmar, R.-J. 2001, A&A, 373, 402
  * Scoville et al. (1987) Scoville, N. Z., Yun, M. S., Sanders, D. B., Clemens, D. P., & Waller, W. H. 1987, ApJS, 63, 821
  * Shan et al. (2012) Shan, W. L., Yang, J., Shi, S. C., et al. 2012, IEEE Transactions on Terahertz Science and Technology, 2, 593
  * Solomon & Rivolo (1989) Solomon, P. M., & Rivolo, A. R. 1989, ApJ, 339, 919
  * Solomon et al. (1987) Solomon, P. M., Rivolo, A. R., Barrett, J., & Yahil, A. 1987, ApJ, 319, 730
  * Stil et al. (2006) Stil, J. M., Taylor, A. R., Dickey, J. M., et al. 2006, AJ, 132, 1158
  * Sun et al. (2015) Sun, Y., Xu, Y., Yang, J., et al. 2015, ApJ, 798, L27
  * Taylor (2005) Taylor, M. B. 2005, in Astronomical Society of the Pacific Conference Series, Vol. 347, Astronomical Data Analysis Software and Systems XIV, ed. P. Shopbell, M. Britton, & R. Ebert, 29
  * Vallée (2014) Vallée, J. P. 2014, ApJS, 215, 1
  * Williams et al. (1994) Williams, J. P., de Geus, E. J., & Blitz, L. 1994, ApJ, 428, 693
  * Xu et al. (2013) Xu, Y., Li, J. J., Reid, M. J., et al. 2013, ApJ, 769, 15
  * Zhang et al. (2014) Zhang, S., Xu, Y., & Yang, J. 2014, AJ, 147, 46

Figure 1.—  12CO ($J$=1–0) integrated intensity map of the MCs on the interval
of $-1.6\times l$+$13.2\,{\rm km}\,{\rm s}^{-1}$ to $-10\,{\rm km}\,{\rm
s}^{-1}$. Only $>3\times$rms points are accounted for.  Figure 2.—  Intensity-
weighted mean velocity (first moment) map of the MCs from 12CO ($J$=1–0)
emission on the interval of $-1.6\times l$+$13.2\,{\rm km}\,{\rm s}^{-1}$ to
$-10\,{\rm km}\,{\rm s}^{-1}$. Only $>5\times$rms points are accounted for.
The black circles and triangles are the positions of the 21 H ii regions and
five 6.7 GHz masers from Table 3. The blue stripes at $l=$41$\fdg$5 and
$b=-$1$\fdg$5 are from bad channels at $-41.59\,{\rm km}\,{\rm s}^{-1}$ (see
the text).  Figure 3.— Typical 12CO ($J$=1–0; black) and 13CO ($J$=1–0; blue,
multiplied by a factor of two) spectra of the distant MCs. The extraction area
of the spectra is shown in the top-left corner of each panel. The $Y$ axis is
the main beam temperature of the MCs.  Figure 4.—  Distribution of the 575 MCs
in the Galactic coordinate system. The filled color circles are 575 MCs in
Table 1 and are weighted with the scale of $L_{\rm CO}^{0.5}$. The thick blue
line (weighted with the luminosity of the MC, $b$=0$\fdg$0377$\times
l-$1$\fdg$0893) shows the best fit by a linear function. Circle-dotted line:
warp model from Hi data ($m=1$ mode, refer to Levine et al., 2006); Box-dotted
line: warp model from 2MASS star counts (Reylé et al., 2009); Triangle-dotted
line: tilted-plane model from Hammersley et al. (1995).

Figure 5.—  Histograms of the 575 MCs with the Galactic longitude (the left
panel) and latitude (the right panel). The dashed blue line in the right panel
shows the best fit by a Gaussian function with the FWHM of $\sim$0$\fdg$7\.
The y axis is the fraction of the MCs weighted with their integrated
intensity.  Figure 6.—  PV diagram of the distant MCs with the Galactic
longitude in the Galactic latitude range of $-$0$\fdg$5 to 1$\fdg$5\. The blue
and red emission is from the 12CO ($J$=1–0) and 13CO ($J$=1–0), respectively.
The white circles and triangles are the same as those in Figure 2. The yellow
dashed lines present the LSR velocity range of the Outer Arm: $V_{\rm
LSR}=-1.6\times l+13.2\,{\rm km}\,{\rm s}^{-1}$ to $-10\,{\rm km}\,{\rm
s}^{-1}$. The green dashed line indicates the longitude$-$velocity relation
from the best fit of the 13CO ($J$=1–0) emission (excluding MCs near $l\sim
44^{\circ}$ and $V_{\rm LSR}\sim-20\,{\rm km}\,{\rm s}^{-1}$): $V_{\rm
LSR}=-2.78\times l+75.9\,{\rm km}\,{\rm s}^{-1}$.  Figure 7.—  Schematic
diagram of the physical mid-plane of the Galaxy with respect to the IAU-
defined plane of $b=0^{\circ}$. Note that the lines and the angles in the map
are not to scale. All solid blue lines are in the IAU-defined plane
$b=0^{\circ}$. The red ellipse indicates the solar circle. The Outer Arm,
which traces the physical mid-plane in the first Galactic quadrant, passes
through the IAU-defined plane at the direction of $l=\beta$ and $b=0^{\circ}$.
$Z_{\rm Sun}$ and $Z_{\rm SgrA^{\star}}$ represent the height of the Sun above
the physical mid-plane and the offset of the Galactic center (Sgr A⋆) below
the IAU-defined plane, respectively. The schematic face-on view of the
geometrical relation, which is the projection onto the IAU-defined plane, is
also shown in the lower left corner. Thus the value of $d_{\rm Crossing}$ is
about 13.69 kpc. The $\alpha$, $\beta$, and the tilted angle $\theta$ are
$-$0$\fdg$046, 28$\fdg$9, and 0$\fdg$072 (see the text), respectively. The
derived $Z_{\rm Sun}$ is about 17.1 pc. In the calculation, the distance from
the Sun to the Galactic center is $d_{\rm SgrA^{\star}}=$ 8.34 kpc.  Figure
8.—  MC velocity dispersion ($\sigma_{v}$=$\Delta V_{\rm FWHM}$/2.355) as a
function of size ($2R$=2(Area/3.14)1/2) for the 575 clouds in Table 1. The
black crosses and the filled circles are for the fitted position and the
scaled size of the MCs, respectively. The thick blue line
($\sigma_{v}\sim\rm{size}^{0.42}$, correlation coefficient (c.c.)=0.34)
indicates the best fit by a power-law correlation.

Figure 9.—  Left: the CO luminosity ($L_{\rm CO}$)–virial mass ($M_{\rm
virial}\sim 2R\times\sigma_{v}^{2}$) relation for the 575 MCs. The black
crosses and the filled circles are the same as those in Figure 8. The thick
blue line ($M_{\rm virial}\sim L_{\rm CO}^{0.87}$, c.c.=0.87) indicates the
best fit by a power-law correlation. Right: the CO intensity ($W_{\rm
CO}$)–virial mass ($M_{\rm virial}$) relation. The thick blue line ($M_{\rm
virial}\sim W_{\rm CO}^{2.11}$, c.c.=0.98) indicates the best fit by a power-
law correlation.  Figure 10.—  Correlation between the size ($2R$) and the CO
luminosity ($L_{\rm CO}$) of the 575 MCs. The black crosses and the filled
circles are the same as those in Figure 8. The thick blue ($L_{\rm
CO}\sim\rm{size}^{2.74}$, c.c.=0.92) and dashed red (weighted with the
luminosity of the MC, $L_{\rm CO}\sim\rm{size}^{2.41}$, c.c.=0.90) lines
indicate the best fit by a power-law correlation.

Table 1Parameters of 575 Molecular Clouds Identified from 12CO($J$=1–0) Emission | ID
---
(1)
| $l$
---
(∘)
(2)
| $b$
---
(∘)
(3)
| $V_{\rm LSR}$
---
($\,{\rm km}\,{\rm s}^{-1}$)
(4)
| $\Delta V_{\rm FWHM}$
---
($\,{\rm km}\,{\rm s}^{-1}$)
(5)
| $T_{\rm peak}$
---
(K)
(6)
| Area
---
(arcmin2)
(7)
| $W_{\rm{}^{12}CO}$
---
(K $\,{\rm km}\,{\rm s}^{-1}$)
(8)
| $L_{\rm{}^{12}CO}$
---
(K $\,{\rm km}\,{\rm s}^{-1}$ arcmin2)
(9)
001 | 34.913 | 0.025 | -23.61 | 2.22 | 2.81 | 1.75 | 3.92 | 6.88
002 | 34.988 | 0.191 | -24.71 | 1.08 | 2.68 | 1.47 | 1.86 | 2.73
003 | 34.991 | 0.147 | -27.85 | 1.50 | 5.66 | 12.23 | 3.24 | 39.60
004 | 35.043 | 0.197 | -12.39 | 0.80 | 2.60 | 2.33 | 1.38 | 3.23
005 | 35.082 | 0.137 | -26.66 | 1.30 | 2.23 | 1.07 | 2.12 | 2.27
006 | 35.086 | 0.041 | -18.33 | 0.85 | 2.28 | 1.86 | 1.45 | 2.70
007 | 35.136 | 0.164 | -26.70 | 2.42 | 3.28 | 4.08 | 4.45 | 18.17
008 | 35.181 | 0.303 | -38.35 | 2.51 | 2.73 | 2.74 | 4.29 | 11.75
009 | 35.248 | 0.152 | -28.02 | 2.64 | 5.59 | 24.06 | 6.51 | 156.68
010 | 35.252 | 0.232 | -22.39 | 2.00 | 4.36 | 8.33 | 4.44 | 36.93
011 | 35.287 | 0.090 | -25.66 | 0.65 | 2.28 | 1.64 | 1.14 | 1.87
012 | 35.374 | 0.186 | -14.85 | 2.64 | 4.53 | 5.44 | 5.87 | 31.94
013 | 35.375 | 0.149 | -22.65 | 3.19 | 5.71 | 10.12 | 7.08 | 71.68
014 | 35.377 | 0.167 | -27.45 | 0.65 | 2.41 | 1.58 | 1.14 | 1.81
015 | 35.421 | 0.472 | -22.28 | 1.41 | 2.39 | 1.18 | 2.51 | 2.96
016 | 35.440 | 0.138 | -24.43 | 0.59 | 2.34 | 1.48 | 1.01 | 1.49
017 | 35.442 | 0.206 | -23.16 | 2.38 | 3.08 | 1.82 | 4.45 | 8.09
018 | 35.448 | 0.121 | -12.25 | 1.41 | 2.72 | 2.00 | 2.60 | 5.20
019 | 35.449 | 0.239 | -24.61 | 0.76 | 2.25 | 1.15 | 1.33 | 1.53
020 | 35.456 | 0.016 | -11.90 | 1.06 | 2.69 | 2.62 | 1.92 | 5.04
021 | 35.471 | -0.517 | -42.42 | 0.68 | 2.35 | 1.11 | 1.19 | 1.32
022 | 35.506 | 0.115 | -11.03 | 1.00 | 3.28 | 5.14 | 2.03 | 10.44
023 | 35.546 | 0.006 | -11.09 | 0.48 | 2.63 | 1.49 | 0.79 | 1.17
024 | 35.609 | -0.326 | -23.95 | 1.09 | 2.18 | 1.02 | 1.79 | 1.83
025 | 35.613 | -0.309 | -26.06 | 2.09 | 3.04 | 3.23 | 3.71 | 12.00
026 | 35.704 | -0.115 | -11.69 | 1.47 | 4.58 | 13.29 | 3.25 | 43.26
027 | 35.905 | -0.213 | -12.57 | 0.53 | 3.25 | 3.96 | 1.06 | 4.21
028 | 35.927 | -0.561 | -11.13 | 1.11 | 3.08 | 1.57 | 1.94 | 3.04
029 | 35.958 | -0.597 | -13.27 | 1.77 | 2.26 | 0.83 | 2.92 | 2.42
030 | 36.210 | -0.242 | -42.84 | 1.90 | 3.24 | 1.81 | 3.44 | 6.24
031 | 36.250 | -0.267 | -37.65 | 1.36 | 2.35 | 1.11 | 2.37 | 2.64
032 | 36.255 | 0.768 | -16.07 | 2.01 | 6.57 | 4.86 | 5.14 | 24.97
033 | 36.281 | -0.315 | -38.17 | 2.72 | 2.91 | 2.32 | 4.88 | 11.29
034 | 36.333 | 0.680 | -26.65 | 1.52 | 3.26 | 1.91 | 2.69 | 5.13
035 | 36.353 | -0.680 | -19.20 | 0.97 | 2.93 | 2.12 | 1.73 | 3.67
036 | 36.368 | 0.631 | -29.17 | 3.11 | 4.57 | 15.22 | 6.88 | 104.72
037 | 36.430 | 0.169 | -26.16 | 1.66 | 2.15 | 0.69 | 2.73 | 1.89
038 | 36.441 | 0.578 | -29.43 | 0.65 | 3.12 | 1.78 | 1.20 | 2.14
039 | 36.534 | 0.562 | -27.23 | 1.96 | 4.19 | 5.66 | 4.05 | 22.94
040 | 36.618 | 0.529 | -28.10 | 1.85 | 2.40 | 0.92 | 3.06 | 2.82
041 | 36.639 | 0.464 | -20.56 | 1.04 | 2.50 | 2.14 | 1.88 | 4.02
042 | 36.703 | 0.567 | -29.76 | 1.19 | 2.62 | 2.92 | 2.09 | 6.10
043 | 36.716 | -0.054 | -44.60 | 1.16 | 2.61 | 0.93 | 2.09 | 1.94
044 | 36.774 | 0.467 | -26.30 | 0.58 | 1.90 | 1.42 | 0.92 | 1.30
045 | 36.822 | -0.259 | -10.48 | 0.68 | 2.69 | 1.69 | 1.30 | 2.20
046 | 36.857 | 0.504 | -30.51 | 3.90 | 4.88 | 4.61 | 8.78 | 40.46
047 | 36.917 | 0.484 | -31.68 | 1.90 | 5.57 | 2.91 | 5.57 | 16.20
048 | 36.929 | 0.485 | -28.19 | 3.90 | 7.38 | 3.58 | 9.94 | 35.62
049 | 37.003 | 0.483 | -25.55 | 0.92 | 2.51 | 0.95 | 1.49 | 1.41
050 | 37.136 | -0.165 | -11.46 | 1.46 | 7.20 | 5.91 | 4.12 | 24.37
051 | 37.174 | -0.150 | -37.53 | 1.38 | 3.62 | 2.42 | 2.83 | 6.85
052 | 37.191 | 0.195 | -33.29 | 2.52 | 2.65 | 1.71 | 4.26 | 7.30
053 | 37.274 | 0.067 | -14.11 | 0.69 | 2.70 | 1.67 | 1.18 | 1.96
054 | 37.506 | 0.053 | -31.60 | 0.97 | 2.29 | 1.72 | 1.64 | 2.82
055 | 37.510 | -0.217 | -35.93 | 0.83 | 2.51 | 1.58 | 1.40 | 2.22
056 | 37.519 | 0.661 | -29.68 | 1.33 | 4.22 | 3.66 | 2.90 | 10.63
057 | 37.594 | -0.147 | -36.81 | 1.32 | 3.67 | 3.13 | 2.77 | 8.68
058 | 37.643 | 1.408 | -46.60 | 0.61 | 2.92 | 2.03 | 1.21 | 2.45
059 | 37.656 | 1.391 | -44.24 | 1.01 | 2.70 | 3.17 | 1.69 | 5.36
060 | 37.747 | 0.709 | -17.34 | 2.31 | 4.61 | 5.85 | 5.40 | 31.62

Table 2Parameters of 131 Molecular Clouds Identified from 13CO($J$=1–0) Emission | ID
---
(1)
| $l$
---
(∘)
(2)
| $b$
---
(∘)
(3)
| $V_{\rm LSR}$
---
($\,{\rm km}\,{\rm s}^{-1}$)
(4)
| $\Delta V_{\rm FWHM}$
---
($\,{\rm km}\,{\rm s}^{-1}$)
(5)
| $T_{\rm peak}$
---
(K)
(6)
| Area
---
(arcmin2)
(7)
| $W_{\rm{}^{13}CO}$
---
(K $\,{\rm km}\,{\rm s}^{-1}$)
(8)
| $L_{\rm{}^{13}CO}$
---
(K $\,{\rm km}\,{\rm s}^{-1}$ arcmin2)
(9)
001 | 34.971 | 0.159 | -27.50 | 0.72 | 1.20 | 1.45 | 0.58 | 0.84
002 | 35.009 | 0.145 | -28.25 | 1.08 | 1.99 | 2.12 | 1.02 | 2.17
003 | 35.145 | 0.152 | -27.60 | 1.13 | 1.13 | 1.14 | 0.93 | 1.06
004 | 35.245 | 0.152 | -28.33 | 2.48 | 1.87 | 5.39 | 2.21 | 11.93
005 | 35.250 | 0.230 | -22.10 | 0.70 | 1.53 | 1.89 | 0.65 | 1.23
006 | 35.359 | 0.212 | -13.66 | 1.21 | 1.29 | 1.13 | 0.97 | 1.09
007 | 35.380 | 0.159 | -22.85 | 3.11 | 1.65 | 2.16 | 2.81 | 6.09
008 | 35.641 | -0.059 | -12.59 | 0.76 | 1.20 | 1.04 | 0.59 | 0.61
009 | 35.737 | -0.148 | -11.39 | 1.13 | 1.46 | 2.39 | 1.00 | 2.39
010 | 36.254 | 0.763 | -16.23 | 0.88 | 1.24 | 1.65 | 0.73 | 1.21
011 | 36.344 | 0.633 | -29.70 | 1.18 | 1.80 | 4.98 | 1.07 | 5.34
012 | 36.516 | 0.564 | -27.04 | 0.98 | 1.72 | 1.78 | 0.86 | 1.53
013 | 36.833 | 0.488 | -28.48 | 0.92 | 1.45 | 1.95 | 0.81 | 1.57
014 | 36.879 | 0.518 | -31.74 | 1.29 | 1.43 | 1.48 | 1.02 | 1.51
015 | 36.919 | 0.488 | -30.59 | 2.87 | 1.54 | 1.49 | 2.53 | 3.76
016 | 37.745 | 0.704 | -17.56 | 1.48 | 2.07 | 3.21 | 1.45 | 4.66
017 | 37.963 | 0.064 | -32.10 | 1.47 | 2.08 | 2.80 | 1.58 | 4.44
018 | 37.970 | -0.089 | -31.74 | 2.25 | 1.90 | 2.28 | 2.13 | 4.86
019 | 38.314 | 0.167 | -17.13 | 0.71 | 1.15 | 1.23 | 0.58 | 0.71
020 | 38.357 | 0.181 | -18.14 | 0.83 | 1.16 | 1.76 | 0.63 | 1.11
021 | 38.360 | 0.559 | -38.12 | 2.03 | 1.68 | 5.32 | 1.76 | 9.35
022 | 38.418 | 0.527 | -40.39 | 1.46 | 1.81 | 2.25 | 1.31 | 2.96
023 | 38.441 | 0.811 | -38.79 | 1.04 | 1.06 | 1.63 | 0.78 | 1.27
024 | 38.632 | -0.150 | -39.75 | 1.98 | 1.66 | 1.70 | 1.79 | 3.04
025 | 38.655 | 0.083 | -38.84 | 2.42 | 2.22 | 6.58 | 2.30 | 15.13
026 | 38.753 | 0.169 | -38.05 | 3.17 | 2.54 | 3.43 | 3.27 | 11.21
027 | 38.771 | 0.100 | -34.12 | 0.85 | 1.26 | 1.27 | 0.67 | 0.86
028 | 38.809 | 0.520 | -40.98 | 1.90 | 2.24 | 2.42 | 1.97 | 4.76
029 | 38.869 | 0.312 | -15.86 | 3.51 | 2.83 | 4.70 | 4.01 | 18.84
030 | 38.879 | 0.210 | -35.50 | 1.15 | 1.36 | 2.16 | 1.00 | 2.17
031 | 38.928 | 0.436 | -19.53 | 0.97 | 1.15 | 2.22 | 0.76 | 1.69
032 | 38.953 | 0.404 | -46.58 | 0.48 | 1.02 | 1.65 | 0.37 | 0.60
033 | 39.013 | 0.387 | -16.76 | 1.20 | 2.16 | 6.49 | 1.24 | 8.06
034 | 39.058 | 0.077 | -16.48 | 0.86 | 1.45 | 1.60 | 0.76 | 1.22
035 | 39.088 | 0.064 | -33.58 | 0.83 | 1.17 | 1.64 | 0.65 | 1.07
036 | 39.094 | 0.293 | -44.99 | 1.42 | 1.32 | 2.05 | 1.26 | 2.57
037 | 39.148 | 0.199 | -15.98 | 0.89 | 1.57 | 1.64 | 0.75 | 1.24
038 | 39.159 | 0.212 | -34.27 | 1.14 | 1.15 | 1.16 | 0.87 | 1.01
039 | 39.198 | 0.208 | -27.45 | 2.07 | 1.53 | 4.76 | 1.79 | 8.51
040 | 39.216 | 0.304 | -27.36 | 0.92 | 1.69 | 2.39 | 0.80 | 1.91
041 | 39.256 | -0.068 | -33.50 | 1.60 | 2.77 | 6.32 | 1.96 | 12.35
042 | 39.265 | 0.352 | -30.35 | 1.74 | 2.38 | 3.77 | 1.76 | 6.61
043 | 39.298 | 0.554 | -43.40 | 1.34 | 1.15 | 1.58 | 1.04 | 1.65
044 | 39.318 | -0.031 | -32.41 | 0.70 | 1.12 | 1.12 | 0.54 | 0.61
045 | 39.323 | 0.552 | -26.92 | 1.63 | 1.89 | 2.01 | 1.50 | 3.02
046 | 39.472 | 0.662 | -32.52 | 1.22 | 1.09 | 1.19 | 0.92 | 1.10
047 | 39.508 | 0.611 | -33.80 | 0.58 | 1.19 | 1.21 | 0.46 | 0.56
048 | 39.537 | 0.878 | -38.26 | 0.84 | 1.42 | 1.39 | 0.72 | 0.99
049 | 39.661 | 1.910 | -28.94 | 0.92 | 1.13 | 2.12 | 0.71 | 1.49
050 | 39.669 | 0.397 | -34.82 | 1.00 | 1.70 | 3.40 | 0.91 | 3.11
051 | 39.727 | 0.511 | -33.34 | 0.56 | 1.21 | 1.84 | 0.48 | 0.88
052 | 39.753 | 0.437 | -47.74 | 1.41 | 1.68 | 2.18 | 1.16 | 2.52
053 | 39.860 | -0.118 | -32.89 | 0.96 | 1.27 | 1.00 | 0.79 | 0.79
054 | 39.870 | 0.658 | -37.95 | 1.89 | 1.52 | 1.54 | 1.65 | 2.55
055 | 39.901 | -0.175 | -32.61 | 0.94 | 1.50 | 1.24 | 0.83 | 1.02
056 | 39.972 | 0.658 | -35.32 | 0.77 | 0.87 | 0.86 | 0.56 | 0.48
057 | 40.017 | 0.540 | -47.02 | 1.05 | 1.15 | 1.11 | 0.83 | 0.92
058 | 40.044 | 0.357 | -44.52 | 0.55 | 1.12 | 1.51 | 0.46 | 0.70
059 | 40.135 | 0.753 | -30.55 | 1.16 | 1.19 | 1.28 | 0.92 | 1.18
060 | 40.586 | -0.093 | -11.36 | 1.58 | 2.14 | 3.72 | 1.51 | 5.61

Table 3Molecular Clouds Associated with Massive Star Formation Regions

| Name a
---
| $l$
---
(∘)
| $b$
---
(∘)
| $V_{\rm MC}$
---
($\,{\rm km}\,{\rm s}^{-1}$)
| Tracers b,c
---
| $l$
---
(∘)
| $b$
---
(∘)
| $V_{\rm LSR}$
---
($\,{\rm km}\,{\rm s}^{-1}$)
| References d
---
MWISP G036.833$+$00.488 | 36.833 | 0.488 | -28.48 |  H ii G036.870$+$00.462 | 36.870 | 0.462 | -27.3 | (4)
MWISP G036.879$+$00.518 | 36.879 | 0.518 | -31.74 |  |  |  |  |
MWISP G036.919$+$00.488 | 36.919 | 0.488 | -30.59 |  H ii G036.914$+$00.489 | 36.984 | 0.489 | -30.3 | (1)
|  |  |  |  H ii G036.914$+$00.485 | 36.914 | 0.485 | -34.1 | (4)
|  |  |  | Maser G36.918$+$0.483 | 36.918 | 0.483 | -35.9 | (6),(9)
MWISP G038.655$+$00.083 | 38.655 | 0.083 | -38.84 |  H ii G038.651$+$00.087 | 38.651 | 0.087 | -40.0 | (4)
|  |  |  | Maser G38.653$+$0.088 | 38.653 | 0.088 | -31.4 | (6),(9)
MWISP G038.809$+$00.520 | 38.809 | 0.520 | -40.98 |  H ii G038.840$+$00.497 | 38.840 | 0.497 | -43.2 | (4)
MWISP G038.869$+$00.312 | 38.869 | 0.312 | -15.86 |  H ii G038.875$+$00.308 | 38.875 | 0.308 | -15.1 | (4)
|  |  |  |  H ii G038.875$+$00.308 | 38.875 | 0.308 | -14.98 | (5)
MWISP G039.198$+$00.208 | 39.198 | 0.208 | -27.45 |  H ii G039.195$+$00.226 | 39.195 | 0.226 | -21.6 | (7)
MWISP G039.870$+$00.658 | 39.870 | 0.658 | -37.95 |  H ii G039.869$+$00.645 | 39.869 | 0.645 | -40.9 | (4)
MWISP G041.727$+$01.433 | 41.727 | 1.433 | -50.88 |  H ii G041.755$+$01.451 | 41.755 | 1.451 | -54.8 | (8)
|  |  |  |  H ii G041.804$+$01.503 | 41.804 | 1.503 | -52.6 | (8)
MWISP G041.982$+$00.342 | 41.982 | 0.342 | -39.95 |  H ii G042.012$+$00.349 | 42.012 | 0.349 | -40.3 | (4)
MWISP G042.120$+$00.521 | 42.120 | 0.521 | -29.76 | Maser G42.13$+$0.52 | 42.13 | 0.52 | -33.3 | (9)
MWISP G042.150$+$00.982 | 42.150 | 0.982 | -29.16 |  H ii G042.068$+$00.999 | 42.068 | 0.999 | -23.6 | (8)
MWISP G042.696$-$00.150 | 42.696 | -0.150 | -44.32 | Maser G42.698$-$0.147 | 42.698 | -0.147 | -42.8 | (6),(9)
MWISP G043.436$+$00.551 | 43.436 | 0.551 | -14.06 |  H ii G043.432$+$00.521 | 43.432 | 0.521 | -12.8 | (4)
MWISP G043.909$+$00.239 | 43.909 | 0.239 | -45.74 |  H ii G043.906$+$00.230 | 43.906 | 0.230 | -38.1 | (4)
MWISP G043.950$+$00.983 | 43.950 | 0.983 | -29.68 |  H ii G043.968$+$00.993 | 43.968 | 0.993 | -21.6 | (4)
|  |  |  |  H ii G043.989$+$01.000 | 43.989 | 1.000 | -17.1 | (8)
MWISP G044.415$+$00.465 | 44.415 | 0.465 | -49.84 |  H ii G044.417$+$00.536 | 44.417 | 0.536 | -55.1 | (4)
MWISP G044.480$+$00.343 | 44.480 | 0.343 | -48.99 |  H ii G044.501$+$00.335 | 44.501 | 0.335 | -43.0 | (4)
MWISP G044.559$+$00.326 | 44.559 | 0.326 | -46.41 |  |  |  |  |
MWISP G044.518$+$00.396 | 44.518 | 0.396 | -48.15 |  H ii G044.518$+$00.397 | 44.518 | 0.397 | -49.7 | (4)
MWISP G043.827$+$00.393 e | 43.827 | 0.393 | -12.17 |  H ii G043.818$+$00.393 | 43.818 | 0.393 | -10.5 | (4)
MWISP G043.957$+$00.998 e | 43.957 | 0.998 | -20.03 |  H ii G043.967$+$00.995 | 43.967 | 0.995 | -19.7 f | (2),(3)

Note. — a Named from Galactic coordinates of the MCs from 13CO ($J$=1–0)
emission (Table 2). The last two MCs in the table are from 12CO ($J$=1–0)
emission (Table 1). b We use H ii regions and 6.7 GHz methanol masers to trace
massive star formation regions. The position and the velocity of the H ii
region is from the radio continuum emission and the radio recombination line,
respectively. The position and the velocity of the 6.7 GHz maser is from the
observations of the maser survey (Green & McClure-Griffiths, 2011; Breen et
al., 2015). c Based on Tables 1 and 2, we do not find the CO counterpart of
the 6.7 GHZ maser G38.565$+$0.538 with the LSR velocity of $-$28.8 $\,{\rm
km}\,{\rm s}^{-1}$ (Table 1 in Breen et al., 2015). However, a point-like MC
with a temperature of $\sim$1.5 K and the LSR velocity of $-$35.40 $\,{\rm
km}\,{\rm s}^{-1}$ is detected from the 12CO ($J$=1–0) data at the maser’s
position. d (1) Lockman, 1989; (2) Bronfman et al., 1996; (3) Gregory et al.,
1996; (4) Anderson et al., 2011; (5) Balser et al., 2011; (6) Green & McClure-
Griffiths, 2011; (7) Bania et al., 2012; (8) Anderson et al., 2015; (9) Breen
et al., 2015. e The parameters of the MCs are from 12CO ($J$=1–0) emission
(Table 1). f The velocity of the H ii region is from the CS ($J$=2–1)
observation.