The SEDIGISM survey: first data release and overview of the Galactic structure
F. Schuller, J. S. Urquhart, T. Csengeri, D. Colombo, A. Duarte-Cabral, M. Mattern, A. Ginsburg, A. R. Pettitt, F. Wyrowski, L. Anderson, F. Azagra, P. Barnes, M. Beltran, H. Beuther, S. Billington, L. Bronfman, R. Cesaroni, C. Dobbs, D. Eden, M.-Y. Lee, S.-N. X. Medina, K. M. Menten, T. Moore, F. M. Montenegro-Montes, S. Ragan, A. Rigby, M. Riener, D. Russeil, E. Schisano, A. Sanchez-Monge, A. Traficante, A. Zavagno, C. Agurto, S. Bontemps, R. Finger, A. Giannetti, E. Gonzalez, A. K. Hernandez, T. Henning, J. Kainulainen, J. Kauffmann, S. Leurini, S. Lopez, F. Mac-Auliffe, P. Mazumdar, S. Molinari, F. Motte, E. Muller, Q. Nguyen-Luong, R. Parra, J.-P. Perez-Beaupuits, P. Schilke, N. Schneider, S. Suri, L. Testi, K. Torstensson, V. S. Veena, P. Venegas, K. Wang, M. Wienen
MMNRAS , 1–18 (2020) Preprint 4 December 2020 Compiled using MNRAS L A TEX style file v3.0
The SEDIGISM survey: first data release and overview of the Galacticstructure ★ F. Schuller , , † , J. S. Urquhart , T. Csengeri , , D. Colombo , A. Duarte-Cabral ,M. Mattern , A. Ginsburg , A. R. Pettitt , F. Wyrowski , L. Anderson , F. Azagra ,P. Barnes , M. Beltran , H. Beuther , S. Billington , L. Bronfman , R. Cesaroni ,C. Dobbs , D. Eden , M.-Y. Lee , S.-N. X. Medina , K. M. Menten , T. Moore ,F. M. Montenegro-Montes , S. Ragan , A. Rigby , M. Riener , D. Russeil ,E. Schisano , A. Sanchez-Monge , A. Traficante , A. Zavagno , C. Agurto ,S. Bontemps , R. Finger , A. Giannetti , E. Gonzalez , A. K. Hernandez ,T. Henning , J. Kainulainen , J. Kauffmann , S. Leurini , S. Lopez , F. Mac-Auliffe ,P. Mazumdar , S. Molinari , F. Motte , E. Muller , Q. Nguyen-Luong ,R. Parra , J.-P. Perez-Beaupuits , P. Schilke , N. Schneider , S. Suri , L. Testi ,K. Torstensson , V. S. Veena , P. Venegas , K. Wang , M. Wienen , Affiliations can be found after the references.
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
The SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) survey used the APEX telescopeto map 84 deg of the Galactic plane between ℓ = − ◦ and ℓ = + ◦ in several molecular transitions, including CO (2 – 1) andC O (2 – 1), thus probing the moderately dense ( ∼ cm − ) component of the interstellar medium. With an angular resolutionof 30 (cid:48)(cid:48) and a typical 1 𝜎 sensitivity of 0.8–1.0 K at 0.25 km s − velocity resolution, it gives access to a wide range of structures,from individual star-forming clumps to giant molecular clouds and complexes. The coverage includes a good fraction of the firstand fourth Galactic quadrants, allowing us to constrain the large scale distribution of cold molecular gas in the inner Galaxy. Inthis paper we provide an updated overview of the full survey and the data reduction procedures used. We also assess the qualityof these data and describe the data products that are being made publicly available as part of this first data release (DR1). Wepresent integrated maps and position-velocity maps of the molecular gas and use these to investigate the correlation between themolecular gas and the large scale structural features of the Milky Way such as the spiral arms, Galactic bar and Galactic centre.We find that approximately 60 per cent of the molecular gas is associated with the spiral arms and these appear as strong intensitypeaks in the derived Galactocentric distribution. We also find strong peaks in intensity at specific longitudes that correspond tothe Galactic centre and well known star forming complexes, revealing that the CO emission is concentrated in a small numberof complexes rather than evenly distributed along spiral arms.
Key words:
ISM: structure – Galaxy: kinematics and dynamics – radio lines: ISM – surveys
Over the last few decades, many systematic continuum surveys of theGalactic plane have been carried out over the full electromagneticspectrum, from the infrared (e.g. GLIMPSE, Churchwell et al. 2009,MIPSGAL, Carey et al. 2009, Hi-GAL, Molinari et al. 2010) to the ★ This publication is based on data acquired with the Atacama PathfinderExperiment (APEX), projects 092.F-9315 and 193.C-0584. APEX is a collab-oration between the Max-Planck-Institut für Radioastronomie, the EuropeanSouthern Observatory, and the Onsala Space Observatory. † E-mail: [email protected] (sub-)millimetre (ATLASGAL, Schuller et al. 2009, BGPS, Aguirreet al. 2011, JPS, Moore et al. 2015; Eden et al. 2017), and radio range(CORNISH, Hoare et al. 2012; Purcell et al. 2013, THOR, Beutheret al. 2016; Wang et al. 2020, GLOSTAR, Medina et al. 2019). Thesecontinuum surveys have been complemented by spectral line surveys,which are essential for estimating distances and determining physicalproperties. Some examples include the CO (1-0) survey from Dameet al. (2001), the CO Boston University-Five College Radio Astron-omy Observatory Galactic Ring Survey (GRS, Jackson et al. 2006),the Census of High- and Medium-mass Protostars (CHaMP, Barneset al. 2011), the CO High-Resolution Survey (COHRS, Dempsey © a r X i v : . [ a s t r o - ph . GA ] D ec F. Schuller et al.
Table 1.
Main characteristics of recent, large scale CO surveys of the inner Galactic plane.Survey Transitions Coverage Angular Velocity rms ReferenceName resolution resolution ( 𝑇 mb )GRS CO (J=1–0) 18 ◦ ≤ ℓ ≤ ◦ , 46 (cid:48)(cid:48) − ∼ | 𝑏 | ≤ ◦ CHaMP CO/ CO/C O/ 280 ◦ ≤ ℓ ≤ ◦ , 37–40 (cid:48)(cid:48) − H + /HNC/ -4 ◦ ≤ b ≤ +2 ◦ HCO + /HCN (J=1–0)COHRS CO (J=3–2) 10 ◦ ≤ ℓ ≤ ◦ , 16 (cid:48)(cid:48) − ∼ | 𝑏 | ≤ ◦ CHIMPS CO/C O 28 ◦ ≤ ℓ ≤ ◦ , 15 (cid:48)(cid:48) − ∼ | 𝑏 | ≤ ◦ FUGIN CO/ CO/C O 10 ◦ ≤ ℓ ≤ ◦ , | 𝑏 | ≤ ◦ (cid:48)(cid:48) − ◦ ≤ ℓ ≤ ◦ , | 𝑏 | ≤ ◦ Mopra-CO CO/ CO/C O/ 305 ◦ ≤ ℓ ≤ ◦ , 35 (cid:48)(cid:48) − O (J=1–0) | 𝑏 | ≤ ◦ ThrUMMS CO/ CO/C O/ 300 ◦ ≤ ℓ ≤ ◦ , 72 (cid:48)(cid:48) − | 𝑏 | ≤ ◦ MWISP CO/ CO/C O -10 ◦ ≤ ℓ ≤ ◦ , 50 (cid:48)(cid:48) − | 𝑏 | ≤ ◦ FQS CO/ CO 220 ◦ ≤ ℓ ≤ ◦ , 55 (cid:48)(cid:48) − ◦ ≤ b ≤ ◦ SEDIGISM CO/C O − ◦ ≤ ℓ ≤ +18 ◦ , 30 (cid:48)(cid:48) − | 𝑏 | ≤ ◦ + W43 Observations for the MWISP survey are still ongoing; the Su et al. (2019) paper focuses on a region covering 25.8 ◦ ≤ ℓ ≤ +49.7 ◦ . Figure 1.
Milky Way coverage of recent CO surveys described in Table 1. The background image is a schematic of the Galactic disc as viewed from the NorthernGalactic Pole (courtesy of NASA/JPL-Caltech/R. Hurt (SSC/Caltech)) showing the known large-scale features of the Milky Way, such as the spiral arms andthe Galactic bar. The survey wedges emanate from the position of the Sun; their respective lengths are arbitrary and do not reflect the sensitivity of each survey.MNRAS , 1–18 (2020)
EDIGISM first data release et al. 2013), the CO Heterodyne Inner Milky Way Plane Survey(CHIMPS, Rigby et al. 2016, 2019), the FOREST unbiased Galac-tic plane imaging survey with Nobeyama (FUGIN, Umemoto et al.2017), the Mopra Southern Galactic Plane CO Survey (Burton et al.2013; Braiding et al. 2018), the Three-mm Ultimate Mopra MilkyWay Survey (ThrUMMS, Barnes et al. 2015), the Milky Way ImagingScroll Painting (MWISP, Su et al. 2019), and the Forgotten QuandrantSurvey (Benedettini et al. 2020). In Table 1 and Fig. 1, we presenta summary of these recently completed CO surveys and show theircoverage on a schematic top-down view of the Milky Way.This wealth of data has greatly enhanced our view of the Galacticstructure and its major components. However, there is still no consen-sus on the exact structure of the Galaxy. For instance, it is not firmlyestablished how many spiral arms are present and what is their exactlocation (e.g. Taylor & Cordes 1993; Reid et al. 2014, 2016; Vallée2017; Drimmel 2000; Siebert et al. 2011; García et al. 2014; GaiaCollaboration et al. 2018), nor what is the exact size and orientationangle of the central bar (e.g. Bissantz et al. 2003; Pettitt et al. 2014;Li et al. 2016b) – thus making it difficult to pin-point and study thelarge-scale distribution of molecular gas in the Galaxy.Progress is also being made in the characterisation of the earliestphases of (high-mass) star formation (e.g. Urquhart et al. 2013a,b,2014b; Csengeri et al. 2014, 2017; Traficante et al. 2017; Elia et al.2017; Urquhart et al. 2018; Pitts et al. 2019), but the role of largescale structures and the interplay between the various phases ofthe interstellar medium (ISM) are still not well constrained. Keyquestions remain, that are also relevant to the study of star formationin external galaxies, such as: what role do the spiral arms play in theformation of molecular clouds and star formation, and what controlsthe star formation efficiency (SFE). Observations of nearby spiralgalaxies have revealed a tight correlation between dense moleculargas and enhancements of star formation activity within spiral arms(e.g. Leroy et al. 2017). Also in the Milky Way, it is clear that spiralarms are rich in molecular gas. For example, recent results from theTHOR survey reveal an increase by a factor 6 of atomic to moleculargas ratio from the arms to inter-arm regions. However, it is not clearwhether this is due to the collection of molecular clouds that fallinto their gravitational potential (e.g. Foyle et al. 2010), or if themolecular gas forms within the spiral arms themselves. Furthermore,it is unclear if the enhanced star forming activity observed (Urquhartet al. 2014a) is directly attributable to the presence of spiral arms oris simply the result of source crowding within the arms (Moore et al.2012).The SFE is the result of a number of stages: the conversion of neu-tral gas to molecular clouds, then to dense, potentially star-formingclumps, and finally to proto-stars and young stellar objects. Each ofthese stages has its own conversion efficiency. Identifying the stagethat is primarily affected by the environment could bring constraintson the dominant SF-regulating mechanism. Some progress has beenmade recently to investigate the effects of environment on the SFE inour Galaxy, based on limited samples of objects (Eden et al. 2012;Moore et al. 2012; Longmore et al. 2013; Ragan et al. 2018). Inorder to extend these studies to much larger samples, we have per-formed a large scale ( ∼
84 deg ) spectroscopic survey of the innerGalactic disc: the SEDIGISM (Structure, Excitation and Dynamicsof the Inner Galactic Interstellar Medium) survey. The spectroscopicdata provide essential information on the distribution of interstellarmatter along the line of sight, thus complementing the existing con-tinuum surveys. These data allow us to achieve an unbiased view ofthe moderately dense ISM over a large fraction of the Galactic disc.The SEDIGISM survey covers a large portion of the fourth quadrant Table 2.
Summary of the APEX observational parameters.Parameter ValueGalactic longitude range -60 ◦ < ℓ < ◦ and 29 ◦ < ℓ < ◦ Galactic latitude range 𝑎 − ◦ < 𝑏 < ◦ Instrument SHeFI (APEX-1)Frequency 217–221 GHzBandwidth 4 GHzAngular resolution 30 (cid:48)(cid:48)
Velocity resolution 0.1 km s − Smoothed velocity resolution 0.25 km s − Mean noise ( 𝑇 mb ) 𝑏 ∼ − 𝜂 mb ) 𝑐 𝑎 The latitude range was extended to − ◦ < 𝑏 < ◦ towards the CentralMolecular Zone, to 𝑏 < − ◦ towards the Nessie filament at ℓ ∼ ◦ , andto 𝑏 < + . ◦ towards the RCW 120 region at ℓ ∼ ◦ . 𝑏 Per 0.25 km s − channel. 𝑐 at high velocity and angular resolution and will make a significantcontribution to our understanding of Galactic structure.The survey has been described in Schuller et al. (2017, hereafterPaper I). It consists of spectroscopic data covering the inner Galacticplane in the frequency range 217 −
221 GHz, which includes the CO (2 – 1) and C O (2 – 1) molecular lines, at 30 arcsec angularresolution. Thus, this survey complements the other spectroscopicsurveys that have been previously mentioned. This is the first of threepapers that describe the survey data and present the initial results. Inthe present paper, we provide an overview of the survey data and afirst look at the connection between the molecular gas and large scalestructural features of the Galaxy (we will refer to this as Paper II). Inthe accompanying papers, we present a catalogue of giant molecularclouds (GMCs) and investigate their properties with respect to theirstar formation activity and their Galactic distribution (Duarte-Cabralet al. submitted; hereafter Paper III); and we investigate the densegas fraction and star formation efficiency as a function of Galacticposition (Urquhart et al. submitted; hereafter Paper IV).The structure of this paper is as follows: we describe theSEDIGISM observations and data quality in Sect. 2. We present thelarge scale distribution of CO and C O in Sect. 3 and investigatethe association between molecular gas and spiral arms. We discusssome interesting regions in Sect. 4, and demonstrate the usability ofother molecular transitions within the spectral range covered by thedata for scientific exploitation in Sect. 5. Finally, we summarise ourconclusions in Sect. 6.
Observations were done with the 12 m diameter Atacama PathfinderExperiment telescope (APEX, Güsten et al. 2006), located at 5100 maltitude on Llano de Chajnantor, in Chile. The observational setupand observing strategy have been described in Paper I and the keyobservational parameters are summarised in Table 2; here we providea brief overview of the most important features of this survey.The prime target lines are CO (2 – 1) and C O (2 – 1) but the4 GHz instantaneous bandwidth also includes a number of transitionsfrom other species (H CO, CH OH, SO, SO , HNCO, HC N, SiO).The observations have been carried out in tiles of 0 . ◦ × . ◦ usingposition-switching in the on-the-fly mapping mode. Each position inthe survey is covered by at least two maps observed in orthogo- MNRAS000
Observations were done with the 12 m diameter Atacama PathfinderExperiment telescope (APEX, Güsten et al. 2006), located at 5100 maltitude on Llano de Chajnantor, in Chile. The observational setupand observing strategy have been described in Paper I and the keyobservational parameters are summarised in Table 2; here we providea brief overview of the most important features of this survey.The prime target lines are CO (2 – 1) and C O (2 – 1) but the4 GHz instantaneous bandwidth also includes a number of transitionsfrom other species (H CO, CH OH, SO, SO , HNCO, HC N, SiO).The observations have been carried out in tiles of 0 . ◦ × . ◦ usingposition-switching in the on-the-fly mapping mode. Each position inthe survey is covered by at least two maps observed in orthogo- MNRAS000 , 1–18 (2020)
F. Schuller et al. nal scanning directions, along galactic longitude and latitude. The0 . ◦ × . ◦ tiles oriented along ℓ or 𝑏 were sometimes observed un-der different conditions. As a result of this plaiting, only 0 . ◦ × . ◦ sub-cubes were observed with roughly constant conditions and showa uniform noise level, as visible in Fig. 2. Some fields were observedin 0 . ◦ × . ◦ maps at the beginning of the survey, also with twoorthogonal scanning directions.The reference positions for each field were selected to be ± . ◦ offthe Galactic mid-plane to avoid contamination, and while this wassufficient to ensure the C O (2 – 1) data were clean, this was notalways the case for the brighter CO (2 – 1) transition. Therefore,we have systematically performed pointed observations towards thereferences points, using an off position further from the Galacticplane. More details can be found in Appendix A and in Table A1 .The full survey coverage is ∼
84 deg (cf. Fig. 2): the main part ofthe survey, as described in Paper I, covers 300 ◦ ≤ ℓ ≤ +18 ◦ , with | 𝑏 | ≤ ◦ (78 deg ). Because additional observing time was available forthis project in 2016 and 2017, we were able to slightly increase thesurvey coverage. We have extended the coverage in latitude to | 𝑏 | ≤ ◦ around the Galactic Centre (358 ◦ ≤ ℓ ≤ +1.5 ◦ ) and mapped a 2 deg region covering the extreme star forming region W43 (+29 ◦ ≤ ℓ ≤ +31 ◦ , with | 𝑏 | ≤ ◦ ). We have also increased the latitude coverageup to +0.75 ◦ at ℓ = ◦ to cover RCW 120, and down to − ◦ for 338 ◦ ≤ ℓ ≤ ◦ to improve the coverage of the Nessie giantfilament (Jackson et al. 2010). The data taken towards W43 allow fora comparison with existing surveys in the northern hemisphere, suchas the HERO survey performed with the IRAM 30-m telescope inthe same spectral lines (Carlhoff et al. 2013). Here we present all thedata that have been taken with APEX for this survey between 2013and 2017. The data provided by the APEX telescope consist of spectra cali-brated in antenna temperature scale ( 𝑇 ★ A ), written in files readableby the CLASS software from the GILDAS package . We have de-veloped a dedicated pipeline in GILDAS/CLASS, which consists ofstandard data reduction steps, such as conversion to 𝑇 mb scale (i.e. 𝑇 mb = 𝑇 ∗ A / 𝜂 mb ), spectral resampling, removing a spectral baseline,and gridding the spectra onto a data cube. In particular, a critical stepconsists in an automatic detection of emission features in order todefine the windows to be masked when subtracting baselines; this isdescribed in detail in Paper I.As previously mentioned, in some cases the initial off-position wasfound to contain emission that would contaminate our maps and sothese were checked against a more distant position, one degree furtheraway perpendicular to the Galactic plane, and reduced independently.Emission in the reference position appears as an absorption featurethat is constant over the extent of a given map. When coinciding withthe velocity range of real emission in the map, it leads to underesti-mating the gas column density, and could also impact the observedvelocity pattern of the emission. Where the reference position hasbeen found to show emission, we corrected for this by adding thespectrum measured for the reference position to each spectrum inthe map. This operation is illustrated in Fig. A1 in Appendix A.Since the rms noise measured on these spectra was typically around Only a small portion of the data is provided here. The fulltable is only available in the online version of this articlehttps://doi.org/10.1093/mnras/staa2369 Table 3.
Fields with issues.Field Comments304.25 − + −
23 km s − from the reference position332.75 + −
23 km s − and −
31 km s − from the reference position336.25 − −
20 km s − from the reference position352.25 + − − from the reference position − ). Therefore, to properlyaccount for the absorption feature in the spectra where necessary, wecomputed this difference between the Doppler corrections computedfor the map centre and for the reference position observed at a dif-ferent time. After shifting the observation of the reference spectrumaccordingly, we then added the modified reference spectrum to eachposition of the map. This step was only necessary where the referenceposition has been found to show significant emission (see Table 3 fordetails). The output of the pipeline is a set of data-cubes calibrated to the 𝑇 mb scale, centred on the most relevant spectral lines possibly detected inthe band-pass, and projected on 9.5 (cid:48)(cid:48) pixels. The velocity resolutionin the final cubes is 0.25 km s − for the CO (2 – 1) and C O (2 –1) lines, and 0.5 km s − for all the other transitions: H CO(3 , − , ) and (3 , − , ), HC N(24 − − ), SiO(5 − , − , ), and CH OH (4 , − , ).Due to variations in weather conditions, the elevation of the tele-scope during observations, and the performance of the instrument,the local noise level 𝜎 rms varies slightly over the survey area, asillustrated in Fig. 2. For each pixel in each CO (2 – 1) data cube, 𝜎 rms is computed as the standard deviation of the signal in the first50 spectral channels (i.e. −
200 km s − < 𝑣 lsr < − . − ), asthis part of the velocity range is almost always devoid of emission,except possibly for some small regions near the Galactic centre. InFig. 3 we show the distribution of the median noise values for all ofthe individual 0 . ◦ × . ◦ sub-cubes, which shows that the rangeof noise values is between 0.5–1.6 K ( 𝑇 mb ), with most fields (61 percent) showing a noise below 1.0 K; the median and mean values are0.94 and 0.96 K respectively. While the sensitivity achieved is suffi-cient to map the distribution of the relatively bright CO (2 – 1) lineemission, we are only able to detect C O (2 – 1) towards the densestregions. Transitions from other species are likely to be detected onlytowards the brightest and most compact dense cores.Another issue worth highlighting is that subtracting baselines inregions with bright, broad emission lines is known to be very ardu-ous and error-prone, particularly when the baseline exhibits varia-tions over a velocity range comparable to the line-width. The centralregion of the Galaxy is clearly the most extreme case in that re-spect, and the data for this region should be used with particular
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EDIGISM first data release Figure 2.
Spatial distribution of the local noise 𝜎 rms over the survey area (see text for details). The median values computed in 0.25 × sub-cubes areshown here in colours (see colour scale on the right) caution. In addition, there are a few other regions that are affectedby baseline ripples and absorption features that we have not beenable to fully remove by adding the spectrum of the correspondingreference position. We identify these regions in Table 3 and recom-mend extreme caution when using data for these fields. However,this issue only emerges when averaging spectra over large regions;correction from the reference position to the individual spectra doesnot show such artefacts. Finally, the C O (2 – 1) data shows a spikeat v lsr ∼ −
48 km s − that appears in a single channel with a varyingintensity over all fields. We have removed this spike from the indi- vidual spectra using a sigma-clipping method, however, an artefactmay still show up when averaging the spectra over large areas.In order to check the consistency of our calibration and the dataquality, we have compared the SEDIGISM data for the W43 complexwith the W43-HERO survey (Carlhoff et al. 2013), which covereda good fraction of the 2 deg field centred at ℓ = ◦ in the sametransitions as SEDIGISM, CO (2 – 1) and C O (2 – 1), with theIRAM 30 m telescope. The results from this comparison show nosystematic differences between the two data sets for the CO (2 –1) line and indicate that the calibration is consistent between the
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Figure 3.
Distribution of the median values of the r.m.s. noise in0.25 × sub-cubes, shown in 0.1 K ( 𝑇 mb ) bins. The mean and me-dian values are both approximately 0.95 K. two surveys, although the distribution of the CO emission revealssome minor differences towards the brightest areas. Since the C OSEDIGISM data is only tracing the most compact, dense clumps butis not sensitive to the more diffuse, extended material, no statisticalcomparison is possible (see Appendix B for more details).
The reduced data, processed with the current version of the dedicatedpipeline, is now available to the community. This first public datarelease (DR1) consists of 78 cubes in CO (2 – 1) and 78 cubes inC O (2 – 1), where each cube covers 2 ◦ in longitude over ± ◦ inlatitude (or more in the few regions with extended 𝑏 coverage, see§ 2.1). Cubes are separated by 1 ◦ in longitude, providing 1 ◦ overlapbetween adjacent cubes. The pixel size is 9.5 (cid:48)(cid:48) , the velocity resolutionis 0.25 km s − , and the velocity range covers −
200 to +200 km s − .The DR1 data can be downloaded from a server hosted at the MPIfRin Bonn (Germany). The 4 GHz of instantaneous bandwidth of the spectral tuningused to observe SEDIGISM includes several transitions from othermolecules (see Table 1 in Paper I). In particular, six lines are de-tected when spatially averaging the data corresponding to the bright-est emission regions in CO (2 – 1); this will be discussed in moredetail in Sect. 5. Therefore, we also provide data cubes covering thesetransitions as part of the DR1, where the spectra have been smoothedto 0.5 km s − velocity resolution in order to increase the signal-to-noise ratio. The velocity range also covers from −
200 to +200 km s − for all lines, except for the SiO (5–4) transition, which is located nearthe edge of the spectral set-up so that only the −
200 to +150 km s − velocity range is available.In Paper III, we have extracted ∼ CO (2 – 1) data from this data release, usingthe Spectral Clustering for Interstellar Molecular Emission Segmen-tation (SCIMES, Colombo et al. 2015) algorithm. The cataloguewith the distance estimates and the derived physical properties for https://sedigism.mpifr-bonn.mpg.de all clouds, as derived in Paper III, as well as the masks represent-ing these clouds in the CO (2 – 1) cubes are also made publiclyavailable alongside the main data release we present here.Some work is still ongoing aimed at improving the data qualityand at solving known issues, such as artefacts for some transitions ora proper estimate of the baselines in regions with complex, extendedemission. We plan to provide data cubes with improved quality aspart of future data releases.
In this section, we use the CO (2 – 1) and C O (2 – 1) data toconstrain the Galactic structure on the largest scale. Some peculiarregions will be discussed in more detail in Sect. 4, while compactobjects and individual molecular clouds are the topic of subsequentpapers.
We show maps of the CO emission integrated over the ±
200 km s − velocity range for the full survey in Fig. 4. These maps reveal thatthe emission from molecular gas is extended over much of the in-ner part of the survey region (330 ◦ < ℓ < ◦ ) but becomes muchmore patchy at larger angular distances from the Galactic centre. Thebrightest emission is associated with the Central Molecular Zone(CMZ; Morris & Serabyn 1996), that extends over | ℓ | ≤ . ◦ or aradius of ∼
200 pc. Outside of the Galactic centre the brightest emis-sion is concentrated in distinct regions, all associated with prominentstar forming regions.Our latitude coverage is rather narrow: only ± ◦ in most direc-tions. By comparing with other surveys with larger latitude coveragelike ThrUMMS or ATLASGAL, it is clear that we miss a numberof molecular clouds and complexes, especially the nearby ones (seealso Alves et al. 2020, regarding nearby clouds in the outer Galaxy).Also a few known complexes up to ∼ ◦ < ℓ < ◦ , and a Galactocentricdistance of 8 kpc and beyond, where the Galactic plane descendsbelow a latitude of 𝑏 < − . ◦ due to the Galactic warp (e.g. Chenet al. 2019; Romero-Gómez et al. 2019), making this area of theGalaxy not well covered by our survey. However, the area coveredby SEDIGISM still encompasses the vast majority of the moleculargas in the inner Galaxy; Rigby et al. (2016) also concluded from acomparison of the CHIMPS data (with the same latitude coverage asSEDIGISM) with the GRS survey ( | 𝑏 | < ◦ ) that the molecular lineemission drops off quickly with distance from the mid-plane. To facilitate the discussion that follows concerning the distributionof molecular material in the Milky Way, Fig. 5 presents a schematicplot of the Galaxy as seen from the northern Galactic pole thatincludes the spiral arms and the Galactic bar. We have chosen to usethe spiral arm loci derived by Taylor & Cordes (1993) and updatedby Cordes (2004) as these have been determined independently ofthe distribution of molecular gas unlike the loci used in the recentwork by Reid et al. (2019), which have been fitted by-hand to the CO (1 − ℓ𝑣 -map of Dame et al. (2001). Moreover, the modelfrom Reid et al. (2019) is poorly constrained in the fourth quadrant MNRAS , 1–18 (2020)
EDIGISM first data release Figure 4.
Integrated emission maps for the full survey (except for W43), showing the CO (2 – 1) spectral cubes integrated over the full ±
200 km s − range ofthe data. due to a lack of reliable maser parallax distances. A single galacticbar is shown for illustrative purposes, with orientation and lengthscales in line with contemporary measurements (Bland-Hawthorn& Gerhard 2016). The near and far 3 kpc arms are not included inthe Cordes (2004) model and have been added in as small, 2-foldsymmetrical arm segments (Dame & Thaddeus 2008) with pitchangles of 1.5 ◦ expanding radially at a velocity of 55 km s − , with thenear 3 kpc arm aligning with the arm segment from Bronfman et al.(2000) (the exact nature of these features is still somewhat unknown,see Green et al. 2011). There is some evidence that the far-3 kpc arm is expanding slightly faster, but this is inconsequential for ouranalysis as there is effectively no emission seen in this region in theSEDIGISM data, due to the limited sensitivity. Fig. 5 shows that theSEDIGISM survey (indicated by the grey shading) covers large partsof three of the main spiral arms (Norma, Sagittarius, and Scutum-Centaurus arms), and almost all of the 3 kpc arms, thus allowing usto refine our understanding of the structure of the Galaxy. MNRAS000
200 km s − range ofthe data. due to a lack of reliable maser parallax distances. A single galacticbar is shown for illustrative purposes, with orientation and lengthscales in line with contemporary measurements (Bland-Hawthorn& Gerhard 2016). The near and far 3 kpc arms are not included inthe Cordes (2004) model and have been added in as small, 2-foldsymmetrical arm segments (Dame & Thaddeus 2008) with pitchangles of 1.5 ◦ expanding radially at a velocity of 55 km s − , with thenear 3 kpc arm aligning with the arm segment from Bronfman et al.(2000) (the exact nature of these features is still somewhat unknown,see Green et al. 2011). There is some evidence that the far-3 kpc arm is expanding slightly faster, but this is inconsequential for ouranalysis as there is effectively no emission seen in this region in theSEDIGISM data, due to the limited sensitivity. Fig. 5 shows that theSEDIGISM survey (indicated by the grey shading) covers large partsof three of the main spiral arms (Norma, Sagittarius, and Scutum-Centaurus arms), and almost all of the 3 kpc arms, thus allowing usto refine our understanding of the structure of the Galaxy. MNRAS000 , 1–18 (2020)
F. Schuller et al.
10 5 0 5 10 x [kpc] y [ kp c ] SagittariusScutum-CentaurusNorma 3kpc-Near3kpc-FarPerseus
Figure 5.
Schematic showing the loci of the spiral arms according to themodel by Taylor & Cordes (1993) and updated by Cordes (2004), with anadditional bisymmetric pair of arm segments added to represent the 3 kpcarms. The grey shaded areas indicate the regions covered by the SEDIGISMsurvey. The star shows the position of the Sun and the numbers identify theGalactic quadrants. The bar feature is merely illustrative and does not play arole in our analysis. The smaller slice in the first quadrant corresponds to theW43 region. ℓ𝑣 maps In the upper and middle panels of Fig. 6 we present a longitude-velocity map of the CO (2 – 1) and C O (2 – 1) transitions pro-duced by integrating the emission between | 𝑏 | < . ◦ . Only voxelsabove a 3 𝜎 rms threshold were considered to produce this ℓ𝑣 map,where the local noise 𝜎 rms is estimated as discussed above (Sect. 2.3).While much of the complex emission seen in the ℓ𝑏 map (Fig. 4)is the result of many giant molecular clouds being blended alongour line of sight across the inner Galactic disc, we find that theseclouds are well separated in velocity, making it easier to break downthe emission into distinct molecular structures. It is clear from thesemaps that while the CO (2 – 1) is detected over a wide range of ve-locities, the C O (2 – 1) emission is much less extended and is likelyto only be tracing dense clumps, which cannot be easily detected be-yond a few kpc due to beam dilution. But even if the C O (2 – 1) isless useful for studies of large scale structures, it is indispensable fordetailed studies of the physical properties of dense structures such asfilaments (Mattern et al. 2018) and clumps (Paper IV).On the CO (2 – 1) ℓ 𝑣 -map shown in Fig. 6, we also overlay thespiral arm loci derived by Taylor & Cordes (1993) and Cordes (2004).The 𝑥 and 𝑦 positions given by Taylor and Cordes have been convertedto ℓ and 𝑣 using a three-component rotation curve (bulge + disc +dark halo) tailored to the data of Eilers et al. (2019). The shape ofthe rotation curve towards the Galactic centre is somewhat uncertain,and so we simply adopt a steeply rising bulge component matchingthat adopted in Eilers et al. (2019, Fig. 3). The Reid et al. (2019)values for solar position and circular velocity at the orbit of the Sun,8.15 kpc and 236 km s − , are used for projection into line-of-sightvelocity, simply assuming pure circular rotation. Comparing the CO emission to the loci of the spiral arms we find very good agreementoutside the Galactic centre region ( | ℓ | ≤ ◦ ). The only significantregion of emission that is not closely associated with a spiral arm isthe CMZ, but this is known to have extreme non-circular velocities(this region is discussed in more detail in Sect. 4). We also note thatthe majority of the CO emission is located within the solar circle (i.e.v lsr < ℓ > ◦ and v lsr > ℓ > ◦ ) and so there is verylittle emission seen towards the far parts of the Perseus, Sagittarius,or Scutum-Centaurus arm. This is likely the result of the sensitivitylimit of the survey and beam dilution, and this means that probablywe are only able to detect the most massive clouds outside the solarcircle on the far-side of the Galaxy.Our simple projection of arms into ℓ𝑣 space assumes purely cir-cular motions for the primary arms, thus will not perfectly alignwith structures like the Norma Arm in the inner galaxy (appearingto move towards us with a v lsr of roughly -30 km s − , Sanna et al.2014). The response of gas to spiral arms alone creates non-circularmotions, forming some peculiar features towards the inner Galaxyseen in ℓ𝑣 space (e.g. Gómez & Cox 2004; Pettitt et al. 2015). Ourmodern, 𝐺𝑎𝑖𝑎 -era understanding of the Galactic bar also suggestsslower pattern speeds than assumed in earlier works, which placecorotation as far out as 6 kpc (Sanders et al. 2019; Bovy et al. 2019).The ISM responds strongly to the motion of the bar out to corotation,and even as far as the more distant Outer Lindblad Resonance forcertain models of bars (Sormani et al. 2015; Pettitt et al. 2020). Anyspiral arm-like features are thus inherently coupled to the bar withinat least corotation and more sophisticated modelling is required tofully understand the kinematics of the gas.In the lower panel of Fig. 6 we show the total CO (2 – 1)and C O (2 – 1) emission as a function of Galactic longitude; the CO (2 – 1) emission profile reveals a number of significant peaks,the most prominent of which is associated with the Galactic centreregion. Many of the others are associated with well known star-forming complexes such as W33, G333 and G305; these are locatedat ℓ = ◦ , ◦ and 305 ◦ , respectively. The integrated C O (2 – 1)emission also reveals peaks that are correlated with the same star-forming regions indicating these regions have either higher opticaldepth and column densities than elsewhere in the Galactic plane,or they contain enough gas at high temperature to produce strongemission in the J=2–1 lines.Analysis of the dense gas traced by the ATLASGAL survey byUrquhart et al. (2018) has shown that approximately 50 per cent ofthe current star formation in the disc of the inner Galaxy is takingplace in a relatively small number of very active regions ( ∼ ℓ ∼
0) as being associated with a complex, we findthat they are responsible for ∼
70 per cent of all the emission. The CO emission traces the lower density diffuse gas in which thedense clumps are embedded and thus allow us to probe the structure,kinematics, and physical properties of these regions. This highlightsthe survey’s ability to conduct detailed studies of the molecular gasassociated with some of the most intense star formation regions inthe Galaxy, and put them in a global setting with respect to the largescale structural features of the Galaxy.In Fig. 7 we show channel maps towards W33, a large complexrepresentative of high-mass star forming regions in the Galactic disc.According to maser parallax measurements, this complex is locatedin the Scutum-Centaurus arm at a distance of 2.4 kpc (Immer et al.2013). In the upper-left panel of this figure we show the integratedemission over the entire velocity range where emission is detected
MNRAS , 1–18 (2020)
EDIGISM first data release Figure 6.
Galactic longitude-velocity distribution of the SEDIGISM survey between 300 ◦ < ℓ < ◦ . The greyscale image shows the distribution of moleculargas as traced by the integrated CO (2 – 1) and C O (2 – 1) emission (upper and middle panels). To emphasis the weaker extended emission we have used a logscale and have masked the emission below 3 𝜎 . The intensity in 𝑇 mb scale has been integrated over the ± ◦ range in Galactic latitude. The location of the spiralarms are shown as curved dotted-dashed lines, coloured to identify the individual arms; colours are as shown in Fig. 5. For the C O (2 – 1) line, the values ofa horizontal row of three pixels centred on − . − have been set to zero due to the presence of a spike that appears at this velocity when large areas areintegrated together (for more details see Sect. 2). Lower panel: Integrated CO (2 – 1) and C O (2 – 1) intensity as a function of Galactic longitude (black andred respectively). The intensities have been integrated over the ±
200 km s − in v lsr and the ± ◦ range in latitude for each longitude. The flux scale has beennormalised to the peak intensity of the CO (2 – 1) emission. The C O (2 – 1) spectrum has been multiplied by 5 and an offset of − .000
200 km s − in v lsr and the ± ◦ range in latitude for each longitude. The flux scale has beennormalised to the peak intensity of the CO (2 – 1) emission. The C O (2 – 1) spectrum has been multiplied by 5 and an offset of − .000 , 1–18 (2020) F. Schuller et al.
Figure 7.
Channel maps of CO (2 – 1) of the ℓ = ◦ cube, which includes the high-mass star forming region W33. The upper-left panel shows the emissionintegrated over the full velocity range where emission is seen in this direction (0 to 60 km s − ), while the other panels show the emission integrated over velocityintervals of ∼ − . The central velocities over which the integration has been performed are given in the upper-left corners of each map. The emission ineach map has been scaled to the brightest emission in each map. (0–60 km s − ). Each of the subsequent panels shows a channel mapwhere the emission has been integrated over 6 km s − in velocity.These maps reveal that the CO (2 – 1) emission seen towards thisregion consists of dense clumps, diffuse larger clouds and numer-ous filamentary structures spread out over 60 km s − . It is worthnoting that there is a wealth of intricate features that emerge in in-dividual channel maps, but that do not appear or are washed out inthe integrated intensity map. This also implies that, even if most ofthe molecular gas is associated with a few major complexes as dis-cussed above, there are plenty of other smaller features detected inthe SEDIGISM data that are not associated with known complexes. The ℓ𝑣 -map presented in Fig. 6 clearly shows that the molecular gasis broadly correlated with the spiral arms. To properly map the dis-tribution of the molecular gas across the disc requires determiningdistances, which is beyond the scope of the current paper but isdiscussed in the accompanying paper by Duarte-Cabral et al. (Pa-per III). However, it is possible to examine the intensity distributionas a function of the Galactocentric distance. This is accomplished bycalculating the kinematic distance for each pixel in the ℓ𝑣 -map abovea 3 𝜎 rms threshold using the three-component rotation curve of Eilerset al. (2019) and the Reid et al. (2019) values for solar position andvelocity, 8.15 kpc and 236 km s − (as described in Sect. 3). Althoughthis produces two distances for sources located within the solar cir-cle (i.e. with a Galactocentric radius 𝑅 gc < .
15 kpc) equally spacedon either side of the tangent distance (referred to as the near andfar distances) it provides a unique distance from the Galactic Centrewhich, therefore, allows us to investigate the distribution of the inte-grated intensity as a function of Galactocentric distance. Given thatthe spiral structure is different in the 1 st and 4 th quadrants, and thatthe SEDIGISM survey has only covered a small portion of the 1 st Figure 8.
Integrated CO (2 – 1) intensity as a function of Galactocentricdistance. The emission has been integrated over intervals of 0.1 kpc and onlyincludes longitudes between 300 ◦ < ℓ < ◦ . quadrant, we have restricted this analysis to the 4 th quadrant. We havealso excluded the Galactic Centre region ( | ℓ | < ◦ ) as kinematicdistances are unreliable in this part of the Galaxy. Even outside thisregion, this approach cannot provide very accurate distances becauseof non-circular motions that deviate from the rotation curve, but itallows us to roughly estimate the fraction of molecular gas that isassociated with the spiral arms.In Fig. 8, we show the integrated CO (2 – 1) intensity as a func-tion of Galactocentric distance, normalised to the peak of the dis-
MNRAS , 1–18 (2020)
EDIGISM first data release tribution. This plot shows that the emission from the molecular gasis highly structured with strong peaks seen at approximately 4.25,5, 5.5 and 6.5 kpc; the first of these roughly corresponds to the tan-gent with the Norma arm, the second and third correspond to the farside of the long-bar where it intersects with the Perseus arm (Bland-Hawthorn & Gerhard 2016), and the fourth with the tangent of theScutum-Centaurus arm. The vast majority of the emission is con-tained between 4 and 7.5 kpc. The lack of emission below 2 kpc isdue to the restricted longitude range selected for this analysis, whilethe lack of emission at distances greater than 8 kpc likely reflectsthe poor sensitivity to molecular material on the far-side of the solarcircle and beyond, where beam dilution certainly plays an impor-tant role. This thick emission zone is analogous to the thick ringof material seen in the 1 st quadrant (often referred to as the 5 kpcmolecular ring), but as pointed out by Jackson et al. (2006), is likelyto arise from a complicated combination of column density and ve-locity fields and may not actually represent a real ringlike structure(see also Dobbs & Burkert 2012). The highly structured nature ofour CO (2 – 1) emission further lends support for a 4-arm model ofthe Galaxy (Urquhart et al. 2014a), which can nevertheless co-existwith a ringlike structure.In order to quantify what fraction of the total emission is associatedwith the spiral arms we have calculated the minimum offset from thearms for each pixel on the ℓ𝑣 -map above 3 𝜎 rms . In Fig. 9 we showthe cumulative distribution of the integrated CO (2 – 1) emissionas a function of velocity offset from the spiral arm loci shown in theupper panel of Fig. 6. When performing the matching of the pixelswith the spiral arms we allowed for a variation of ± . ◦ in Galacticlongitude as the spiral arm tangents are not well constrained. Weconsider pixels within Δ 𝑣 <
10 km s − , which is of the order ofthe amplitude of streaming motions around the spiral arms ( ∼ − ; Burton 1971; Stark & Brand 1989; Reid et al. 2009), tobe associated with a spiral arm. This plot reveals that approximately60 per cent of the molecular emission is closely associated with aspiral arm. This proportion is a little lower than the value of 80 percent derived by Urquhart et al. (2018) from a similar analysis of GRSclouds identified by Rathborne et al. (2009). However, as pointed outby Roman-Duval et al. (2009) only approximately two-thirds of theemission in the GRS was accounted for in the source extraction withdiffuse emission below the detection threshold accounting for the rest.The strong correlation we have found between the CO emissionand the spiral arms is consistent with the findings of Roman-Duvalet al. (2009) and Rigby et al. (2016). Nevertheless, this analysis alsoindicates that a significant amount of molecular gas (up to 40 per cent)is located in the inter-arm regions.
Although it is clear that the majority of the CO emission outside theCMZ is closely associated with the spiral arms, there are a numberof interesting features seen in the ℓ𝑣 map that are worth discussingin some detail. In Fig. 10 we show the ℓ𝑣 -map of the Galactic Centre region. Theemission in this region can be separated into two distinct types: a nar-row horizontal strip of emission centred around v lsr = 0 that stretchesacross the whole map, and a region of more complex emission be-tween 359 ◦ < ℓ < ◦ and velocities −
150 km s − < v lsr <
150 km s − .The horizontal strip around v lsr = 0 is the result of foreground and Figure 9.
Cumulative integrated CO (2 – 1) intensity as a function of veloc-ity offset from the nearest spiral arm for longitudes between 300 ◦ < ℓ < ◦ .The velocity offsets have been calculated by finding the minimum velocitydifference to a spiral arm for each pixel in the ℓ𝑣 map with a flux above 3 𝜎 . background emission within the Galactic disc, while the CMZ itselfis responsible for emission over a large range of v lsr . The CMZ is apeculiar region of the inner Galaxy that includes a number of largemolecular complexes such as Sagittarius A ( ℓ =
0, v lsr = 50 km s − ),Sagittarius B ( ℓ = . ◦ , v lsr = 50 km s − ), Sagittarius C ( ℓ = . ◦ ,v lsr < − ) and Sagittarius D ( ℓ = .
9, v lsr = 80 km s − ), eachcovering more than 10 arcmin ; these molecular complexes are la-belled in Fig. 10. This map nicely shows the complex kinematics inthe Galactic centre region, in particular the presence of non-circularmotions and gas emission at forbidden velocities (negative for ℓ > ◦ and positive for ℓ < ◦ ; Riquelme et al. 2010 and references therein).In addition to these two large-scale features, we can also see somefiner scale detail such as the narrow absorption features at ℓ = . ◦ with v lsr (cid:39) −
50 km s − , −
30 km s − and 0 km s − ; these features aredue to absorption of the strong emission emanating from the hot gasin the CMZ by the colder foreground segments of the 3 kpc, Norma,and Sagittarius arms (previously observed in HCO + and HCN; e.g.Fukui et al. 1977, 1980; Linke et al. 1981; Riquelme et al. 2010). Onthis plot we have also overlaid the loci of the 3 kpc arms. Comparingthe molecular emission with the loci of the near 3 kpc arm (indicatedby the lower dashed-dotted yellow line shown in Fig. 10), we findgood agreement with the absorption feature seen at ℓ = . ◦ and −
50 km s − , which we have already attributed to this arm. We also seesome association with molecular emission along its length, althoughthis emission is weak and rather sporadic. It is also interesting to notethe velocity of the absorption feature associated with the Norma arm( ∼ −
30 km s − ), while the model loci of this arm pass very close tov lsr = 0 at this longitude. In Fig. 11 we show the Bania complex of molecular clouds (Bania1977; Bania et al. 1986), which consist of three large (40–100 pc) dis-tinct molecular complexes located in a narrow longitude range closeto the Galactic centre ( ℓ between 354.5 and 355 . ◦ ) with v lsr veloc-ities of 68, 85 and 100 km s − . Adopting the nomenclature from theoriginal papers, these are known as Clump 4, Clump 3 and Clump 1, MNRAS000
30 km s − ), while the model loci of this arm pass very close tov lsr = 0 at this longitude. In Fig. 11 we show the Bania complex of molecular clouds (Bania1977; Bania et al. 1986), which consist of three large (40–100 pc) dis-tinct molecular complexes located in a narrow longitude range closeto the Galactic centre ( ℓ between 354.5 and 355 . ◦ ) with v lsr veloc-ities of 68, 85 and 100 km s − . Adopting the nomenclature from theoriginal papers, these are known as Clump 4, Clump 3 and Clump 1, MNRAS000 , 1–18 (2020) F. Schuller et al.
Figure 10.
Longitude-velocity map of the Galactic centre region. The lower and upper yellow dashed-dotted lines show the loci of the near and far 3 kpc armsrespectively (see text for details). On this map we label some of the more significant molecular clouds and absorption features that have been attributed toforeground spiral arms; some of these are discussed in the text.
Figure 11.
Longitude-velocity map of the Bania Clouds. The features de-scribed in the text are labelled following the nomenclature used by Bania(1977). as labelled in Fig. 11 (the reference Clump 2 was given to anotherobject located at ℓ (cid:39) ◦ ). This region is unusual in that it is associatedwith velocities that are forbidden by Galactic rotation models. Evenif this complex was located outside the solar circle on the far-sideof the Galaxy at a distance of 40 kpc, the maximum velocity thatwe would expect in this direction would be ∼
16 km s − (Burton &Gordon 1978).These clouds were originally mapped in CO (1-0) where all ofthe clouds were detected with good signal to noise (Bania 1980,1986). However, in the less abundant CO (2 – 1) tracer, Clump 3and Clump 4 are only weakly detected. Clump 1 is much brighterand appears to be elongated, extending over 1 ◦ in longitude. Thiscloud is also the only one of the three that is associated with an H iiregion (G354.67+0.25, Caswell & Haynes 1982), which is located atthe western edge of the cloud.Bania (1986) suggested that this complex could be associated with a feature that he refers to as the 135 km s − arm, which can bereproduced by a Galactocentric ring of material with a radius of 3 kpcrotating at a velocity of 222 km s − and expanding from the Galacticcentre at a speed of 135 km s − . Clump 1 is located at the southernterminus of this structure, at a distance of 11.4 kpc. This large scalestructure is not seen in our ℓ𝑣 -map but is clearly seen in the ℓ𝑣 -map ofDame et al. (2001) (see Fig. 2 from Jones et al. 2013). However, thenature of this 135 km s − arm is contentious (see discussion by Joneset al. 2013) and it is not clear if Clump 1 is part of this structure oris entering the dust lane (Liszt & Burton 1980). Modern simulationefforts often attribute these features to stem from gas approachingthe far end of the bar, about to begin the journey back towards theGalactic centre (Baba et al. 2010; Li et al. 2016b; Sormani et al.2018). Examination of the ℓ𝑣 -map has revealed the existence of a populationof unusual clouds. These appear as very narrow horizontal linesin the ℓ𝑣 -map (see Fig. 12 for some examples), so much so, thatwe initially thought them to be artificial, perhaps caused by spikesin the spectrometers or due to artefacts introduced during the datareduction procedure. However, on closer examination these werefound to be extended over large areas ( ∼ ◦ in diameter) and tohave morphologies typical of molecular clouds (see Fig. 13 for anexample of their structure).These clouds have three primary characteristics; they are large insize, they have very narrow line-widths (FWHM ∼ − ) andthey tend to have velocities close to the solar one (i.e. v lsr close tozero). In Table 4, we summarise the positions and velocities for sevenof these clouds clearly seen in the ℓ𝑣 -map. Their velocities and largeangular sizes would suggest that the majority of these are local clouds.However, we note that one (Cloud 2) has a velocity that would place it MNRAS , 1–18 (2020)
EDIGISM first data release Figure 12.
This map is a zoom of a region of the CO (2 – 1) ℓ𝑣 -map presented in Fig. 6, which contains three elongated clouds that have very narrowline-widths (FWHM ∼ − ). We have classified them as wispy clouds. Figure 13. CO (2 – 1) emission integrated over the line-width for Cloud 4(see Table 4 for more details). at a larger distance. Given their narrow line-widths it is possible thatthese types of clouds have been missed in previous surveys where thevelocity resolutions were > . − as they would only be 1 or 2velocity channels wide and discarded as artefacts. It would thereforebe interesting to investigate these objects in more detail, however,their near proximity to the Sun makes kinematic distances unreliableand, without these, determining physical properties is not possible.Typical molecular clouds have FWHM line-widths of a few km s − (e.g. Paper III) and given that the thermal contribution is of the or-der of 0.3 km s − (assuming a temperature of 10 −
20 K) most of themotion in these clouds is non-thermal in nature, and often attributedto turbulence. The clouds identified in this Section are unusual inthat their line-widths are much narrower than typically found formolecular clouds, and, therefore, the thermal and non-thermal com-ponents appear roughly balanced. In Fig. 14 we show an exampleof line-width for Cloud 4; this has been produced by integrating theemission seen in the ℓ𝑣 -map in longitude (i.e. along its length). Giventhat the non-thermal energy can work to support clouds against grav-itational collapse, such low values could indicate that these cloudswould be potentially unstable to collapse, if they were associatedwith sufficient mass. In the absence of a robust distance estimate, wecannot determine the masses of the clouds, and thus are limited inour ability to make any further analysis on the nature of these clouds.Nevertheless, the fact that these are not seen in the C O data sug-gests that they either have low excitation temperatures or low columndensities and are, therefore, rather diffuse and perhaps transient.We note that the most striking of these clouds are located in the4 th quadrant. This potentially highlights a subtle difference between Figure 14. CO (2 – 1) emission integrated along the length of Cloud 4 lo-cated at ℓ = . ◦ and v lsr = 5.25 km s − . The black line shows the integratedemission, which has been centred on the v lsr of the source, while the red lineshows the result of a Gaussian fit to the profile (FWHM 0.66 km s − , seeTable 4). Table 4.
Measured properties of the diffuse clouds.Cloud id. ℓ min ℓ max v lsr FWHM( ◦ ) ( ◦ ) (km s − ) (km s − )1 351 . . . .
652 347 . . − .
75 0 .
983 346 . . .
75 0 .
754 343 . . .
25 0 .
665 342 . . .
25 0 .
616 340 . . . .
717 338 . . − . . the distribution of molecular gas in the 1 st and 4 th quadrants inthat there is very little material with a v lsr close to zero in the 1 st quadrant, and thus fewer local molecular clouds in the portion ofthe 1 st quadrant mapped by SEDIGISM than in the 4 th quadrant.Interestingly, similar clouds with narrow line-widths can also be seenin the CO(3–2) ℓ𝑣 map of CHIMPS (Rigby et al. 2016, Fig. 6), for MNRAS000
717 338 . . − . . the distribution of molecular gas in the 1 st and 4 th quadrants inthat there is very little material with a v lsr close to zero in the 1 st quadrant, and thus fewer local molecular clouds in the portion ofthe 1 st quadrant mapped by SEDIGISM than in the 4 th quadrant.Interestingly, similar clouds with narrow line-widths can also be seenin the CO(3–2) ℓ𝑣 map of CHIMPS (Rigby et al. 2016, Fig. 6), for MNRAS000 , 1–18 (2020) F. Schuller et al. instance a chain of clouds running from +10 to +15 km s − in v lsr over the 32 . ◦ ≤ ℓ ≤ . ◦ longitude range. As previously mentioned (Sect. 2.1), transitions from several othermolecules are included in the spectral tuning used by SEDIGISM,and the data cubes are also publicly available as part of the cur-rent data release. This includes spectral cubes for H CO(3 , -2 , ) and (3 , − , ), HC N(24-23), SO(6 − ), SiO(5 – 4) andHNCO(10 , -9 , ), with 0.5 km s − velocity resolution. These tran-sitions have lower state energies between 10 K and 340 K (cf. table 1in Paper I), and thus probe a wide range of physical conditions.In particular, spectral lines from formaldehyde (H CO), SO, andcyanoacetylene (HC N) are commonly detected toward high-massstar-forming regions (e.g. Sutton et al. 1985; Belloche et al. 2013;Tang et al. 2018; Duronea et al. 2019).These lines are much weaker than the CO (2 – 1) and C O (2 –1) lines discussed so far and are likely only detected towards thedensest regions. To explore the utility of these additional lines inour survey we have searched for the possible detection of thesemolecules in the 2 ◦ × ◦ field centred at 334 ◦ , which is neither toocrowded nor too empty and as such representative of the full surveydata. Within this field, there are only two positions where non-COlines are detected: toward the ATLASGAL massive dense clumpsAGAL G333.604 − − CO (2 –1) and C O (2 – 1) in this field, we detect H CO (3 , − , ) and(3 , − , ), HC N (24 − − ) with peak brightnesstemperatures of 1 − , − , ), are not detected.In order to improve our sensitivity to weaker emission we havealso performed a stacking analysis to search for emission from otherspecies using s pectral-cube . We analysed the signal-to-noise ratioin the extracted lines vs. intensity threshold in the CO (2 – 1) linein steps of 5 K, and we found that most detected lines peaked at athreshold of 30 K. Then, we selected all voxels within the CO (2 –1) cube covering the G334 field where the CO (2 – 1) brightnesswas above a set threshold of 30 K. For each pixel in those regions, weused the first moment map to select the peak velocity. The weak linespectra (i.e., HNCO and SiO) corresponding to these positions werethen shifted by the CO (2 – 1) velocity and averaged, such that anysignal is expected to have velocity offset close to zero. In Fig. 15 weshow the results of our stacking analysis, which reveals that we haveindeed detected a weak feature in the HNCO(10 – 9) line at 5- 𝜎 inintegrated intensity. We have also detected a stronger feature in theSiO transition ( ∼ 𝜎 ), but this feature is not peaked at 0 km s − andis much broader; since high-excitation SiO primarily traces outflows,this broad, offset feature may represent the average of several outflowfeatures over the G334 field.Using this approach, we can also increase the signal-to-noise ofthe detection of the more prominent species (e.g. SO, H CO andHC N; see Fig. 15). The success of this semi-blind stacking approachsuggests that detailed studies of Galactic-scale cloud chemistry willbe possible despite the survey’s relatively short integration times perposition.Finally, we want to highlight that the most extreme star forming https://spectral-cube.readthedocs.io/ regions of our Galaxy exhibit extended emission in several of theseweaker lines. We illustrate this in Fig. 16, where we show an exampleof the integrated intensity maps of all lines towards a region aroundSgr B2, one of the most active star forming sites in our Galaxy,located close to the Galactic Centre (see Fig. 10 for location), wherewe detect extended emission in the SO (5–4), SiO (5–4), HNCO(10 , -9 , ), CH OH (4 , -3 , ) and the H CO 3 , -2 , lines. Here we have presented the first public data release of the SEDIGISMsurvey, which covers 84 deg of the inner Galactic plane in CO (2 –1) and C O (2 – 1), at 30 (cid:48)(cid:48) angular resolution and a typical noise levelof order 1 K ( 𝑇 mb ) at 0.25 km s − resolution. Future data releases mayaddress remaining issues such as the baseline subtraction in complexregions and other artefacts not fully addressed with the current re-duction pipeline (e.g. a spike near v lsr -48 km s − in C O (2 – 1)spectra). All data products extracted from this data set, in particulara catalogue and masks of molecular clouds (Paper III) are also beingmade publicly available alongside this data release, thus providingthe community with high added-value products that are complemen-tary to other surveys. This will constitute an invaluable resource forMilky Way studies in the southern hemisphere.In this paper we have provided an up-to-date description of thedata reduction procedure and data products, and highlighted someknown issues with some of the fields. We also discussed the Galacticdistribution of the molecular gas and investigated its correlation withknown star-forming complexes and the large scale structural featuresof the Galaxy such as the spiral arms. Overall, the data appearsconsistent with a 4-arm model of the Galaxy. Using the model fromTaylor & Cordes (1993) and updated by Cordes (2004), we foundthat ∼
60 per cent of the CO (2 – 1) emission is tightly associatedwith the spiral arms (i.e. within 10 km s − of an arm) and very clearpeaks can be seen in the distribution of intensities with Galactocentricdistance, that can be attributed to specific spiral arms.We have also shown how the velocity information allows us toanalyse the complex nature of the molecular gas, which can be sepa-rated into different scale structures (filamentary, diffuse and compactstructures). This also allows us to investigate the large scale dynamicsof the interstellar medium. We have also demonstrated the feasibilityof using transitions from less abundant molecular species for scienceexploitation. Finally, we have highlighted some interesting regionswhere the SEDIGISM survey can provide either a new perspective(i.e. Galactic centre region and the Bania molecular complex) oridentify a new population of local molecular clouds, which appearas large ( ∼ ◦ ) features near v lsr = 0 km s − and with very narrow( < − ) line-widths.A systematic exploitation of the full survey data is now underway. We have also used the SEDIGISM data to confirm the natureof filaments previously identified in ATLASGAL (Li et al. 2016a)and to explore their kinematics and mass per unit length (Matternet al. 2018). In parallel to this work we are publishing a catalogueof nearly 11,000 molecular clouds and complexes extracted withSCIMES (Colombo et al. 2015), for which we derived distancesand physical properties (Paper III), and a systematic assessment ofthe dense gas fraction and star formation efficiency as a function ofenvironment (Paper IV).In addition to these studies there are a number of ongoing projectsaimed at extracting and characterising very long filaments directlyfrom the spectral cubes. We also plan to constrain further the large-scale Galactic structure (number and position of the spiral arms, MNRAS , 1–18 (2020)
EDIGISM first data release Figure 15.
Stacked spectra over the G334 field for the transitions SO(5 – 4), H CO(3 , –2 , ), H CO(3 , –2 , ), HC N(24 – 23), HNCO(10 – 9) and SiO(5 – 4).The red line in each panel shows the best-fit Gaussian profile.
Figure 16.
Integrated intensity maps between +
40 and +
90 km s − towards a region around Sgr B2 showing extended emission in several molecular tracers. Thelinear scale corresponds to 3–120 K km s − for each panel, except the CO line, where it goes up to 240 K km s − . The weakest line of H CO (3 , -2 , is notshown here. orientation of the bar), by comparing the SEDIGISM data with theresults of simulations. By exploiting the SEDIGISM and ThrUMMS(Barnes et al. 2015) data together, we will also characterise the ex-citation conditions of the interstellar medium over a large fraction ofthe Galaxy, extending the exploratory work presented in Paper I tothe full survey coverage. Other topics under study include the anal-ysis of the dynamical properties and turbulence in giant molecularcomplexes.Clearly, a lot of novel studies can be carried out based on theSEDIGISM data alone, and the exploitation of this survey combinedwith data from other spectroscopic or continuum surveys opens new perspectives for a detailed investigation of the structure and physicalconditions of the interstellar medium. ACKNOWLEDGEMENTS
We thank the anonymous referee for their positive report and con-structive comments . We are very grateful for the continuous sup-port provided by the APEX staff. FS acknowledges support froma CEA/Marie Sklodowska-Curie Enhanced Eurotalents fellowship.ADC acknowledges support from the Royal Society University Re-search Fellowship URF/R1/191609. LB acknowledges support from
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DATA AVAILABILITY
The data presented in this article is available from a dedicated web-site: https://sedigism.mpifr-bonn.mpg.de
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Figure A1. CO (2 – 1) spectrum averaged over a small region of the0.5 × sub-field 303.25+0.25, before correction (black) and after cor-rection (red). The spectrum measured toward the reference position (bottom,shown in green) has been added to each observed spectrum in the data-cube. Dept. of Space, Earth and Environment, Chalmers University of Technol-ogy Onsala Space Observatory, 439 92 Onsala, Sweden Haystack Observatory, Massachusetts Institute of Technology, 99 MillstoneRoad, Westford, MA 01886, USA INAF - Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047Selargius (CA), Italy Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France School of Engineering, Macquarie University, NSW 2109, Australia McMaster University, 1 James St N, Hamilton, ON, L8P 1A2, Canada European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748Garching bei München, Germany Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yi-heyuan Road, Haidian District, Beijing 100871, People’s Republic of China
APPENDIX A: SPECTRA OBSERVED TOWARDSREFERENCE POSITIONS
As described in Sect. 2.1, position-switching observations were doneusing a fixed reference position for each map. These reference posi-tions were located at ± ◦ in galactic latitude. We have done pointedobservations in CO (2 – 1) (also in position-switching mode) to-wards all these reference positions, using an off position located onedegree further away from the galactic plane (i.e. at ± ◦ in lati-tude). In some rare cases where we have found that emission was stillpresent in the off position (detectable as an absorption feature in thespectrum), we have repeated the observations using another, nearbyoff position.The data were processed using standard procedures inGILDAS/CLASS. The spectra were smoothed to 0.2 km s − res-olution. We list in Table A1 the measured rms at that resolution,and the number of lines detected and their v lsr velocities. Only thelines detected at more than 5- 𝜎 (in integrated intensity) are listed.The spectrum observed towards the reference position was then sub-tracted from the on-source data only when at least one line wasdetected. Fig. A1 shows an example case where such a correctionwas required. APPENDIX B: COMPARISON BETWEEN SEDIGISM ANDHERO DATA ACROSS THE W43 REGION
In this section we compare the observations of the W43 region fromSEDIGISM and from the W43 Hera/EmiR Observations (HERO,
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Table A1.
Spectra observed towards the reference positions: measured rms and spectral lines detected. The positions (Col. 1) give the galactic coordinates ofthe associated on-source maps. The full table is available in the online version of the paper.Position 𝜎 rms [K] Lines detected( 𝛿 V = − )303.25+0.25 0.066 two lines: T peak = 0.69 K at v lsr = -37.7 km s − , T peak = 0.85 K at -3.5 km s − peak = 0.90 K at v lsr = -1.1 km s − peak = 0.42 K at v lsr = -40.4 km s − , T peak = 2.60 K at -29.7 km s − , T peak = 0.43 K at -3.3 km s − peak = 1.51 K at v lsr = -26.3 km s − Carlhoff et al. 2013) project, observed with the IRAM 30m telescope.Both surveys imaged this region in CO(2-1) and C O(2-1) lines.The HERO data have better angular (12 (cid:48)(cid:48) versus 30 (cid:48)(cid:48) ) and spectral(0.15 km s − versus 0.25 km s − ) resolutions than SEDIGISM, witha similar sensitivity (typically 1 K 𝑇 mb per 0.15 km s − channel, butwith smaller pixels; Carlhoff et al. 2013).To compare the data from the SEDIGISM and HERO surveys wefirst smoothed and interpolated both data sets to a common resolu-tion of 35" and 0.5 km s − . To do so we used the python packagespectral_cube and the smoothing procedures described in its doc-umentation . Since the two data sets do not cover exactly the sameregion, we regridded the HERO data on the SEDIGISM data grid,using the astropy reproject function.To consider only the significant emission in the comparison, wemask the data using a dilate masking technique (Rosolowsky & Leroy2006). This technique consists of generating two masks, at relativelylow and high signal-to-noise ratio (SNR). Connected regions in theposition-position-velocity (PPV) space in the low signal-to-noiseratio mask that do not contain a region of the high signal-to-noiseratio mask are eliminated from the final mask. The result is an actualexpansion of the high signal-to-noise regions to lower significantemission, without the inclusion of noisy peaks. For our purposeswe consider 𝑆𝑁 𝑅 ≥
10 for the high signal-to-noise ratio mask and
𝑆𝑁 𝑅 ≥ lsr ) for the two data sets in Fig. B1 for CO (left column) and C O(right column).It is clear from Fig. B1, that the HERO data have a lower noisethan SEDIGISM, since less pixels are masked. Beside this, the COintegrated maps from the two surveys appear largely alike (Fig. B1,left column). To avoid biases due to the different sensitivity in thetwo surveys, we have also computed integrated CO maps consid-ering only voxels above a fixed value of 𝑇 mb = 5 K (Fig. B2).The ratio between the two integrated intensity maps appears mostlyconsistent around unity (within the calibration uncertainty), whichdemonstrates that both data sets are consistent. Since the C O (2 –1) line emission is much weaker than CO, we can only comparethe brightest regions between both data sets (Fig. B1, right column).We notice some slight changes for the distribution of the C O emis-sion between SEDIGISM and HERO, but there is no pronouncedsystematic difference visible. https://spectral-cube.readthedocs.io/en/latest/smoothing.html MNRAS , 1–18 (2020)
EDIGISM first data release Figure B1.
Integrated intensity maps from SEDIGISM (top) and HERO (bottom) CO (left) and C O (right) data. SEDIGISM and HERO data have beenhomogenized and masked as described in Section B.
Figure B2.
Integrated CO (2 – 1) intensity maps from SEDIGISM (left) and HERO (middle), and absolute ratio between the two maps (right). Only voxelsabove a threshold of 5 K have been considered. MNRAS000