The Milky Way Imaging Scroll Painting (MWISP): Project Details and Initial Results from the Galactic Longitude of 25.8deg to 49.7deg
Yang Su, Ji Yang, Shaobo Zhang, Yan Gong, Hongchi Wang, Xin Zhou, Min Wang, Zhiwei Chen, Yan Sun, Xuepeng Chen, Ye Xu, Zhibo Jiang
aa r X i v : . [ a s t r o - ph . GA ] J a n Draft version January 3, 2019
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THE MILKY WAY IMAGING SCROLL PAINTING (MWISP): PROJECTDETAILS AND INITIAL RESULTS FROM THE GALACTIC LONGITUDE OF+25 . ◦ . ◦ Yang Su, Ji Yang, Shaobo Zhang, Yan Gong,
1, 2
Hongchi Wang, Xin Zhou, Min Wang, Zhiwei Chen, Yan Sun, Xuepeng Chen, Ye Xu, and Zhibo Jiang Purple Mountain Observatory and Key Laboratory of Radio Astronomy, Chinese Academy ofSciences, Nanjing 210034, China Max-Planck Institute f¨ur Radioastronomy, Auf dem H¨ugel 69, 53121 Bonn, Germany
ABSTRACTThe Milky Way Imaging Scroll Painting (MWISP) project is an unbiased Galacticplane CO survey for mapping regions of l = − ◦ to +250 ◦ and | b | < ∼ . ◦ CO, CO, and C O ( J =1–0) lines simultaneously with full-samplingusing the nine-beam Superconducting SpectroScopic Array Receiver (SSAR) systemwith an instantaneous bandwidth of 1 GHz. In this paper, the completed 250 deg data from l = +25 . ◦ . ◦ ′′ and a typicalrms noise level of ∼ CO at the channel width of 0.16 km s − and ∼ CO and C O at 0.17 km s − . The high-quality data with moderate resolution( ∼ ′′ ), uniform sensitivity, and high spatial dynamic range, allow us to investigatethe details of molecular clouds (MCs) traced by the three CO isotope lines. Threeinteresting examples are briefly investigated, including distant Galactic spiral armstraced by CO emission with V LSR < − , the bubble-like dense gas structure nearthe H ii region W40, and the MCs distribution perpendicular to the Galactic plane. Keywords:
Galaxy: structure – ISM: clouds – ISM: molecules – radiolines: ISM – stars: formation – surveys
Corresponding author: Yang [email protected] INTRODUCTIONMolecular gas plays a crucial role in star formation. Molecular hydrogen (H ) is thedominant component of molecular clouds (MCs) in the interstellar medium (ISM).Unfortunately, H radiates inefficiently in the cold, dense molecular ISM due to alack of a permanent dipole moment and corresponding dipolar rotational transitionsin the radio band (e.g., Bolatto et al. 2013). Carbon monoxide (CO), which is thenext most abundant molecule in the ISM, is widely used to trace the molecular gasbecause CO emission is easily excited in the molecular ISM environment and the CO J =1–0 transition at 2.6 mm (or 115 GHz) is readily observed on the ground (e.g.,Combes 1991; Dame et al. 2001; Heyer & Dame 2015 and references therein).Systematic CO surveys of the Milky Way are helpful for improving our knowledgeof the molecular ISM, the physics of star formation, and the Galactic structures. Sofar, many Galactic CO surveys have been done using single-dish telescopes, such asthe FCRAO CO ( J =1–0) survey of the Outer Galaxy (Heyer et al. 1998), the CfA1.2 m complete CO ( J =1–0) survey (see Figure 1 and Table 1 in Dame et al. 2001),the Bell Laboratories 7 m CO ( J =1–0) survey (Lee et al. 2001), the NANTEN4 m CO ( J =1–0) survey (Mizuno & Fukui 2004), the FCRAO 14 m CO ( J =1–0) Galactic Ring Survey (GRS, Jackson et al. 2006), the Mopra 22 m CO, CO,C O ( J =1–0) survey (Burton et al. 2013), and the Three-mm Ultimate Mopra MilkyWay Survey (ThrUMMS) for CO, CO, C O, and CN ( J =1–0) lines (Barnes et al.2015).Detailed information for large CO surveys was summarized in Figures 1–2 inHeyer & Dame (2015), in which the authors enumerate the major surveys of COand CO ( J =1–0) emission along the Galactic plane from 1970–2015. The FOR-EST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope(FUGIN; Umemoto et al. 2017) plans to conduct the simultaneous CO, CO, andC O ( J =1–0) observations toward the Galactic plane. This survey will achieve ratherhigh angular resolution ( ∼ ′′ ) data of CO ( J =1–0) lines for the Galactic plane.The CO ( J =3–2) High-Resolution Survey of the Galactic plane (COHRS,Dempsey et al. 2013) and the CO / C O ( J =3–2) Heterodyne Inner Milky WayPlane Survey (CHIMPS, Rigby et al. 2016) were made using the 15 m James ClerkMaxwell Telescope (JCMT) in the submillimeter wavelengths, with an angular reso-lution of ∼ ′′ for COHRS and ∼ ′′ for CHIMPS, respectively. The SEDIGISMsurvey (Structure, Excitation, and Dynamics of the Inner Galactic ISM, Schuller et al.2017) covers 78 deg in the fourth quadrant using the CO and C O ( J =2–1) linesat the 1 mm band. All of these CO spectral line surveys, together with other Galacticmultiwavelength legacy data, will lead to significant advances in our understandingof the ISM physics of our Galaxy. WISP CO survey between 25 . ◦ . ◦ ) project is an ongoing northernGalactic plane CO survey using the 13.7 m millimeter-wavelength telescope at Del-ingha, China (hereafter DLH telescope). The MWISP project is led by the PurpleMountain Observatory (PMO), with the full support from the staff members at Del-ingha. The survey observes CO, CO, and C O ( J =1–0) lines simultaneously forregions of l = − ◦ to +250 ◦ and | b | < ∼ . ◦ l =+25 . ◦ . ◦ | b | < ∼ . ◦
2. Section 2 mainly describes the DLH telescope, theobserving strategy and data reduction, and the details of the MWISP project. Section3 displays the initial results for the large-scale CO maps along the Galactic plane witha large coverage in latitude. Three interesting examples, including the most distantCO arm, the bubble-like C O structure near the H ii region W40, and the verticaldistribution of CO gas, are further investigated in that section. Finally, Section 4gives a summary and future prospects. THE MWISP CO SURVEY2.1.
Telescope and Multibeam Receiver System
The DLH telescope is situated at approximately (37 ◦ . ′ ◦ . ′ ≈ µ m. The tracking accuracy of the telescope isapproximately 1 ′′ –3 ′′ in both azimuth and elevation. The pointing accuracy for thewhole sky is better than 5 ′′ , about one-tenth of the telescope half-power beam width(HPBW) at 115 GHz ( ∼ ′′ ).A 3 × × ′′ (see Figure 16 in Shan et al. 2012), which are also measured before every observa-tional season. The SSAR system employs two-sideband-separating superconductor-insulator-supercoductor mixers with a typical single sideband noise temperature of60 K and image rejection ratio above 10 dB over the frequency range of 85–115 GHz.The instrument includes Fast Fourier Transform Spectrometers (FFTSs), digital LocalOscillator (LO) sources, digital bias power supplies, and an independent intermediatefrequency (IF) module.The IF band is 2.64 ± CO, CO, and C O ( J =1–0) lines at115.271 GHz, 110.201 GHz, and 109.782 GHz, respectively, are all within the 1 GHzband when the LO is set at 112.6 GHz. Eighteen high-resolution FFTS digital spec- http://english.dlh.pmo.cas.cn/ic/ trum analyzers work at 1 GHz or 200 MHz bandwidth with 16,384 channels. The1 GHz bandwidth covers a wide velocity range for the CO line survey, while the200 MHz bandwidth provides higher spectral resolution observations for further study(e.g., see Table 1 in Gong et al. 2018).Typical system temperatures, which include noises from the receiver, the antenna(the optical system, the dome, and the membrane), and the atmosphere, are ∼
250 Kfor CO at the upper sideband and ∼
140 K for CO and C Observations and Data Reduction
The sky coverage of the MWISP project is divided into 10,941 cells for the region of l = − ◦ to +250 ◦ and | b | < ∼ . ◦
2. Each cell with 30 ′ × ′ is scanned along the Galacticlongitude ( l ) and the Galactic latitude ( b ) at least twice to reduce the fluctuation ofnoise. We chose 30 ′ × ′ as the cell’s size based on three factors. One is that the cellcan be completed in time with the suitable scanning parameters. This considerationis important to ensure the atmospheric stability during the 1–1.5 hr observation fora full mapping (see below). The other factor is to limit the final data size of thecompleted cell for one line, e.g., ∼ ×
91 pixels × × ′′ (or 75 ′′ ) per second, with a dump time of 0.3s (or 0.2 s), depending on a cell’s elevations, off-positions, and weather conditions.The sampling interval is 15 ′′ (= scan rate × dump time) and the spacing between scanrows is 10 ′′ , fulfilling the oversampling of the 50 ′′ beam of the 13.7 m telescope. A cellthus can be fully mapped in one half an hour (or an hour) along l or b . Generally,we used the (scan rate= 50 ′′ s − , dump time=0.3 s) mode in observations. Mappingtwice (or three hours) can well satisfy the expected rms noise level. The (scan rate=75 ′′ s − , dump time=0.2 s) mode is often used for some special regions such as, forexample, the low elevation cells toward l < ◦ − ◦ (e.g., near the direction of theGalactic center), or some cells with far off positions (e.g., the Aquila Rift region andthe Cygnus region). Sometimes, scanning twice (e.g., three hours) cannot reach theexpected rms noise level, so further mapping with the (scan rate= 75 ′′ s − , dumptime=0.2 s) mode is also used to reduce the rms noise for the observed cells.The most important consideration is to maintain uniform sensitivity, which essen-tially depends on the weather or the total system temperature. During observations,standard sources are thus observed about every two hours in the position-switch mode.The spectral profiles and intensities of standard sources are monitored daily to checkthe stability of the observations. WISP CO survey between 25 . ◦ . ◦ ∼ ′ × ′ (i.e., the sky coverageof the 3 × ≤ ◦ ) is carefully checked to ensurefree of emission for the CO line. However, the CO emission in the range of 0–40 km s − is widely distributed toward the inner Galaxy, especially for the centralmolecular zone, the Aquila Rift, and the Cygnus region. When we observe theseregions, some off-positions with a little of CO emission ( T MB . CO lines may show some weakabsorption features at certain local standard of rest (LSR) velocities. The relevantinformation was recorded for all observed cells.The total bandwidth of 1 GHz, with 16,384 channels, provides a channel frequencyinterval of 61 kHz, resulting in a velocity separation of about 0.16 km s − for COand 0.17 km s − for CO and C O. A first-order (or linear) baseline was fitted forthe CO spectra. Most of the bad channels in the spectra were removed. The antennatemperature, T ⋆ A , was converted to the main beam brightness temperature, T MB , withthe formula of T MB = T ⋆ A / ( f b × η MB ). The beam-filling factor of f b is assumed to be1 for the extended CO emission and the main beam efficiency η MB varied between40% and 50% in the past seven seasons. All intensities shown in this paper are onthe T MB scale.Due to the field rotation, a larger cell size of 45 . ′ × . ′ ± BB.B, all CO spectra were summed together with the rms as the weight.Finally, the three-dimensional (3D) FITS data cubes of each cell were made with agrid spacing of 30 ′′ for CO, CO, and C O ( J =1–0) lines. All data were reducedusing the GILDAS software (Pety 2005).2.3. Characteristics and Advantages of the MWISP
The MWISP project is a large-scale, unbiased, and high-sensitivity triple CO isotopeline survey for the northern Galactic plane (e.g., l = − ◦ to +250 ◦ and | b | < ∼ . ◦ l and b ). That is, theMWISP project can map about 240–250 square degrees of the Galactic plane peryear. The MWISP survey started in 2011 November and is expected to be completedin 2022.The characteristics and advantages of the MWISP CO survey are summarized asfollows:
1. Large-scale CO mapping with a high spatial dynamic range. The MWISP projectwill provide us with large-scale CO maps of ∼ , with a moderate angularresolution of ∼ ′′ . The final full-sampling 3D CO datasets have a grid spacingof 30 ′′ . Researchers thus can investigate the detailed structures of local MCs (e.g.,1 pixel < ∼ < ∼ ∼ > ∼
10 kpc).2. Unbiased CO survey with high sensitivity. The unbiased survey with high sensi-tivity, wide velocity coverage, and high velocity resolution has a uniform sensitivity,providing us a clearer picture of the molecular gas distribution and properties ofthe Milky Way. Through systematic studies of the CO emission of the molecular gas,many new MCs will be revealed due to the high-quality data of the new survey, whichhave an rms sensitivity of ∼ . .
3) K, a velocity coverage of ∼ − , anda velocity resolution of ∼ .
16 (0 .
17) km s − for CO ( CO and C O) lines.3. Simultaneous observations of CO, CO, and C O ( J =1–0) transitions. The CO emission tracing the total molecular gas can reveal structures and distributionsof the diffuse gas with a typical density of 10 cm − . The optically thinner COand C O lines trace denser molecular gas with a typical density of 10 –10 cm − because of their less abundance with respect to H and therefore less optical deptheffects. Therefore, the MWISP survey is adequate for revealing the diffuse gas of MCenvelopes, denser molecular gas of giant molecular clouds (GMCs), and the changesin abundance ratio between CO and C O due to isotope-selective photodestructionof the rarer CO species.With a spatial resolution of ∼ ′′ and a grid spacing of 30 ′′ , the full-samplingMWISP survey provides a rich CO data set for b > ◦ regions, which are less coveredby other CO surveys, excluding the CfA 1.2 m complete CO ( J =1–0) survey with abeam size of ∼ ′ (Dame et al. 2001). Figure 1 gives a comparison between the CfA1.2 m CO map and our MWISP map for the Serpens / Aquila Rift MC complex. Notethat the MWISP integrated map reveals more detailed structures than that of the1.2 m CO data. Furthermore, the MWISP survey has a wider velocity coverage thanthat of the previous CO survey (e.g., − − for the GRS CO survey,Jackson et al. 2006), leading to more completed velocity coverage for MCs in the firstquadrant of the Milky Way (e.g., the most distant MCs beyond the solar circle).The ongoing FUGIN survey plans to investigate the distribution and propertiesof molecular gas in the Galaxy with the CO, CO, and C O ( J =1–0) lines(Umemoto et al. 2017). The region of l = +10 ◦ to +50 ◦ and b = +1 ◦ to +1 ◦ isfully covered using the multibeam (2 × versus 160 deg ) and the highervelocity resolution (0 .
16 km s − versus 0 .
65 km s − ). Meanwhile, the sensitivity of theMWISP survey is better than that of the FUGIN project (e.g., T rms ( T MB ) ∼ WISP CO survey between 25 . ◦ . ◦ CO and ∼ CO / C O at 8 . ′′ × . ′′ × . − in the first quadrantregions, Umemoto et al. 2017). On the other hand, the spatial resolution of theNobeyama 45 m telescope is about 2.5 times higher than that of the DLH 13.7 mtelescope. Therefore, the FUGIN data with the final grid spacing of 8 . ′′ | b | < ◦ .In summary, the MWISP CO survey gives us a good opportunity to study the Galac-tic structures, the MC properties and the star formation, and the associations betweenthe molecular gas and the extended radio sources such as, for example, H ii THE SURVEY DATAIn this section, we present the CO data for the completed region of l = +25 . ◦ . ◦ | b | < ∼ . ◦
2. Figure 2 shows the rms distribution of the ∼
250 square degreedataset. The typical rms noise level of the spectra is ∼ CO ( J =1–0) at achannel width of 0.16 km s − and ∼ CO ( J =1–0) and C O ( J =1–0) at0.17 km s − , with a spatial resolution of ∼ ′′ . Generally, the rms noise of each cellis uniform for the three CO isotope lines in the 30 ′ × ′ region. As shown in Figure 3,however, the variation in rms indeed exists between cells, e.g., ∼ COand ∼ CO/C O. Some of bright stripes with somewhat larger rmsnoises (e.g., ∼ CO and ∼ CO/C O) also can be seenalong l and b , with lengths of several tens of arcminutes. These features are due tothe bad weather (or the large system temperature) in the OTF scanning. Table 1lists the parameters of the CO data used in this paper.The distributions of the CO emission are shown through intensity maps, intensity-weighted mean velocity maps, and position − velocity (PV) diagrams. Some interestingresults of the new CO survey are also investigated. The CO data are smoothed to the0.5 km s − velocity resolution in order to improve the sensitivity for weak emission.The improved rms levels of the analyzed data are of ∼ CO line and ∼ CO and C O lines, respectively.3.1.
CO gas with V LSR < − Very recently, Su et al. (2016) and Sun et al. (2017) presented the results of distantMCs traced by CO emission between l =34 . ◦
75 and l =45 . ◦
25. These MCs withnegative velocities are divided into the distant Outer Arm and the Extreme OuterGalaxy (EOG), respectively. These two parts of the molecular gas are very likely theextension of the Norma–Cygnus Arm and the Scutum–Centaurus Arm in the firstquadrant (i.e., molecular gas structures from the fourth quadrant of the inner Galaxyto the first quadrant of the outer Galaxy). Actually, the more negative velocityfeature at V LSR < − was proposed to be from the Outer Scutum-CentaurusArm (Dame & Thaddeus 2011).With the progress of the MWISP project, larger mapping was completed for therange of +25 . ◦ < ∼ l < ∼ +49 . ◦ | b | < ∼ . ◦
2. Figure 4 shows the spatial distribution ofthe CO gas with V LSR < − , together with the corresponding LSR velocityinformation. A large amount of new CO emission is revealed because of the highsensitivity and the large coverage of the unbiased survey. Some of the MCs withrelatively strong CO emission have the corresponding CO emission, but none ofthem show significant C O emission under the improved rms level of ∼ − . Higher resolution and sensitivityobservations are expected to detect the weak emission of the dense gas far away fromus.Some of MCs with V LSR < ∼ − are likely the local molecular gas due to their ∼ − LSR velocities and relatively high | b | values (e.g., | b | > ∼ . ◦
5; see theAquila Rift region near l ∼ ◦ − ◦ in the lower panel of Figure 4). However,most of V LSR < − MCs in Figure 4 are believed to lie beyond the solar circle.Obviously, CO emission is mainly concentrated near the Galactic plane, excludingthe possible local gas in the Aquila Rift region at high | b | . In spite of this, the distantCO gas seems to also be slightly displaced from b = 0 ◦ , which was explained by thewarped plane at larger Galactocentric distances and the apparent tilted structurecaused by the Sun’s z -height above the physical midplane of the Galactic disk (seeSu et al. 2016; Sun et al. 2017).Due to the large distances, many MCs have small angular sizes and weak CO emis-sion. These MCs are relatively isolated in l - b - v space. Despite these properties, someinteresting distant MCs with brighter CO emission also display extended concentra-tions, which usually have special morphologies such as filaments, arcs/shells, and WISP CO survey between 25 . ◦ . ◦ CO gas with V LSR > − Figure 5 shows the CO ( J =1–0) intensity map in the 0–130 km s − interval,overlaid with radio continuum contours from the Effelsberg 11 cm survey (Reich et al.1990). Some bright and/or extended radio sources discussed below are labeled on theguide map.Figures 6 and 7 show CO and CO channel maps, respectively. The channelmaps are made by integrating emission over a 10 km s − velocity bin from 0 km s − to 120 km s − . The 120–130 km s − CO and CO gas, which displays emissiononly near l ∼ ◦ − ◦ , is presented in Figure 8. A large number of MC structuresand features are seen in both of the CO and CO channel maps.For the 0–20 km s − maps, the most prominent features are the Aquila Rift (e.g.,see Figure 3 in Dame & Thaddeus 1985), which displays large-scale, diffuse, andenhanced CO emission in the field of view (FOV). The MWISP survey, with the widespatial dynamic range, reveals lots of detailed structures for the large-scale moleculargas not far from us (i.e., d < ∼
450 pc; Ortiz-Le´on et al. 2017). Large-scale extendedstructures are also discernible from the PV diagrams of CO and CO emission(see the region of V LSR ∼ − and l ∼ ◦ –40 ◦ in Figure 9). The CO J =2–1 and CO J =2–1 emission from the Aquila Rift region was investigated byNakamura et al. (2017) using the 1.85 m telescope. Combinations of our CO J =1–0data and the 1.85 m CO J =2–1 data will help us understand the detailed moleculargas properties of such local regions.Many of bright CO concentrations in the velocity interval of ∼ − arefound to be near the Galactic plane of b ∼ − . ◦ . ◦
6, excluding the diffuse andextended emission from the local MCs (e.g., the Aquila Rift region discussed above).These concentrations, which have typical angular sizes of several arcminutes, are likelyrelated to the distant Perseus Arm in the first quadrant of the Galaxy.For 20–40 km s − gas, the CO and CO emission is found to be extended over alarge region from l ∼ ◦ − ◦ and | b | < ∼ ◦ in the channel maps. This GMC complexis less studied in the literature. A 170 pc long giant molecular filament (GMF) ofG40.82 − ∼ b > ∼ ◦ of the GMC is related to the H ii regions Sh 2-75 (at l =40 . ◦ b =1 . ◦
50) and Sh 2-76 (at l =40 . ◦ b =2 . ◦ ii regions, Sh 2-76,is exactly located at a distance of 1.92 +0 . − . kpc from the parallax measurements of0.521 ± .
024 mas (Chibueze et al. 2017).At 30–60 km s − , significant CO emission appears at l ∼ ◦ –36 ◦ and b ∼ − ◦ to+1 ◦ , which is roughly perpendicular to the Galactic plane. Enhanced CO emission inthe GMC complex displays complicated morphologies, such as multi-shells, bubbles,and cometary bright-rimmed structures. The GMC complex is associated with thehigh-mass star-forming region W48 (G35.2 − +0 . − . kpc(Zhang et al. 2009). The very bright SNR G34.7 − ∼ V LSR > ∼
50 km s − molecular gas, CO emission is mainly confined within | b | < ∼ ◦ . In addition to the CO emission close to the Galactic plane, considerableMCs are detected in 1 ◦ < ∼ | b | < ∼ ◦ , even for velocities near the tangent point (see 80–120 km s − maps in Figures 6 and 7). These features have not been revealed byprevious CO surveys (e.g., FCRAO GRS project, Jackson et al. 2006) due to the lim-ited Galactic latitude coverage of | b | < ∼ ◦ toward the first quadrant of the Milky Way.From Figure 9, we find that the PV diagram of the MWISP CO data is consistentwith the previous PV map of the GRS CO survey (see Figure 3 in Jackson et al.2006). Furthermore, some new features in the CO PV diagram are unveiled for the V LSR < ∼
50 km s − range (e.g., see structures at l ∼ ◦ –42 ◦ and V LSR ∼ − ,the rectangle in the diagram) because of the larger latitude coverage of the MWISPsurvey. The CO emission of the structure is from the molecular gas associated withH ii regions Sh 2-75 and Sh 2-76.Figure 10 displays the spatial and velocity distribution of the C O ( J =1–0) emissionin the velocity range of 0–40 km s − . Enhanced C O emission is clearly seen near l ∼ . ◦ b ∼ . ◦
7, which is probably associated with the feedback of massivestars in the Serpens south and the W40 complex (see Section 3.3.2). Excluding C Oemission from the local MCs such as the Aquila Rift, the rest of C O gas is mainlyconfined in b ∼ − ◦ to b ∼ ◦ . In the figure, two other prominent C O features,which are located at l ∼ ◦ and l ∼ ◦ , are found to be away from the b = 0 ◦ plane.The two C O features are related to the massive star-forming region W48 and H ii regions Sh 2-75 and Sh 2-76, respectively.Figure 11 shows the C O emission in the velocity interval of 40–80 km s − and80–120 km s − , which is mainly within | b | < ∼ ◦ and | b | < ∼ ◦ , respectively. For the 40–80 km s − maps, the W48 GMC complex at l ∼ ◦ , which is roughly perpendicularto the b = 0 ◦ plane, is clearly seen in Figures 11a and 11b. On the other hand, theW51 GMC complex at a distance of 5.41 +0 . − . kpc (e.g., the trigonometric parallaxfrom Sato et al. 2010) also can be seen at l ∼ ◦ . The massive star-forming regionW51 was recently reviewed by Ginsburg (2017, and references therein). WISP CO survey between 25 . ◦ . ◦ V LSR =80–120 km s − maps (see Figures 11c,d), the most enhanced C Oemission with ∼
10 K km s − is associated with the mini-starburst region W43 at( l ∼ . ◦ b ∼ . ◦ +0 . − . kpc (Zhang et al. 2014a), is probably at or close to the near end of theGalactic long bar. The ongoing star-forming activity of the global mini-starburstregion is likely the result of the massive gas clouds accumulated by the bar potentialsand kinematics. The gas property of the interesting GMC complex W43 was studiedby many groups (e.g., Nguyen Luong et al. 2011; Carlhoff et al. 2013; Motte et al.2014; Sofue et al. 2018).For the survey data presented here, the C O emission is relatively discrete in com-parison with the extended CO and CO emission. Researchers thus can identifythe main structures of MCs using C O emission due to the less velocity crowdingand line blending (e.g., see the W40 region in Section 3.3.2).As shown in Figure 11, the distribution of the C O emission is below the planeof b = 0 ◦ , especially for the 40–80 km s − MCs. Near the tangent point, where thedense gas is more concentrated in the plane, the dominant part of them is also belowthe plane (e.g., the 80–120 km s − maps). This interesting feature is explained by theSun’s offset above the physical midplane of the Milky Way (e.g., z Sun ∼ V LSR >
60 km s − (Section 3.3.3). 3.3. Interesting Examples
CO gas at the Edge of the Milky Way
The CO emission of the distant molecular gas is very weak. Due to the warping andflaring of the outer gas disk, the distant MCs with negative velocities are concentratedat the somewhat higher latitudes, which is different from the molecular gas in thefirst quadrant of the inner Galaxy. The unbiased MWISP CO survey has a widevelocity and areal coverage, meanwhile there is also a high sensitivity, allowing us tosystematically investigate weak CO emission far away from the Sun (see Figure 4).Compared with the CfA 1.2 m CO survey (Dame et al. 2001), the MWISP CO datafrom the 13.7 m telescope obviously reveal detailed distributions and structures ofthe distant MCs.Figure 12 shows the longitude–velocity ( l – v ) diagram of the CO emission forthe V LSR < ∼ − molecular gas. Note that the CO intensity is multiplied by afactor of 100 for the corresponding signal velocity range but not the whole velocityrange. The noise is therefore suppressed and the tiny features of weak CO emissionare enhanced in the l – v diagram. Based on the diagram, we find that the brightemission (thick blue parts in the figure) exhibits a large-scale molecular gas structureoutside the solar circle. On a large scale, the structure shows a velocity gradient of ∼ − degree − and has relatively enhanced CO emission, which is likely2from MCs within the Outer Arm (or the Norma–Cygnus Arm) in the first quadrant.Moreover, the large-scale structure is comprised of several interesting substructuresin the l – v space, which themselves are worthy of further investigation.In addition to the enhanced CO emission, considerable molecular gas with weakerCO emission at more negative LSR velocities is also discerned from Figure 12 (see theregions between the red lines, i.e., V LSR = − . × l + 4 . ±
12 km s − ). The mostnegative velocity is at ∼ −
75 km s − with low surface brightness. These MCs areprobably in a distant section of the Scutum–Centaurus Arm (designated as the OSCby Dame & Thaddeus 2011), which is referred to as the EOG region by Sun et al.(2017). The trend of the molecular gas in the l – v space is approximately describedby a linear fit of V LSR = − . × l + 4 .
34 km s − , which is in good agreement withprevious studies (see Dame & Thaddeus 2011; Sun et al. 2017). The velocity gradientof ∼ . − degree − for the EOG gas is smaller than that of the molecular gasof the Outer Arm in the l – v space (e.g., ∼ . − degree − in Su et al. 2016).The uncertainty of the tentative fit is largely due to the limited longitude coverage.On the other hand, the OSC Arm may consist of several broken lines with somewhatdifferent slopes in the l – v space. Further MWISP data from l = 0 ◦ to 26 ◦ and l = 50 ◦ to 100 ◦ can give us a firmer conclusion (e.g., see Figure 3 in Sun et al. 2015).Figure 13 displays the distribution of the EOG CO emission. Samples in the figureare selected from the MCs with V LSR < ∼ − . × l +4 .
34 km s − , which reduces possiblecontamination from the Outer Arm gas. We find that these most distant MCs arequite sparse in the l − b space and indeed located above the b = 0 ◦ plane. For regionsof l ∼ ◦ − ◦ , the lack of the EOG CO emission in the map indicates that the slopeof − .
57 in our fitting is probably too steep for the distant MCs at larger longitudes(e.g., a slightly flat slope of > ∼ − . l > ∼ ◦ CO gas in the l – v space). Twointensity peaks are located at b ∼ . ◦ b ∼ . ◦
8, or ∼
370 pc and ∼
870 pc above the b = 0 ◦ plane, assuming a median heliocentric distance of 17.8 kpc (Sun et al. 2017).These values are several times larger than that of the Outer Arm gas (e.g., 0 . ◦
42 or ∼
110 pc at a heliocentric distance of 15 kpc; Su et al. 2016), indicating the differentdistributions of the two MC groups for the distant V LSR < − gas. Indeed, themore negative the value of the LSR velocity is, the higher the Galactic latitude of thedistant molecular gas.3.3.2. Bubble-like C O Structure near the H ii region W40 At similar distances, there may be multiple MCs with different velocity fields in theISM, leading to complicated molecular gas structures. Simultaneous CO, CO, andC O ( J =1–0) line observations can provide a more complete picture of the moleculargas properties, such as structures, kinematics, and dynamics. Star formation is asso-ciated with molecular gas. The MWISP survey is helpful for studying the relationshipbetween MCs and star-forming activity in the Milky Way.An intriguing case is the W40 region, which is part of the Serpens / Aquila Rift MCcomplex. The region is located at a distance of 436.0 ± . WISP CO survey between 25 . ◦ . ◦ CO, CO, andC O ( J =2–1) lines with the 1.85 m telescope. The gas toward W40 within anarea of ∼ O intensity map (e.g., see theupper right corner of Figure 10).Figure 14 displays the close-up view toward the W40 region, where the blue, green,and red colors represent the CO, CO, and C O emission, respectively. The cir-cular morphology of the dense gas in the three-color image is centered at ( l =29 . ◦ b =3 . ◦ . ◦
45. Obviously, the emission of the dense gas traced by CO and C O lines is mainly concentrated in the western half of the bubble-likestructure, while the C O structure in the eastern half is broken. The LSR velocity ofC O emission peaks at V LSR ∼ − on the bubble-like structure, where boththe CO and CO lines show the striking self-absorption features (see the typicalspectra to the right of the figure).The strongest CO emission of T MB ∼
37 K at V LSR ∼ − is located at( l =28 . ◦ b =3 . ◦ ii region W40 (see thepurple circle in Figure 14). The typical peak temperature of the optically thick COline is nearly 10 K in some bright regions, while considerable CO emission has alower peak temperature of ∼ − and 10–20 km s − maps in Figure 6).Figure 15 displays the gas distribution in velocity intervals of 3–5, 5–7, 7–9, and 9–11 km s − toward the complex. For the 3–5 km s − gas, the very strong CO emissionis found to surround the W40 H ii region (the 9 ′ purple circle in the map), while COemission seems to be lacking toward the center of the H ii region. The enhanced densegas, which is represented by the bright CO emission, exhibits a bubble-like structurein the 5–7 and 7–9 km s − maps. We use a large circle with a diameter of 0 . ◦ O bubble-like structure (i.e., near l ∼ . ◦ b ∼ . ◦ CO and CO emis-sion are found to point to the W40 H ii region (see the V LSR ∼ − mapin Figure 15). Moreover, the CO data reveal many interesting features, includingrim-bright MCs, molecular gas bubbles / cavities, and shells / arcs in the complicatedregion. These features may be related to the strong feedback from the H ii regionW40.Figure 16 exhibits the velocity distribution of the C O gas toward the W40 region.Three main velocity components are recognized, such as the partial shell structuresurrounding W40 at V LSR ∼ − (thick blue), the western half bubble-likestructure at V LSR ∼ − (thin blue to green), and the eastern broken bubble-4like structure at V LSR ∼ − (red). Some line-broadening features of CO gas(e.g., V LSR < ∼ − and V LSR ∼
12 km s − ) are also revealed in the whole region,indicating the potential outflow candidates. Detailed analysis of the nearby regionwill be helpful for understanding the relationship between the molecular gas and theongoing star formation.3.3.3. CO gas distribution perpendicular to the Galactic plane
Dame & Thaddeus (1994) suggested that a thick molecular disk traced by faint COemission is well above the central thin CO disk in the inner Galaxy. Their resultsare based on CO observations of the 1.2 m telescope for narrow regions of Galacticlongitudes l = 30 ◦ , ◦ , and 50 ◦ and Galactic latitudes | b | < ◦ . Benefiting from theunbiased, high-sensitivity, and large-area MWISP CO survey, we have a pretty goodchance of studying the second, faint, and thick disk component of the molecular gastoward the inner Galaxy.Channel maps of CO and CO (Figures 6 and 7) show that there are many MCsnear the terminal velocity extending up to ∼ ◦ –3 ◦ from the plane. Some MCs haveextremely b values of > ◦ . These MCs at | b | > ∼ ◦ and V LSR ≥
60 km s − are relativelyisolated with respect to the molecular gas near the Galactic plane. The MCs withweak CO emission, which are often associated with the enhanced H i emission (e.g.,to compare with the GALFA data, Peek et al. 2011), have little velocity crowding atregions of relatively high b .Peak CO velocities of V LSR ≥
60 km s − molecular gas are extracted pixel bypixel for the whole region of +25 . ◦ < ∼ l < ∼ +49 . ◦ | b | < ∼ . ◦
2. The pixels with threeconsecutive channels ≥ × rms (or ∼ − ) would be considered to be the valid signal. The near kinematic distancesof the CO signal are calculated assuming the flat Galactic rotation curve model (e.g.,Reid et al. 2014). Then, the distances from the b = 0 ◦ plane (i.e., z ) are estimated foreach pixel. To reduce sample fluctuations, z values are combined every 10 pc per binfrom −
500 pc to +400 pc. CO intensities are averaged for all valid CO signal per bin.The error bars are estimated from the express of I mean (CO)/(number of pixels perbin) . . As a consequence, the error bars of the high- z samples are obviously largerthan those of points near the Galactic plane.Figure 17 displays the vertical distribution of the CO gas. We find that one Gaussianmodel produces a poor fit to the histogram of the CO distribution from the Galacticplane (e.g., χ /dof=94.8/86). The full width at half maximum (FWHM) of the oneGaussian component is 162.7 pc, which is larger than results from other studies (e.g.,FWHM ∼ z ∼ −
11 pc and the prominent broad wing at | z | > ∼
100 pc cannotbe fitted by the one Gaussian model.On the contrary, the model of two Gaussian components works better( χ /dof=60.5/84, also see the red-thick line in Figure 17). Table 2 shows the pa- WISP CO survey between 25 . ◦ . ◦ ∼ − , whichis roughly 5 σ of the CO integrated intensity. The value of the zero-point depends onthe current rms level of the MWISP survey, in which samples with three consecutivechannels greater than 3 × rms ( ∼ . CO signal of theMCs. A few points near the zero-point are from the noise (e.g., bad channels andbaseline fluctuations in the velocity axis of the 3D data cubes). These bad points,which are randomly distributed in the area, are readily discerned by checking theirspectra.Samples away from the Galactic plane with values above 3.8 K km s − are actuallyfrom the high- z CO emission. An example is the unusual peak at z ∼ −
435 pc(Figure 17). CO emission of this feature is mostly from MC G40.331 − V LSR ∼ − . The high | b | MC is associated with the W50 nebula, whichhas a near kinematic distance of 4.9 kpc, as estimated from the large-scale gas at V LSR =77 km s − (see the detailed H i and CO analysis in Su et al. 2018). At thedistance of 4.9 kpc, the true displacement from the b = 0 ◦ plane is z ≈ −
370 pcfor the b = − . ◦
302 molecular gas, which is likely related to the energetic jets of themicroquasar SS 433 in W50 (i.e., jet–ISM interactions discussed in Su et al. 2018).The two Gaussian components correspond to the central thin molecular gas disk(FWHM=88.5 pc) and the extended thick CO disk (FWHM=276.8 pc), respec-tively. The thickness ratio of the two Gaussian components is FWHM(Thickplane)/FWHM(Thin plane) ∼ R GC ∼ ∼ | z | > ∼
100 pc and | z | < ∼
100 pc is about 0.03,indicating very faint CO emission for the second (or thick) molecular gas component.In the above calculation, we used the A5 rotation curve model of Reid et al. (2014),in which the values of R =8.34 kpc and V =240 km s − are adopted. The thicknessof the molecular gas disk will increase by a factor of ∼ R =8.5 kpc and V =220 km s − .We have shown that CO emission in the first quadrant of the inner Galaxy issomewhat below the Galactic plane based on channel maps in Figure 6. Furtherquantitative analysis indicates that the distribution of molecular gas is actually belowthe b = 0 ◦ plane for the V LSR > ∼
60 km s − CO emission (Figure 17 and Table 2).Assuming that the inner disk of our Galaxy is flat and tilted, we derive the Sun’s offsetfrom the Galactic physical midplane based on the molecular gas distribution along theGalactic latitude. Considering z Sun R ≈ z peak R GC (mean) , the Sun is about 8 . × . . ≈ ∼ SUMMARY AND FUTURE PROSPECTS6The MWISP project is a large, systematic, and unbiased CO survey of the northernGalactic plane using the 13.7 m millimeter-wavelength telescope with a 3 × ∼
250 deg regions of the first quadrant of l = +25 . ◦ . ◦ | b | < ∼ . ◦ CO, CO, and C O ( J =1–0) lines. Using the OTF mode, we achievedthe high-quality CO data with an angular resolution of ∼ ′′ and grid samplingof 30 ′′ . The rms noise level of the data is ∼ CO at a channel width of0.16 km s − and ∼ CO and C O at 0.17 km s − .Using the new CO data, we investigate the distant molecular gas in the first quadrantof the Milky Way. The CO distribution of the most distant gas, which is weak andisolated in space and velocity, is described by the equation of V LSR < ∼ − . × l +4 .
34 km s − . We believe that these MCs are from the Scutum–Centaurus Arm in theouter Galaxy, which are mainly concentrated in b > ∼ ◦ regions of the first quadrantof the Milky Way due to the warping of the Galactic outer disk.A bubble-like structure with a diameter of 0 . ◦ O emission is revealed towardthe Serpens / Aquila Rift MC complex. We suggest that the interesting structure,together with the surrounding rim-bright MCs and molecular shells / arcs, may berelated to the nearby H ii region W40, which is just located at the southwestern edgeof the bubble-like dense gas structure.In addition to a thin CO disk with an FWHM of ∼ ∼ i thickness for regions of R GC ∼ ∼ ∼ z = − . ii regions and SNRs, which have profound effectson their surrounding ISM. Therefore, the large-scale CO survey with high dynamicrange is also an excellent dataset for studying the effects of stellar feedback on MCs(i.e., outflows, H ii regions, stellar winds, and SNRs on the surrounding molecular gasenvironment).Further MC identification is important for the extended CO emission with veloc-ity crowding. New methods, such as SCIMES (i.e., Spectral Clustering for Inter-stellar Molecular Emission Segmentation, Colombo et al. 2015), may be useful for WISP CO survey between 25 . ◦ . ◦ CO emission and the dense gas traced by CO and CS emission in the innerGalaxy. Based on CO data of multiple surveys, they showed that CO emission is veryimportant for studying the dense star-forming MCs and the diffuse molecular ISM inthe Galaxy.Using the CO, CO, and C O ( J =1–0) lines in combination with the wide ve-locity coverage, the high-velocity resolution, and the large-scale mapping provided bythe MWISP, we can separate distinct clouds at similar velocities, investigate detailedkinematic information of the molecular gas at complicated regions, and provide theMC catalog traced by CO emission. More sophisticated methods will be explored tosystematically analyze the spatial and kinematic features of MCs in the Milky Waybased on the new CO survey.We gratefully acknowledge the staff members of the Qinghai Radio Observing Sta-tion at Delingha for their support of the observations. We thank the anonymousreferee for a careful reading of the manuscript and several critical suggestions thatimproved the paper. This work is funded by the National Key R&D Program ofChina through grants 2017YFA0402701 and 2017YFA0402702. J.Y. acknowledgesCAS support through grant QYZDJ-SSW-SLH047. X.C. acknowledges support bythe NSFC through grant 11473069. Y.S. was supported by the NSFC through grant11773077. Facility:
PMO 13.7m
Software:
GILDAS/CLASS (Pety 2005)REFERENCES
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Parameters of the MWISP Survey Data
Molecular linesHPBWGrid spacingVelocity separationSystem temperaturerms ( T MB )Mapping regions CO ( J =1–0) ∼ ′′ ′′ − ∼
250 K ∼ . . ◦ < ∼ l < ∼ . ◦ | b | < ∼ . ◦ CO and C O ( J =1–0) ∼ ′′ ′′ − ∼
140 K ∼ . . ◦ < ∼ l < ∼ . ◦ | b | < ∼ . ◦ Table 2.
Two Gaussian Components for the Inner Galactic CO Plane
Narrow Broad z (pc) FWHM (pc) I (K km s − ) FWHM (pc) I (K km s − ) χ /dof − . a a The residual is calculated with a weight of the mean CO intensity per bin.
Note — z is the displacement of the CO gas from the Galactic plane of b = 0 ◦ ; FWHMis the thickness of the CO layer; I is the peak of the best fit. Note that the zero-pointis at 3.8 K km s − , i.e., I = I peak = I ( z ) + 3.8 K km s − (see Figure 17 and thetext). Note that the z value of the broad component is fixed to that of the narrow one.Samples are from the CO emission near the tangent point, i.e., V LSR >
60 km s − . WISP CO survey between 25 . ◦ . ◦ Figure 1.
Top: The CO integrated intensity map of the Serpens / Aquila Rift MCcomplex from the Columbia-CfA CO survey (Dame et al. 2001). Bottom: The MWISP CO integrated intensity map for the same region. In both panels, all the integratedvelocity ranges are 0 to 30 km s − , and the color bars represent the integrated intensity inunits of K km s − . N u m be r pe r b i n C u m u l a t i v e f r a c t i on Figure 2. (a): Distribution of rms noise values of the three lines for the whole dataset.The long-dashed lines indicate the median values of CO ( J =1–0) (blue), CO ( J =1–0)(green), and C O (red) emission, respectively. (b): Cumulative distribution of rms noisevalues for the three CO lines.
WISP CO survey between 25 . ◦ . ◦ Figure 3.
Rms noise maps for the CO, CO, and C O ( J =1–0) lines. The mapsdisplay the variation in rms between cells. There are some stripes along l and b in the mapsdue to the bad weather (or the high system temperature) in the OTF mapping. Figure 4. (a) Integrated emission of the CO ( J =1–0) emission in the interval of −
77 to0 km s − . (b) Intensity-weighted mean velocity (first moment) map of the CO emissionfor V LSR ≤ − MCs.
WISP CO survey between 25 . ◦ . ◦ Figure 5.
MWISP CO ( J =1–0) intensity map in the 0–130 km s − interval, overlaidwith the Effelsberg 11 cm radio contours (250, 500, 750, 1 × , 2.5 × , 5 × , 7.5 × ,and 1 × mK) from Reich et al. (1990). Some bright and/or extended radio sources arelabeled on the map. Figure 6.
Channel maps of CO with velocity intervals of 10 km s − for V LSR ≥ − gas. WISP CO survey between 25 . ◦ . ◦ Figure 6. (Continued) Figure 6. (Continued)
WISP CO survey between 25 . ◦ . ◦ Figure 7.
Channel maps of CO with velocity intervals of 10 km s − for V LSR ≥ − gas. Figure 7. (Continued)
WISP CO survey between 25 . ◦ . ◦ Figure 7. (Continued) Figure 8.
Left panel: The integrated emission of the CO emission in the interval of120 to 130 km s − . Right panel: The integrated emission of the CO emission in the sameinterval.
WISP CO survey between 25 . ◦ . ◦ Figure 9.
Longitude–velocity diagrams of the CO and CO emission for V LSR ≥ − gas. The rectangle in the right panel shows the region of the densemolecular gas associated with H ii regions Sh 2-75 and Sh 2-76, which was not fully coveredin latitude by the GRS survey (Jackson et al. 2006). Figure 10.
Intensity map and velocity distribution of C O emission for V LSR =0–40 km s − gas. WISP CO survey between 25 . ◦ . ◦ Figure 11.
Intensity map and velocity distribution of C O emission for V LSR =40–80 km s − and 80–120 km s − gas. Figure 12.
Longitude–velocity diagram of the CO emission for V LSR < ∼ − gas.The region between the red lines ( V LSR = − . × l + 4 . ±
12 km s − ) contains mostof the detected CO gas in the EOG. The Outer Arm and the EOG regions are labeled onthe map. Note that the CO signal is multiplied by a factor of 100 for the correspondingvelocity range due to the weak emission of the distant MCs (see the text).
WISP CO survey between 25 . ◦ . ◦ -4 -2 0 2 4Galactic Latitude (Degree)02004006008001000 T o t a l I n t en s i t y ( K k m / s ) Figure 13.
Distribution of the EOG CO emission with V LSR < ∼ − . × l + 4 .
34 km s − .The CO intensity less than 1.2 K km s − (or ∼ σ for a typical velocity range of 4 km s − ) isnot shown on the map. Two intensity peaks are found to be at b ∼ . ◦ b ∼ . ◦
8. Note thatthe MC at ( l =40 . ◦ b =1 . ◦ V LSR ∼ −
57 km s − ; e.g., Table 1 in Dame & Thaddeus2011) cannot be seen in the map because of the velocity criterion here. But the MCprobably belongs to the EOG. The corresponding CO emission of the distant MC can beseen in Figures 4 and 12. Figure 14. CO ( J =1–0, blue), CO ( J =1–0, green), and C O ( J =1–0, red) intensitymap in the 1–14 km s − interval toward the H ii region W40. The H ii region W40 is markedas a purple circle centered at l =28 . ◦ b =3 . ◦ ′ (e.g., Quireza et al. 2006).Typical spectra, which are extracted from the three white boxes, are shown to the right ofthe map. WISP CO survey between 25 . ◦ . ◦ Figure 15.
Channel maps of CO ( J =1–0, blue), CO ( J =1–0, green), and C O( J =1–0, red) with velocity intervals of 3–5, 5–7, 7–9, and 9–11 km s − toward the W40complex. The large circle, which is centered at ( l =29 . ◦ b =3 . ◦
82) with a radius of 0 . ◦ Figure 16. C O velocity field toward the W40 region. The big and small circles are thesame as those in Figure 15. Samples with T MB (C O) > ∼ σ are considered here. -400 -200 0 200 400z (pc)0102030405060 M ean I n t en s i t y ( K k m / s ) pe r b i n Figure 17.
CO distribution along the distance from the Galactic plane of b = 0 ◦ . Thetwo red dashed lines indicate the narrow and broad Gaussian components, respectively.The error bars, which were calculated from I mean (CO)/(number of pixels per bin) . , areshown in blue. Note that the zero-point of the best fitting is at 3.8 K km s − accordingto the MWISP CO data (see Table 2). The unusual peak at z ∼ −
435 pc probably hasa connection with the dynamical interactions between the jets of the unique microquasarSS 433 in W50 and the surrounding high- zz