Molecular Gas Structures traced by $^{13}$CO Emission in the 18,190 $^{12}$CO Molecular Clouds from the MWISP Survey

After the morphological classification of the 18,190 $^{12}$CO molecular clouds, we further investigate the properties of their internal molecular gas structures traced by the $^{13}$CO($J=$ 1$-$0) line emissions. Using three different methods to extract the $^{13}$CO gas structures within each $^{12}$CO cloud, we find that $\sim$ 15$\%$ of $^{12}$CO clouds (2851) have $^{13}$CO gas structures and these $^{12}$CO clouds contribute about 93$\%$ of the total integrated flux of $^{12}$CO emission. In each of 2851 $^{12}$CO clouds with $^{13}$CO gas structures, the $^{13}$CO emission area generally does not exceed 70$\%$ of the $^{12}$CO emission area, and the $^{13}$CO integrated flux does not exceed 20$\%$ of the $^{12}$CO integrated flux. We reveal a strong correlation between the velocity-integrated intensities of $^{12}$CO lines and those of $^{13}$CO lines in both $^{12}$CO and $^{13}$CO emission regions. This indicates the H$_{2}$ column densities of molecular clouds are crucial for the $^{13}$CO lines emission. After linking the $^{13}$CO structure detection rates of the 18,190 $^{12}$CO molecular clouds to their morphologies, i.e. nonfilaments and filaments, we find that the $^{13}$CO gas structures are primarily detected in the $^{12}$CO clouds with filamentary morphologies. Moreover, these filaments tend to harbor more than one $^{13}$CO structure. That demonstrates filaments not only have larger spatial scales, but also have more molecular gas structures traced by $^{13}$CO lines, i.e. the local gas density enhancements. Our results favor the turbulent compression scenario for filament formation, in which dynamical compression of turbulent flows induces the local density enhancements. The nonfilaments tend to be in the low-pressure and quiescent turbulent environments of the diffuse interstellar medium.

Molecular clouds usually present complex and hierarchical structures. Since its discovery by Wilson et al. (1970), CO line emission has been widely used as a tracer of molecular gas. The boundaries of MCs are usually defined by either the low-J rotational CO emission or extinction above some threshold (Heyer & Dame 2015). The unbiased lxyuan@pmo.ac.cn Corresponding author: Ji Yang jiyang@pmo.ac.cn Galactic plane CO survey, the Milky Way Imaging Scroll Painting (MWISP), is performed using the 13.7m millimeterwavelength telescope of Purple Mountain Observatory (PMO) and observes 12 CO, 13 CO, and C 18 O (J = 1 − 0) spectra, simultaneously (Su et al. 2019). The first phase of the MWISP CO project covering the Galactic longitude from l = 9 • .75 to 230 • .25 and the Galactic latitude from b = -5 • .25 to 5 • .25, has been completed. The second phase of MWISP has begun and intend to extend the Galactic latitude from b = -10 • .25 to 10 • .25. This high-quality CO survey provides us with opportunities to promote the analysis of the molecular clouds properties to a large sample spanning wide spatial scales, different evolutionary stages and various environments.
After observations with sufficient sensitivity and high spatial resolution carried out using the Herschel telescope, filaments became known to play an important role in the star formation of MCs (André et al. 2010;Molinari et al. 2010;André et al. 2014André et al. , 2016Yuan et al. 2019Yuan et al. , 2020Peretto et al. 2022). Our researchs in Yuan et al. (2021) (Paper I) use the 18,190 MCs identified by the 12 CO lines data from MWISP survey and classfied them as filaments and nonfilaments. We found that the filaments make up about 10% of the total number of molecular clouds, while contributing about 90% of the total integrated flux of 12 CO line emission. Despite the systematic difference between the filaments and nonfilaments in their spatial areas, their averaged H 2 column densities do not vary significantly. Neralwar et al. (2022a) have classified the SEDIGISM clouds into four morphologies and found that most of molecular clouds present elongated structures. In addition, the ringlike clouds show the peculiar properties, which are speculated to be related to the physical mechanisms that regulate their formation and evolution (Neralwar et al. 2022b). Following our paper I, several questions can be asked, for instance, is there any possible evolution sequence between filaments and nonfilaments? What are the physics behind the molecular clouds presenting filaments or nonfilaments? Quantifying the amount, distribution, and kinematics of the diffuse and dense gas among them may provide new clues to answering these questions.
Compared with the 12 CO(1-0) line emission having a critical density of ∼ 10 2 cm −3 , the less abundant isotope 13 CO(1-0) lines can trace the denser gas with a density of ∼ 10 3 cm −3 . The large-scale, unbiased, and highly sensitive data on CO and its isotopic lines from the MWISP survey provides us with opportunities to systematically investigate the spatial distribution and properties of the diffuse and dense molecular gas in a large sample of Molecular clouds.
In this paper, we use the 13 CO(1-0) line emission to trace relatively dense gas components within 18,190 12 CO molecular clouds and reveal the relationship between the 13 CO gas fractions and morphologies in molecular clouds. In section 2, we describe the data set, including the 13 CO line emission data and 12 CO molecular cloud catalog; Section 3 introduces three different methods used to extract 13 CO molecular gas structures in 12 CO molecular clouds and compares their results. In Section 4, we present the distribution of the physical parameters of the extracted 13 CO gas structures within the 12 CO MCs and systematically investigate the correlation between 12 CO(1-0) and 13 CO(1-0) line emission in the 12 CO clouds having 13 CO structures, in addition, we also link the the 13 CO gas structures and the morphologies of 12 CO molecular clouds to reveal the possible relation between them. Section 5 discusses how our observational results provide the clues for us to understand the molecular clouds' formation and evolution. We conclude with a summary in Section 6.
2. DATA 2.1. 13 CO J = 1 -0 data from MWISP survey The 13 CO data is from the MWISP survey, which is an ongoing northern Galactic plane CO survey. This survey is conducted by the 13.7m telescope at Delingha, China. The detailed introductions for the telescope, the multibeam receiver system, observation mode, and data reduction procedures are described in Su et al. (2019). The half-power beamwidth (HPBW) for the telescope at 115 GHz is about 50 . The velocity separation of 13 CO lines is about 0.17 km s −1 . The main beam efficiency (η MB ) varied between 40% and 50%.
In this work, we focus on the 13 CO emission in the Second Galactic Quadrant with 104 • .75 < l < 150 • .25, |b| < 5 • .25, and −95 km s −1 < V LSR < 25 km s −1 . Figure 1 presents the large-scale 13 CO gas distribution as a velocity-integrated intensity map and a latitude-integrated intensity map.

Catalog and morphology classification
We define a molecular cloud as a contiguous structure in the position-position-velocity (PPV) data cube with 12 CO(1-0) line intensities above a certain threshold. As described in Yan et al. (2021), a total of available 18,190 molecular clouds have been identified from the 12 CO data cube in the range of 104 • .75 < l < 150 • .25, |b| < 5 • .25, and −95 km Near Far Figure 1. Top Panel: the velocity-integrated intensity map of 13 CO(1-0) emission in the second Galactic quadrant with 104.75 • < l < 150.25 • and |b| < 5.25 • . This map is derived by integrating the 13 CO emission over the velocity range between -95 km s −1 and 25 km s −1 . The distribution of noise RMS for the spectrum in each pixel is presented in Figure A1, the mean noise RMS is about 0.25 K. The sensitivity for this velocity-integrated map can be calculated as σ × √ N × dv = 1.2 K km/s, where σ is the mean noise RMS (0.25 K), dv represents the velocity resolution, its value is 0.17 km/s, N = 760 is the number of the velocity channels. Bottom Panel: the latitude-integrated intensity map of 13 CO(1-0) emission. This map is derived by integrating the 13 CO emission over the latitude range from −5.25 • to 5.25 • . The white dashed line at VLSR = -30 km s −1 divides the molecular clouds into two groups, i.e., the Near and Far groups, as described in Section 4.1. The velocity-integrated intensity maps and the latitude-integrated maps for the 13 CO emission in the near and far velocity ranges, are shown in Figure  A2 and A3, respectively. s −1 < V LSR < 25 km s −1 , using the Density-based Spatial Clustering of Applications with Noise (DBSCAN) algorithm (Ester et al. 1996;Yan et al. 2020).
In the paper I, we have compeleted the morphological classification for these 12 CO molecular clouds, which are mainly classified into filaments and nonfilaments (Yuan et al. 2021). In this work, we aim to analyze the properties of high column density gas traced by the 13 CO line emission in these molecular cloud samples.
3. EXTRACTING 13 CO J = 1 -0 EMISSION STRUCTURES WITHIN 12 CO MOLELCULAR CLOUDS In this work, the 13 CO emission structures are defined as molecular structures within the 12 CO molecular clouds whose spectral voxels have the 13 CO line intensities above a certain threshold. We utilize three different methods, i.e. clipping, DBSCAN (Ester et al. 1996;Yan et al. 2020), and moment mask (Dame 2011) to extract the 13 CO gas structures.

Background noise
Before extracting the 13 CO line emission within the 12 CO molecular clouds, it is necessary to determine their RMS noises (σ). The 13 CO spectral line data are chopped into the separate data cubes with sizes equivalent to the extent of 12 CO emission in the PPV space for 12 CO molecular clouds.  of 12 CO molecular cloud, the 13 CO emission structures, and the background noise in a separate data cube. The voxels within the background region in each 13 CO data cube are utilized to estimate the RMS noise of 13 CO lines for each molecular cloud. The distribution of the resultant 13 CO rms noises for all of 18,190 molecular clouds are presented in Figure 2. The typical value is about 0.25 K. The corresponding RMS noise for each molecular cloud is used for the 13 CO emission extraction. Furthermore, the values of voxels in the background region of each 13 CO data cube are set to zeros, so that the 13 CO structures are extracted within the 12 CO emission boundaries. We extract the 13 CO emission structures in these chopped 18,190 13 CO data cubes, which are correspond to the 18,190 12 CO molecular clouds.

Clipping
Clipping is a common technique, which directly extracts the structures containing the 13 CO spectral channels above the statistical significance level. Spectral channels in each 13 CO data cube having intensities above the defined clipping levels are extracted as the significant 13 CO emission. Whereas the clipping can not avoid the positive noise spikes with values above the clipping level, unless the clipping level is enough high. The high threshold used may lead to loss of the faint emission with intensities below the cutoff levels.

Moment Mask
The main point of a moment mask is that the 13 CO emission is extracted on the smoothed 13 CO spectral line data, to reduce the effects of noise spikes. The extracted structures are further extended to the adjacent voxels, whose ranges are determined by the smoothed spatial and velocity resolutions. The MWISP 13 CO data has a spatial resolution of ∼ 50 arcsec and a velocity resolution of 0.17 km s −1 , we smooth the data with two times the beam size (FWHM S ∼ 100 ) in position space and with four times the velocity channels in velocity space (FWHM V ∼ 0.7 km s −1 ).
In the smoothed 13 CO data, we calculate the noise RMS (σ sm ) and extract the 13 CO emissions with intensities higher than the defined σ sm levels. After that, the extracted 13 CO structures are extended to the structures containing the voxels, which are adjacent to the extracted voxels. The adjacent voxels are defined as the ns pixels in spatially and nv pixels in velocity. Among that, ns = 0.5×FWHM S /ds, nv = 0.5×FWHM V /dv, ds and dv are the spatial and velocity resolutions for the raw data, respectively (Dame 2011). According to the resolutions of the smoothed data, we obtain ns=1 and nv=2. Thus, a total of 3×3×5 voxels are determined to be adjacent to an extracted 13 CO voxel. Based on the boundaries of enlarged structures, we obtain the determined 13 CO structures from the raw 13 CO data cube.

DBSCAN
DBSCAN algorithm, which was designed to discover clusters in arbitrary shape (Ester et al. 1996), has been developed to extract a set of contiguous voxels in the PPV space with 12 CO emission above a certain threshold as a molecular cloud (Yan et al. 2020). This method is based on both the intensity levels and the connectivity of signals. We utilize the identical parameters to extract the 13 CO emission as that used for the 12 CO molecular clouds (Yan et al. 2020), except for the post-selection criteria of the peak values. For the 12 CO emission, its peak intensity in a 12 CO molecular cloud needs to be larger than the intensity of its boundary threshold adding 3σ. Owing to the relatively lower value for the 13 CO line intensity, its peak intensity in a 13 CO structure is larger than the intensity of its boundary threshold adding the 2σ, where σ is the background noise. The parameters used for the DBSCAN extraction are described in detail in Appendix B. The chopped 13 CO data cube without any smoothing procedures is used for identifying the 13 CO structures by the DBSCAN algorithm.

Comparison among different methods
3.3.1. Test on a Case: G139.73 We take a 12 CO molecular cloud G139.725-0.507-038.33 (hereafter G139.73) as an sample to compare the performances of the three different methods. In Figure C4 and C5, we present the velocity-integrated intensity maps and the latitude-integrated maps of 12 CO and 13 CO emission for the G139.73, which are integrated by the chopped data cubes without any clipping. Figure 3 shows the 13 CO emission fluxes of the G139.73 extracted by three techniques at the cutoff levels from 2σ to 10σ. We should note the noise σ used by the moment mask is estimated using the smoothed data (σ sm = 0.05 K), but that for the DBSCAN and clipping, their background noise σ is calculated using the raw data without any smoothing procedures (σ = 0.27 K). The distribution of integrated fluxes extracted by the DBSCAN and clipping algorithms have a similar trend and the values steep up from 4σ to 2σ. For the moment mask with the noise σ sm of 0.05 K, its extracted fluxes are higher than that from two other methods at the same cutoff levels. To quantify the contribution of the background noises to the 13 CO emission fluxes using the three methods, we  . The distribution of 13 CO emission fluxes within the 12 CO cloud G139.73 extracted by three techniques at the cutoff levels from 2σ to 10σ. The σ for the clipping and DBSCAN is 0.27 K, which is calculated using the raw 13 CO line data. Whereas the σ for the moment mask is 0.05 K, estimated from the smoothed 13 CO line data. Note-The number detection rate is the number of 12 CO clouds having 13 CO structures divided by the total number of 18,190. The area ratio is the ratio between the total angular areas of the extracted 13 CO structures and the total 12 CO angular areas of 18,190 12 CO clouds. The flux ratio is the total integrated fluxes of the extracted 13 CO line emission divided by that of 12 CO line emission.
use a 13 CO data cube without the significant 13 CO emission to represent the pure background noises. Further, three methods are performed for the extraction of the 13 CO line emission in this noise cube at the cutoff levels from 2σ to 10σ. This noise data have the same sizes and spatial positions as that of the G139.73 data cube but in the different radial velocity range of (-90.3 -78.4) km s −1 . The extracted noise fluxes are shown as the error bars in Figure 3. We find that the background noises contribute about 15% of the integrated fluxes by the moment mask and clipping at the cutoff level of 2σ. Whereas, the DBSCAN almost completely avoids the background noise. We should note that these effects of the background noises on the integrated fluxes are based on a case of G139.73, whose 13 CO line emissions have relatively high intensities. For the molecular clouds with faint emission and small spatial scales, the effects of noises can be magnified.
In Figure C6, we present the averaged 12 CO and 13 CO spectral lines for pixels along the boundaries, which are determined by the clipping at the cutoff level from 2σ to 5σ, as well as the corresponding mean 13 CO spectral lines of the extracted 13 CO structures. From the cutoff level of 3σ, the averaged 13 CO spectrum along the determined boundary begins to have a significant signal. In Figure C7 and C8, we show the velocity-integrated map and latitudeintegrated map of the extracted 13 CO structures at cutoff level of 3σ and 4σ. We find that there are a lot of positive spikes extracted by the clipping at the cutoff level of 3σ. The same spectral lines for the 13 CO structures but extracted using the DBSCAN are presented in Figure C9. For the DBSCAN, the mean 13 CO spectrum along its boundary determined at 2σ begins to have a significant ratio of signal to noise (S/N). We also show the integrated maps of the extracted 13 CO structures by the DBSCAN at the threshold of 2σ and 3σ in Figure C10 and C11. Figure C12 presents the same 13 CO spectral lines for 13 CO structures but identified using the moment mask at the cutoff level from 8σ sm to 11σ sm . The averaged-boundary 13 CO spectrum begins to show the effective signal at 9σ sm . The maps for the extracted 13 CO structures by the moment mask at 8σ sm and 9σ sm are shown in Figure C13 and C14. We find that there are tiny 13 CO structures extracted by the DBSCAN, not presented in the structures from the moment mask. We should note that the S/N for the averaged-boundary spectrum is related to the number of the spectrum along the boundaries. For the molecular clouds with smaller spatial scales, the averaged-boundary spectrum for 13 CO structures extracted using the same method at the same threshold may not have a significant S/N. The cutoff levels of 4σ for the clipping, 2σ for the DBSCAN, and 9σ sm for the moment mask are adopted to extract the 13 CO structures.

Number detection rate
We determine to extract the 13 CO structures within a large sample of 12 CO molecular clouds, which have the angular areas spanning from 1 to 10 4 arcmin 2 and the integrated fluxes ranging from ∼ 1 to 10 5 K km s −1 arcmin 2 , using the clipping at the cutoff of 4σ, the DBSCAN at the cutoff of 2σ, and the moment mask by a threshold of 9σ sm , to compare the extracted results from the three different methods.
For the clipping, there are 4,390 molecular clouds detected 13 CO line emission. The extracted structure with values above the threshold, whose spatial size is one pixel (0.25 arcmin) or its velocity span is just one channel (0.17 km s −1 ), is determined as the noise spike. After removing these noise spikes, 2,462 molecular clouds are regarded as having the significant 13 CO line emission. However, the DBSCAN and Moment mask algorithms do not extract individual noise spike. For the DBSCAN algorithm, 2,851 molecular clouds are identified to have the 13 CO emission. The moment mask extracts the 13 CO line emission in the 2,735 molecular clouds. The number detection rates in the total 18,190 MCs by three methods are listed in Table 1. 3.3.3. Area ratios between the 13 CO and 12 CO line emission The total angular area for the 18,190 12 CO molecular clouds is about 228.2 deg 2 . The total angular area for the extracted 13 CO structures within the 18,190 12 CO clouds by the clipping is 24.7 deg 2 . The value is 46.2 deg 2 extracted by the DBSCAN and 47.2 deg 2 from the moment mask. The ratios between the total 13 CO angular areas of the extracted 13 CO structures and the total 12 CO angular areas of the 18,190 12 CO clouds are listed in Table 1. 3.3.4. Integrated flux ratios between the 13 CO and 12 CO line emission The total 12 CO(1-0) emission fluxes for the 18,190 12 CO molecular clouds is 3.7×10 6 K km s −1 arcmin 2 . The total extracted 13 CO(1-0) emission flux in this catalog is 1.5×10 5 K km s −1 arcmin 2 by the clipping. The value is 2.3×10 5 K km s −1 arcmin 2 by the DBSCAN algorithm and 2.7×10 5 K km s −1 arcmin 2 by the moment mask method. These total integrated flux ratios between the 13 CO and 12 CO line emission by three methods are listed in Table 1.
We find that the number detection rates of the DBSCAN and moment mask are consistent with ∼ 15%, the value of the clipping is about 2% lower than that of the other two techniques. In addition, the clipping extracts plenty of positive noise spikes. For the total angular areas of the extracted 13 CO structures, the values from the clipping is about 50% of that from the DBSCAN and moment mask. For the extracted integrated fluxes of 13 CO line emission, the values from the clipping are about 60% of that from the DBSCAN or Moment mask. That indicates the clipping method extracts amounts of noise spikes with intensities larger than the threshold of 4σ, meanwhile, it also loses substantial faint 13 CO emission. For either the total angular areas or the total integrated fluxes of the extracted 13 CO structures, the values from the moment mask are a bit higher than that from the DBSCAN, while the number detection rate of the moment mask is a bit lower than that from the DBSCAN. Overall, these values from the DBSCAN and moment mask are close.

Differences of extracted basic parameters
The 13 CO emissions are extracted within the 12 CO clouds by three methods. All the 13 CO emissions within the boundary of a 12 CO cloud belong to the same cloud. We take all the 13 CO line emissions within a 12 CO cloud as a whole, referred to as 13 CO molecular gas structures. The equivalent angular area of 13 CO molecular structures (A13 CO ) for a 12 CO cloud is the sum of the pixel areas of the extracted 13 CO emission regions projected in the l-b panel. The mean velocity span of 13 CO molecular structures (V aver, 13 CO ) within a 12 CO cloud is the averaged extracted velocity span of each pixel in the 13 CO emission regions, weighting by its corresponding velocity-integrated intensity, i.e. V aver, 13 CO = Σ(V span (l, b) × W13 CO (l, b))/ΣW13 CO (l, b). The total 13 CO integrated flux (F13 CO ) is the integrated flux of all the 13 CO structures in an individual molecular cloud, is the 13 CO line intensity at the coordinate of (l, b, v) in PPV space, dv = 0.166 km s −1 is the velocity resolution. The peak value (T peak, 13 CO ) is the maximal value of the 13 CO line intensities for the extracted 13 CO structures.
The number distributions of angular sizes (A13 CO ), mean velocity spans (V aver, 13 CO ), integrated fluxes (F13 CO ), and peak intensities (T peak, 13 CO ) of 13 CO structures, which are identified by three techniques, are presented in Figure 4. We find the angular sizes of 13 CO structures extracted by the clipping are systematically smaller than that from the DBSCAN and moment mask. We compare the angular sizes of 13 CO structures from the DBSCAN and moment mask, their distributions in the range of the angular sizes larger than 6 arcmin 2 are similar. There are more structures from the DBSCAN in the range of 1 -6 arcmin 2 . To check the reliability of these extra structures, which are identified by the DBSCAN, but not by the moment mask, we present their 13 CO line intensity maps integrated along with three different directions (l, b, v) and their averaged 13 CO line spectra. As shown in Figure D15, we find that these structures usually are located around the regions contoured at the levels of half of the peak velocity-integrated intensity of 12 CO emission. In addition, their 13 CO spectral profiles usually present the Gaussian-like profiles. Thus, we determine these 13 CO structures are valid. The number distributions of T peak, 13 CO of 13 CO line emission from the three techniques are similar in the range of T peak, 13 CO > 2.0 K. After the smooth procedure, a portion of tiny structures with T peak, 13 CO < 2.0 K may be missed by the moment mask. For the distribution of the mean velocity span of 13 CO structures, we find the values from the moment mask tend to be larger than that from the DBSCAN, and the values from the DBSCAN  are prone to be larger than that from the clipping. That may be due to that the extracted voxels in the smoothed data by the moment mask are further extended to the adjacent voxels, while the intensities of voxels extracted by the DBSCAN are larger than 2σ, and the 13 CO structures from the clipping only contain the velocity channels with intensities larger than 4σ. Thus the velocity span of 13 CO emission in each spatial pixel is derived from the velocity channels with intensities larger than 2σ for the DBSCAN and 4σ for the clipping. The number distributions of F13 CO have a similar trend with that of A13 CO . That means the differences of the F13 CO distributions from three techniques are mainly attributed to their A13 CO distributions.

Summary of methods
Above all, the clipping extracts plenty of noise spikes with values larger than 4σ and meanwhile loses the faint significant emission having intensities less than 4σ. That leads to the extracted parameters being systematically smaller than that from the other two methods. In addition, the moment mask leaves out a part of faint and tiny 13 CO structures, owing to the smooth procedure. The moment mask is more suitable for the structures with relative large angular sizes and high emission intensities. The DBSCAN algorithm can not only avoid the noise spikes but also preserve the faint and tiny 13 CO structures not identified by the moment mask. Each voxel in the PPV space of the structures extracted by the DBSCAN is larger than 2σ. We take the resultant 13 CO structures from the DBSCAN, which is consistent with the extraction algorithm used for the 12 CO clouds identification, for the follow-up analysis.

14.01
Note-The central Galactic coordinates (lcen, bcen) for each 12 CO cloud are the averaged Galactic coordinates in its velocity-integrated 12 CO(1-0) intensity map, weighting by the value of the velocity-integrated 12 CO(1-0) intensity. The central velocity (VLSR) for each cloud is the averaged radial velocity in its radial velocity field, weighting by the value of the velocity-integrated 12 CO(1-0) intensity. The A12 is the velocity resolution, dldb = 0.5 arcmin × 0.5 arcmin = 0.25 arcmin 2 , the angular size of a pixel is 0.5 arcmin. The F13 CO is calculated using the 13 CO(1-0) line emission within each cloud through the same formula, but T mb (l, b, v) is the 13 CO line intensity. This table is available in its entirety from the online journal. A portion is shown here for guidance regarding its form and content.

Comparing the physical properties of molecular clouds with and without 13 CO molecular structures
The whole catalog of 18,190 molecular clouds is identified using the 12 CO(1-0) line data by the DBSCAN algorithm (Yan et al. 2021). Among that, the 2,851 12 CO clouds have 13 CO structures, which are also extracted by the DBSCAN algorithm. Since the boundary of a molecular cloud is defined by the 3D surface of 12 CO(1-0) line emission in the PPV space, all the 13 CO emissions within this surface belong to the same 12 CO cloud. Thus its internal 13 CO emission components are characterized as its substructures, which are referred to as 13 CO molecular structures. All the extracted 12 CO(1-0) line emission data of 18,190 molecular clouds and the extracted 13 CO(1-0) line emission data within the 2,851 12 CO clouds have been published in ScienceDB (Yuan et al. 2022).
We should note that a single molecular cloud may have more than one individual 13 CO molecular structure. We take all the separate 13 CO molecular structures in a single 12 CO molecular cloud as a unity. The 13 CO emission parameters are derived from all the 13 CO structures in a 12 CO cloud as a whole. The equivalent angular area of 13 CO molecular structures for a 12 CO cloud is the sum of the pixel areas of the extracted 13 CO emission regions projected in the l-b panel. The velocity span of 13 CO molecular structures within a 12 CO cloud is the range between the maximal and minimal velocity of the extracted 13 CO structures along the velocity axis. For a 12 CO cloud with multiple 13 CO structures, the minimal velocity is the minimal value in the velocity ranges for all the extracted 13 CO structures and the maximal velocity is the maximal value of that. Figure C9 illustrates the velocity span for the extracted 13 CO structures in the 12 CO cloud G139.73. The 13 CO integrated fluxes are the integrated fluxes of all the 13 CO structures in an individual molecular cloud. The peak value is the maximal value of the 13 CO line intensities within the boundary of a 12 CO MC. The parameters of 13 CO molecular structures for the 2,851 12 CO molecular clouds are listed in Table  2. The rest 15,339 12 CO molecular clouds do not have the significant 13 CO structures. We systematically compare the basic physical parameters of the 12 CO molecular clouds having 13 CO structures ( 13 CO-detects) to that of molecular clouds without 13 CO structures (Non 13 CO-detects).  . The number distributions of angular areas (A12 CO ), velocity spans (V span, 12 CO ), the peak intensities (T peak, 12 CO ) and the integrated fluxes (F12 CO ) of 12 CO line emission for 12 CO molecular clouds with and without 13 CO structures. The green histgrams represent 12 CO molecular clouds not having the 13 CO structures (Non 13 CO-detects). The magenta ones are the 12 CO molecular clouds having the 13 CO structures ( 13 CO-detects).  Figure 6. The number distributions of angular areas (A12 CO ) and the averaged velocity-integrated intensities (W aver, 12 CO ) of 12 CO line emission for 12 CO molecular clouds in near and far groups, respectively. The near 12 CO clouds are in a velocity range of (-30 25) km s −1 , the far 12 CO clouds are in (-95 -30) km s −1 . The green histgrams represent 12 CO molecular clouds not having the 13 CO structures (Non 13 CO-detects). The magenta ones are the 12 CO molecular clouds having the 13 CO structures ( 13 CO-detects).
In Figure 5, we present the number distributions of angular areas (A12 CO ), velocity spans (V span, 12 CO ), peak intensities (I peak, 12 CO ), and integrated fluxes (F12 CO ) of 12 CO line emission for the 13 CO-detects and Non 13 CO-detects, respectively. The quantiles at 0.05, 0.25, 0.5, 0.75, and 0.95 of these parameters for the 13 CO-detects and Non 13 COdetects are listed in Table 3, respectively. We find that these quantiles of the parameters in the 13 CO-detects are systematically larger than that from the Non 13 CO-detects. We calculate the total F12 CO of the whole Non 13 COdetects. The value is about 2.5×10 5 K km s −1 arcmin 2 , which makes up about 6.8% of that of the total 18,190 12 CO molecular clouds (3.7×10 6 K km s −1 arcmin 2 ). The rest ∼ 93% are from the 13 CO-detects. That indicates the 13 CO-detects are the main contributor of 12 CO emission fluxes, although their number only take a percentage of ∼ 15% in the total number. The total A12 CO for the 18,190 12 CO clouds is about 228.2 deg 2 . Among that, the sum of the A12 CO from the 13 CO-detects take a percentage of 76.2% and that from the Non 13 CO-detects take about the rest of 23.8%.
Following the paper I (Yuan et al. 2021), the 12 CO clouds are divided into two groups by a V LSR threshold of -30 km s −1 , shown as a white-dashed line in Figure 1. The 12 CO clouds with central velocities in a range of (-30 25) km s −1 are in the near group, and the 12 CO clouds with central velocities ranging from -95 km s −1 to -30 km s −1 are in the far group. In the near group, there are 9,544 12 CO molecular clouds, among which the 13 CO-detects take a percentage of 10.4%. In the far group, there are 8,646 12 CO molecular clouds and 21.5% of them have 13 CO structures.
The number detection rate of the 13 CO-detects in the near is lower than that in the far group. That may be due to that there are more MCs with faint 12 CO emission, but no 13 CO emission detected in the near group. The number distributions of the A12 CO of 13 CO-detects and Non 13 CO-detects in the near and far group are presented in Figure  6, respectively. The quantiles at 0.05, 0.25, 0.5, 0.75, and 0.95 of their A12 CO values are listed in Table 4. According Note-The quantiles at 0.05, 0.25, 0.5, 0.75 and 0.95 for each parameter in its sequential data and its mean value.   to the spiral structure of the Milky Way, the kinematical distances, which are estimated using the Bayesian distance calculator in Reid et al. (2016), center on about ∼ 0.5 kpc for molecular clouds in the Local arm and ∼ 2 kpc for that in the Perseus arm. Considering these typical distances, the molecular cloud in the local region with an angular size of 1 has a physical scale of ∼ 0.15 pc, the value is ∼ 0.6 pc for that in the Perseus arm.
In addition, we estimate the averaged velocity-integrated intensities of 12 CO lines emission, W aver, 12 CO = T mb (l, b, v)dvdldb/ dldb, for molecular clouds in the near and far gourps, respectively. Their distributions are shown in Figure 6. The H 2 column density (N H2 ) can be calculated through the N H2 = X CO W12 CO , where X CO = 2×10 20 cm −2 (K km s −1 ) −1 is the CO-to-H 2 conversion factor (Bolatto et al. 2013). The quantile values of N H2 for Non 13 CO-detects and 13 CO-detects are also listed in Table 4. We find that the typical values of A12 CO and N H2 of 13 CO-detects are larger than those of Non 13 CO-detects, either in the near or far groups. Thus the 13 CO emission seems to be related to the properties of 12 CO emissions independent of the distance.
Compared with the Non 13 CO-detects, the 13 CO-detects tend to have larger A12 CO , higher T peak, 12 CO , and W aver, 12 CO , either in near or far group. 4.2. 12 CO and 13 CO lines emission in 12 CO molecular clouds having 13 CO structures To systematically analyze what properties of MCs determine the 13 CO line emission in the 13 CO-detects, we examine the correlation between their physical properties of 13 CO line emission with that of the 12 CO line emission. Figure 7 presents the number distributions of angular sizes (A12 CO , A13 CO ), velocity spans (V span, 12 CO , V span, 13 CO ), the integrated fluxes (F12 CO , F13 CO ), and peak intensities (T peak, 12 CO , T peak, 13 CO ) of 12 CO and 13 CO lines emission of the 2,851 13 CO-detects. The quantiles at 0.05, 0.25, 0.5, 0.75, and 0.95 of these parameters for 13 CO line emission are listed in Table 5. Those for 12 CO line emission are listed in Table 3. We find that the values of angular areas, velocity spans, peak intensities, and the integrated fluxes of 13 CO structures in the MCs are systematically smaller than that of their 12 CO line emission. ratios between the parameters of 13 CO structures and that of 12 CO line emission for the 2,851 13 CO-detects, these parameter ratios include their angular sizes (black), velocity spans (blue), the peak intensities (cyan) and the integrated fluxes (magenta).

Ratios between 13 CO and 12 CO emission parameters
Since only a portion molecular gas components in a 12 CO cloud have 13 CO emission, what are the specific fractions of 13 CO gas in these 12 CO MCs? Figure 8 presents the distributions of the ratios between the 13 CO emission parameters and 12 CO emission parameters in the each 12 CO cloud with 13 CO structures. The quantiles at 0.05, 0.25, 0.5, 0.75, and 0.95 of these ratio values are listed in Table 5. We find that the median and mean values of A13 CO /A12 CO and T peak, 13 CO /T peak, 12 CO are close to ∼ 0.25. In addition, the 95% of the 13 CO-detects have the A13 CO /A12 CO with values less than 0.53 and the T peak, 13 CO /T peak, 12 CO with values less than 0.44. For the V span, 13 CO /V span, 12 CO in the 13 CO-detects, their median and mean values are about 0.5 and 95% of them are less than 0.76. The median and mean values of F13 CO /F12 CO , are 0.04 and 0.05, respectively. Moreover, the F13 CO /F12 CO for the 95% of 13 CO-detects is not larger than 0.13. That implies the fractions of 13 CO gas in the 12 CO molecular clouds are typically less than 13%. Considering the 12 CO lines are more optically thick, this value should be much lower. . The correlations of angular sizes, velocity spans, peak intensities, and integrated fluxes of 12 CO(1-0) emission between that of 13 CO(1-0) line emission. Each dot represents a 12 CO cloud with 13 CO structures. The 12 CO emission parameters of a 12 CO cloud represent the global physical parameters of a 12 CO clouds. The 13 CO emission parameters are derived from all the 13 CO structures inside a single 12 CO cloud. The black lines represent the upper limits (A13 CO vs A12 CO , F13 CO vs F12 CO ). The colors on the points represent the distribution of the probability density function of the 13 CO-detects counts (2D-PDF), which are calculated utilizing the Kernel-density estimation through Gaussian kernels in the PYTHON package scipy.stats.gaussian kde.

Correlation between 13 CO and 12 CO emission parameters
Since the parameters of 13 CO line emission are distributed in a certain range, how do the global properties of molecular clouds affect their 13 CO line emission? Figure 9 presents the correlations between the parameters of 13 CO emission and that of 12 CO emission in each 13 CO-detects. The spearman's rank correlation coefficients (R-value) for these relations are estimated and the resultant R-values are noted in the Figure 9. We find the R-value (0.75) for A12 CO and A13 CO is consistent with that for F13 CO and F12 CO , a little higher than the R-value (0.73) for T peak, 13 CO and T peak, 12 CO and the value of 0.72 for the V span, 12 CO and V span, 13 CO . That implies there are roughly positive correlations between them, but they still present a little dispersed.
There are sharp upper limits for the ratios of A13 CO /A12 CO , and F13 CO /F12 CO independent of the angular areas and CO integrated fluxes in wide ranges. As shown in Figure 9, we outline their upper limits with slopes of 0.7 and 0.2, respectively. That indicates the area of 13 CO emission in a molecular cloud generally does not exceed the 70% of the 12 CO emission area, independent of the 12 CO emission area. For the integrated fluxes, the 13 CO emission fluxes are usually less than 20% of the 12 CO emission fluxes.
Overall, the global physical parameters of molecular clouds, such as the angular areas and integrated fluxes of 12 CO emission, show roughly positive correlations and provide upper limits for that of 13 CO emission. Whereas they cannot critically determine the 13 CO emission.
Since the correlation between the global physical properties of each 12 CO cloud and that of its interior 13 CO structures exhibits a bit scattered, we further focus on the local areas having both 12 CO and 13 CO line emissions in each 12 CO cloud with 13 CO structures. Figure 10 presents the correlations between the parameters of 12 CO and 13 CO emission towards the same areas where both the 12 CO and 13 CO emissions are detected in each 13 CO-detects. The T averl, 12 CO is calculated by averaging the 12 CO line intensities within the 13 CO emission region. The T aver, 13 CO 10 1 2 3 4 5 6 7 8 9 T averl, 12 CO (K) 1e 7 Figure 10. The correlations between the averaged line intensities (T averl, 12 CO vs T aver, 13 CO ), peak (W peak, 12 CO vs W peak, 13 CO ) and averaged (W averl, 12 CO vs W aver, 13 CO ) velocity-integrated intensities, and the integrated fluxes (F loc, 12 CO vs F13 CO ) of 12 CO and 13 CO emission towards the same areas where both the 12 CO and 13 CO emissions are detected in each 13 CO-detects. Each dot represents a single 12 CO cloud with 13 CO structures. The parameters of 12 CO line emission are the physical parameters toward the 13 CO-emitting regions within 12 CO clouds. The 13 CO emission parameters are derived from all the 13 CO structures inside a single 12 CO cloud. The black lines show the linear least-squares fits to the data. The colors on the points show the distribution of the probability density function of the 13 CO-detects counts (2D-PDF).
is the mean value of the 13 CO spectra intensities. The W aver, 13 CO and W averl, 12 CO are the averaged values of the velocity-integrated intensities of 13 CO line and that of 12 CO line in the 13 CO emission region, respectively. The W peak, 13 CO and W peak, 12 CO are the peak values of the velocity-integrated intensity of 13 CO lines and that of 12 CO lines at the same positions, respectively. The F13 CO and F loc, 12 CO represent the integrated fluxes of 13 CO line emission and that of 12 CO emission in the 13 CO emission area, respectively. We calculate their spearman's rank correlation coefficients (R-value) and the resultant R-values are noted in Figure 10. Based on these relations between W aver, 13 CO and W averl, 12 CO (R-value = 0.8), W peak, 13 CO and W peak, 12 CO (R-value = 0.91), and F13 CO and F loc, 12 CO (R-value = 0.96), their correlations are more and more tightly. That indicates the properties of 12 CO line emission in the area where both 12 CO and 13 CO are detected, its velocity-integrated intensity (W12 CO ) in this area, is a more direct link for that of 13 CO line emission. Furthermore, we also implement the linear least-squares to these linear relations and the fitted slopes are noted in Figure 10. These relations indicate that the 13 CO fluxes linearly increase as the 12 CO fluxes incrases in the region, where the W12 CO is larger than a value of ∼ 1 K km s −1 , which is close to the sensitivities of MWISP data.

The counts of 13 CO molecular structures in a single 12 CO molecular cloud
The boundary of a molecular cloud is defined by its 12 CO(1-0) line emission, its internal 13 CO molecular structures can present several individual structures, as shown in Figure C10. The counts of 13 CO molecular structures in a single 12 CO cloud can provide essential clues to the development of dense gas content and the internal sub-structures of molecular clouds. We statistic the number of separate 13 CO molecular structures in each 13 CO-detects. Owing to that the molecular cloud distance may affect the spatial physical resolution of 13 CO molecular structures, we divide the 2,851 13 CO-detects into the near and far groups, as introduced in Section 4.1. Figure 11 presents the distributions Near Far Figure 11. The distribution of the 13 CO structure counts in a single 12 CO cloud for the 13 CO-detects in the near (left panel) and far (right panel) groups, respectively. In the right-up corner of each panel, the pie chart illustrates the percentages of 12 CO clouds in this sample with one 13 CO structure, two 13 CO structures, and multiple (more than two) 13 CO structures, respectively. Note-The number detection rate is the number of extracted 13 CO-detects divided by the number of molecular clouds in the total samples, nonfilaments and filaments, respectively. The flux ratio is the total integrated fluxes of 13 CO line emission divided by that of 12 CO line emission for the total molecular clouds, nonfilaments and filaments, respectively.
of 13 CO molecular structure counts in a single 12 CO cloud for the 13 CO-detects in near and far groups, respectively. We find that the 13 CO-detects with one 13 CO structure are dominant and take a percentage of about 60%. Then the molecular clouds having two 13 CO molecular structures occupied about 15% of the 13 CO-detects. The rest ∼ 20% of 13 CO-detects have more than two 13 CO structures, the 13 CO structure counts in a single 12 CO cloud can be up to ∼ 600. It should be noted that the number fraction of 12 CO clouds with one 13 CO velocity structure varies about 10% for that in the near and far groups, as well as the fraction of 12 CO clouds with multiple 13 CO structures.

4.3.
Linking the internal 13 CO gas structures to the morphologies of 12 CO clouds 4.3.1. The 13 CO structures detection rates and morphologies In paper I (Yuan et al. 2021), we took the morphological classification for the total 18,190 molecular clouds, which were classified as unresolved, non-filaments (11,680), and filaments (2,062). Among the 2,851 13 CO-detects, 1,641 12 CO molecular clouds belong to nonfilaments and 1,166 clouds are classified as filaments. We find that the number detection rate of 13 CO structures is 15.7%(2851/18190) in the whole 12 CO molecular clouds, 14% (1641/11680) in the 12 CO clouds classified as nonfilaments, and 56.5% (1166/2062) in the 12 CO clouds classified as filaments. For the ratio of the total integrated fluxes of 13 CO line emission to that of 12 CO line emission, the value is 6.3% for the total molecular clouds, 2.9% for the nonfilaments, and 6.7% for the filaments. Those values are listed in Table 6. Thus, compared with nonfilaments, the filaments tend to have higher density gas structures, which are traced by 13 CO lines.

The 13 CO structures parameters and morphologies
Furthermore, we focus on the properties of the 13 CO molecular gas structures in the filaments and nonfilaments of the 2,851 13 CO-detects. Figure 12 compares the 13 CO structures parameters of these filaments and nonfilaments. We find that the angular areas (A13 CO ), velocity spans (V span, 13 CO ) and integrated fluxes (F13 CO ) of 13 CO structures in these filaments tend to be larger than that in these nonfilaments. While the number distributions of their peak intensities are similar. We find that filaments not only tend to have 13 CO gas structures, but also their internal 13 CO structures have larger angular sizes, velocity span, and integrated fluxes. That indicates the 12 CO cloud classified as filaments gather more high-density gas structures in the local areas where both 12 CO and 13 CO emissions are detected.
We also link the 13 CO molecular structure counts in a single 12 CO cloud to its morphology. Figure 13 illustrates the distribution of 13 CO molecular structure counts within one 12 CO cloud for the 13 CO-detects classified as filaments and nonfilaments. We find that about 85% of these nonfilaments have one 13 CO molecular structure and only 3% of nonfilaments have more than two 13 CO molecular structures. While for these filaments, those with one 13 CO molecular structure only occupy about 35%, and those having more than two 13 CO molecular structures take about 44%. Moreover, only filaments could harbor more than ten 13 CO structures. That indicates the filament tend to have more separate 13 CO molecular structures in its interior. That implies the development of dense gas content in filaments is separate and inhomogenous. Nonfilaments Filaments Figure 13. The distribution of the 13 CO molecular structure counts in a single 12 CO cloud for the 13 CO-detects classified as nonfilaments (left panel) and filaments (right panel) groups, respectively. In the right-up corner of each panel, the pie chart illustrates the percentages of MCs in this sample with one 13 CO molecular structure, two 13 CO molecular structures, multiple (larger than two) 13 CO molecular structures, respectively.
In the Milky Way, Torii et al. (2019) used the CO J = 1 − 0 data from the FUGIN project and found that the ratio of the integrated intensities of 13 CO and 12 CO lines (W( 13 CO)/W( 12 CO)) along the Galactical longitude l = 10 • -50 • were distributed in a range of 1% -10%. While the W( 13 CO)/W( 12 CO) in the star-forming cloud Orion B can achieve about 15% (Gratier et al. 2021). Roman-Duval et al. (2016) investigated the Galactic distribution of molecular gas components traced by 12 CO and 13 CO lines along the Galactic radius. The values of W( 13 CO)/W( 12 CO) have an approximately constant value of 20% out to the galactic radius of 6.5 kpc, decrease to ∼ 8% -10% in the solar neighborhood and about 5% -10% out to the radius of 14 kpc.
In the nearby galaxies, Cormier et al. (2018) carried out the CO observations for nine nearby spiral galaxies using IRAM 30-m telescope, which has a spatial resolution of ∼ 1.5 kpc, the resultant W( 13 CO)/W( 12 CO) ratio has a median value of ∼ 9% and varies by a factor of 2. For the five nearby star-forming galaxies, Gallagher et al. (2018) combined the IRAM 12 CO(1-0) maps and ALMA observations of 13 CO(1-0) lines and presented the distribution of the W( 13 CO)/W( 12 CO) along the radius, which have a mean value of 8.8% with a radius less than 1 kpc and 3.9% in a radius larger than 1 kpc. Furthermore, Méndez-Hernández et al. (2020) presented the ALMA observations towards 27 low-redshift (0.02 < z < 0.2) star-forming galaxies, their averaged value of W( 13 CO)/W( 12 CO) is 5.6±1% and varies by a factor of 2. Figure 14 compares the above literature results with our results. Overall, the W( 13 CO)/W( 12 CO) depends on not only the molecular cloud conditions but also their positions in the galaxy. Moreover, the W( 13 CO)/W( 12 CO) in the local molecular clouds with active star formation rates is higher.

Implications of molecular clouds formation and evolution
The questions of how molecular clouds form and what mechanisms determine their physical properties still remain open. Several mechanisms have been invoked to explain the gathering mass of molecular clouds. The agglomeration of smaller clouds (Oort 1954;Field & Saslaw 1965;Kwan & Valdes 1983;Tomisaka 1984), the turbulence flows in the diffuse ISM Passot et al. 1995;Ballesteros-Paredes et al. 1999a), and the large-scale gravitational instability of the Galactic disk (Lin & Shu 1964;Roberts 1969;Tasker & Tan 2009).
From our observations of 18,190 molecular clouds using 12 CO and 13 CO lines. In terms of their morphologies, i.e., nonfilaments and filaments, filaments tend to have larger spatial scales. Whereas their averaged H 2 column densities do not vary significantly (Yuan et al. 2021). Furthermore, we find that 13 CO gas emission determined by the its H 2 Orion B Gratier et al. 2021Torii et al. 2019

Rgal > 1 kpc
Meńdez-Hernández et al. 2020 Figure 14. Comparisons among the integrated intensity ratios between 13 CO and 12 CO lines (W( 13 CO)/W( 12 CO)). The colored points show the average or median values of W( 13 CO)/W( 12 CO). The error bars on the individual points reflect the distributions for the ratios. From left to right, we present the literature results of W( 13 CO)/W( 12 CO) in the local regions of Milky Way and in the nearby galaxies, respectively. The W( 13 CO)/W( 13 CO), which is presented in Figure 12 of Torii et al. (2019), is calculated through the total velocity-integrated intensities of 13 CO emission in a Galactic longitude of 1 degree divided by that of 12 CO emission in the same bin. The presented values of this work is the ratio of the total integrated intensities of 13 CO emission to that of 12 CO emission in each molecular cloud. The W( 13 CO)/W( 13 CO) for the Orion B is the mean integrated intensity of 13 CO emission divided by that of 12 CO emission, whose values are listed in Table 2 of Gratier et al. (2021). column density is primarily detected in the filaments. That indicates the filament gathers more mass on a global scale and meanwhile has local density enhancements where both 12 CO and 13 CO emissions are detected. In addition, the filament also tends to have more than one individual structure traced by 13 CO lines in its interior. That implies the development of dense gas content in filaments is separate and inhomogenous. The formation of filament often arises from the shock compression in the ISM (Arzoumanian et al. 2018;Abe et al. 2021;Arzoumanian et al. 2022). The shock compression may be caused by supersonic turbulence in the molecular clouds (Padoan & Nordlund 1999;Pudritz & Kevlahan 2013;Matsumoto et al. 2015), cloud collisions (Inoue & Fukui 2013;Inoue et al. 2018;Tokuda et al. 2019), feedback from massive stars, and galactic spiral shock. While the supercritical filaments may be driven by the gravitational contraction/accretion (Arzoumanian et al. 2013;Gong et al. 2021;Yuan et al. 2020) and further fragment into smaller components owning to turbulence and gravitational instabilities (Hacar & Tafalla 2011;Henshaw et al. 2016;Kainulainen et al. 2017;Lu et al. 2018;Lin et al. 2019;Yuan et al. 2019). We try to investigate the relation between the filaments and nonfilaments. If the filaments fragment into nonfilaments due to the gravitational instability, the high-density gas fraction in nonfilaments should be comparable to that of filaments. That is unlikely owing to our observational results of nonfilaments with less dense gas. Our observed properties of filaments and nonfilaments favor that molecular clouds be explained as the density fluctuations induced by the turbulent compression in the diffuse ISM and broken up by the combination of dynamical and thermal instabilities, like the physical processes of shear, rotation, cooling, and magnetic fields (Ballesteros-Paredes et al. 1999b;Koyama & Inutsuka 2002;Heitsch et al. 2006;Vázquez-Semadeni et al. 2006;Beuther et al. 2020). Filaments tend to be under shock compressions and nonfilaments tend to be in low-pressure environments. Meanwhile they present the different spatial scales and internal structures. In addition, we are not able to rule out the hypothesis that nonfilament collisions to form filaments to some degree.

SUMMARY AND CONCLUSIONS
We identify the 13 CO gas structures in the 18,190 12 CO molecular clouds and systematically compare the physical properties of 12 CO clouds having 13 CO gas structures ( 13 CO-detects) and those of 12 CO clouds without 13 CO gas structures (Non 13 CO-detects). Furthermore, we systematically analyze the 13 CO and 12 CO emission parameters in the 2851 13 CO-detects, and link the internal 13 CO gas structures of each 12 CO cloud with its morphology, i.e., filament or nonfilament. The main conclusions are as follows: 1. In the whole sample of 18,190 12 CO molecular clouds, ∼ 15.7% 12 CO clouds (2851) have the 13 CO molecular gas structures. The total integrated fluxes of 12 CO line emission for the 13 CO-detects are about 93% of that for the whole sample of molecular clouds.
2. In the 2851 13 CO-detects, we find the 13 CO structures' area in a 12 CO cloud generally does not exceed 70% of the 12 CO emission area, independently of the 12 CO emission area, and its interior integrated fluxes of 13 CO emission are usually less than 20% of those of its 12 CO emission.
3. In the 2851 13 CO-detects, we find a strong correlation between the velocity-integrated intensities of 12 CO lines and those of 13 CO lines emission in the same areas where both the 12 CO and 13 CO emissions are detected.
4. In the 2851 13 CO-detects, we find that there are ∼ 60% of 12 CO clouds have one individual 13 CO structure, about 15% of 12 CO clouds have two separate 13 CO structures, and the rest of them have more than two separate 13 CO structures.
5. We link the 13 CO gas fractions in the 13 CO-detects with their morphologies, i.e., filaments or nonfilaments, and find that the 13 CO line emissions are primarily detected in the 12 CO clouds classified as filaments. In addition, a filament tends to have more than one individual 13 CO structure in its interior.
We gratefully thank the anonymous referee for the constructive comments that helped improve the quality of this paper. This research made use of the data from the Milky Way Imaging Scroll Painting (MWISP) project, which is a multi-line survey in 12 CO/ 13 CO/C 18 O along the northern galactic plane with PMO-13.7m telescope. We are grateful to all of the members of the MWISP working group, particulaly to the staff members at the PMO-13.7m telescope, for their long-term support. This work was supported by the National Natural Science The DBSCAN algorithm extracts the consecutive structures in the PPV space of CO lines data, based on a line intensity threshold and two parameters, i.e. and MinPts. Two parameters of and MinPts define the connectivity of structures in the PPV space. Each point within the extracted consecutive structure is called a core point. For a core point, its adjacent points contained in its neighborhood has to exceed a threshold. The parameter of MinPts determines the threshold of the number of adjacent points and the represents the radius of the neighborhood. A border point in the consecutive structure is defined as a point inside the -neighborhood of a core point, but not necessarily contain the MinPts neighbors, as shown in Figure 2 of (Ester et al. 1996). In the PPV space of CO data, Yan et al. (2020) has examined all the choices of parameters and 12 CO line intensities cutoffs to identify molecular clouds. The parameters of cutoff = 2σ, minPts = 4, = 1 are used in the DBSCAN algorithm to extract the 12 CO molecular clouds in the 12 CO data cube, as well as the 13 CO molecular structures within the 12 CO molecular cloud. The post-selection criteria are examined and utilized to avoid the noise contamination (Yan et al. 2020). These criteria for a extracted structure include: (1) the minimum voxel number is 16; (2) the peak intensity is larger than the value of cutoff + 3σ for 12 CO or cutoff + 2σ for 13 CO; (3) the angular area is large than one beam size (2×2 pixels); (4) the number of velocity channels are larger than 3.  Figure C5. The velocity-integrated intensity map (upper) and latitude-integrated intensity map (lower) of 13 CO emission for the molecular cloud G139.73, which are derivied by the raw chopped 13 CO data cube without any clipping.  Figure C6. Upper panels: the mean spectrum of 12 CO (black) and 13 CO (magenta) spectral lines along the boundaries of 13 CO structures extracted in the molecular cloud G139.73 by the clipping at the cutoff level of 2σ, 3σ, 4σ, and 5σ, respectively. The 13 CO spectral lines are from the raw chopped 13 CO data cube without any clipping, as shown in Figure C5. The 12 CO spectral lines are from the extracted 12 CO cloud of G139.73. The noted Num in each panel is the number of the spectrum along the corresponding boundary. The S/N is the ratio of the peak intensity to the noise RMS for the averaged-boundary 13 CO spectrum. Lower panels: the mean spectrum of the extracted 12 CO cloud (blue) in Yan et al. (2021) and the mean spectrum of the 13 CO structures (red) extracted by the clipping at the cutoff level of 2σ, 3σ, 4σ, and 5σ. The vertical dashed lines illustrate the velocity span for the extracted 13 CO structures within G139.73 by the clipping at the corresponding cutoff levels of 2σ, 3σ, 4σ, and 5σ, respectively. All the 13 CO spectra are multiplied by a factor of 3. The σ value is estimated from the raw data and equal to 0.27 K  Figure C12. Same as Figure C6, but for the 13 CO structures identified by the moment mask at the cutoff level of 8σsm, 9σsm, 10σsm and 11σsm. The noise σsm is calculated from the smoothed data. Its value is 0.05 K.  Figure C14. Same as Figure C7, but using the moment mask at the cutoff level of 9σsm (0.45 K). Figure D15. 13 CO structures identified by the DBSCAN, but not by the moment mask. The colormaps represent the distributions of the velocity-integrated intensities of 13 CO line emission. The black contours indicate the boundaries and the level of 50% at the 12 CO line velocity-integrated intensities. Figure D16. 13 CO structures identified by the DBSCAN. The colormaps represent the distributions of the velocity-integrated intensities of 13 CO line emission. The black contours indicate the boundaries and the level of 50% at the 12 CO line velocityintegrated intensities.