The Three-mm Ultimate Mopra Milky Way Survey. II. Cloud and Star Formation Near the Filamentary Ministarburst RCW 106
Hans Nguyen, Quang Nguyen Luong, Peter G. Martin, Peter J. Barnes, Erik Muller, Vicki Lowe, Nadia Lo, Maria Cunningham, Frédérique Motte, Stefan N. O'Dougherty, Audra K. Hernandez, Gary A. Fuller
DDraft version August 14, 2018
Preprint typeset using L A TEX style emulateapj v. 5/2/11
THE THREE-MM ULTIMATE MOPRA MILKY WAY SURVEY. II. CLOUD AND STAR FORMATION NEARTHE FILAMENTARY MINISTARBURST RCW 106
Hans Nguy ˜ˆe n , Quang Nguy ˜ˆe n Lu’o’ng , Peter G. Martin , Peter J. Barnes
5, 6 , Erik Muller , Vicki Lowe ,Nadia Lo , Maria Cunningham , Fr ´e d ´e rique Motte, Balthasar Inderm¨uhle , Stefan N. O’Dougherty , AudraK. Hernandez , Gary A. Fuller (Dated: Received 2014 July10; accepted 2015 February 9; published 2015 MM DD) Draft version August 14, 2018
ABSTRACTWe report here a study of gas, dust and star formation rates (SFRs) in the molecular cloud complexes(MCCs) surrounding the giant H II region RCW 106 using CO and CO (1-0) data from the Three-mm Ultimate Mopra Milky way Survey (ThrUMMS) and archival data. We separate the emissionin the Galactic Plane around l = 330 ◦ -335 ◦ and b = − ◦ -1 ◦ into two main MCCs: the RCW 106(V LSR = −
48 km s − ) complex and the MCC331-90(V LSR = −
90 km s − ) complex. While RCW 106(M ∼ . × M (cid:12) ) is located in the Scutum-Centaurus arm at a distance of 3.6 kpc, MCC331-90 (M ∼ . × M (cid:12) ) is in the Norma arm at a distance of 5 kpc. Their molecular gas mass surface densitiesare ∼
220 and ∼
130 M (cid:12) pc − , respectively. For RCW 106 complex, using the 21 cm continuumfluxes and dense clump counting, we obtain an immediate past ( ∼ -0.2 Myr) and an immediate future( ∼ +0.2 Myr) SFRs of 0 . +0 . − . M (cid:12) , yr − and 0 . ± . (cid:12) yr − . This results in an immediate pastSFR density of 9 . +3 . − . M (cid:12) yr − kpc − and an immediate future SFR density of 4 . +3 . − . M (cid:12) yr − kpc − .As both SFRs in this cloud are higher than the ministarburst threshold, they must be undergoinga ministarburst event although burst peak has already passed. We conclude that this is one of themost active star forming complexes in the southern sky, ideal for further investigations of massive starformation and potentially shedding light on the physics of high-redshift starbursts. Subject headings: stars: formation, stars: protostars, ISM: clouds, ISM: structure, ISM: HII regions INTRODUCTION
Studying the earliest phases of star formation involvesexamining the morphological and kinematic structure ofmolecular clouds and the transformation between differ-ent states of materials. Massive star formation often oc-curs in molecular cloud complexes (MCCs) with radius ∼
70 pc or more (e.g., W43; e.g., Nguyen Luong et al.2011a; Motte et al. 2014), Cygnus X with radius ∼
80 pc Canadian Institute for Theoretical Astrophysics, Universityof Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel69, 53121 Bonn, Germany EACOA Fellow at NAOJ, Japan & KASI, Korea National Astronomical Observatory of Japan, Chile Obser-vatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Astronomy Department, University of Florida, P.O. Box112055, Gainesville, FL 32611, USA School of Science and Technology, University of New Eng-land, NSW 2351, Australia CSIRO Astronomy and Space Science, P.O. Box 76, Epping,NSW 1710, Australia School of Physics, University of New West Wales, NSW 2052Australia Departamento de Astronom´ıa, Universidad de Chile, CaminoEl Observatorio 1515, Las Condes, Santiago, Casilla 36-D, Chile Laboratoire AIM Paris-Saclay, CEA/IRFU - CNRS/INSU- Universit´e Paris Diderot, Service d’Astrophysique, Bˆat. 709,CEA-Saclay, F-91191, Gif-sur-Yvette Cedex, France College of Optical Sciences, University of Arizona, 1630 E.University Blvd., P.O. Box 210094, Tucson, AZ 85721, USA Astronomy Department, University of Wisconsin, 475 EastCharter St., Madison, WI 53706, USA Jodrell Bank Centre for Astrophysics, Alan Turing Build-ing, School of Physics and Astronomy, University of Manchester,Manchester, M13 3PL., UK † [email protected] (e.g., Schneider et al. 2006; Motte et al. 2007), and theCentral Molecular Zone (CMZ) with radius ∼
180 pc(e.g., Miyazaki & Tsuboi 2000; Jones et al. 2012). MCCsassociated with massive star formation reside mainlyalong the mid-plane of the Galaxy, may be distant andheavily obscured in the visible regime. Though not be-ing obscured in the radio wavelength, Large-field radiosurveys of MCCs generally yield data sets with a sub-stantial field of view but insufficient resolution to con-duct case-by-case studies of particular clouds. This isparticularly true for low resolution surveys such as thecarbon monoxide (CO) survey of the Milky Way (Bronf-man et al. 1989; Dame et al. 2001). While they catalogmolecular clouds, the structure and properties of eachcloud cannot be fully resolved. However, recent high-angular-resolution spectroscopic surveys of the Galacticplane such as the Galactic Ring Survey (GRS; Jacksonet al. 2006) and the Three-mm Ultimate Mopra Milkyway Survey (ThrUMMS; Barnes et al. 2013) help to ad-dress this problem.Of particular interest here is the MCC of gas surround-ing the bright giant H II region RCW 106, which was dis-covered in the H α emission line survey of the southernMilky Way (Rodgers et al. 1960). The giant H II re-gion RCW 106 hosts a cluster with mass ∼ M (cid:12) anda Lyman continuum photon emission of 10 s − , likelyoriginating from dozens of O-type stars ( M > (cid:12) ).It resides in the Scutum–Centaurus arm at a distanceof ∼ . a r X i v : . [ a s t r o - ph . GA ] A p r H. Nguyen, Q. Nguyen Luong, P. G. Martin, et al.
Table 1
Observation parameters of the data used in this study
Instrument Tracer Frequency Ang. Res Velocity ∆ v res σ rmsor Survey (GHz) ( (cid:48)(cid:48) ) (km s − ) (km s − )CfA CO 115.27 510 -165 to 165 1.3 0.1 K km s − ThrUMMS CO 115.27 72 -140 to 10 0.33 ∼ − ThrUMMS CO 110.20 72 -140 to 10 0.34 ∼ − SGPS H I − SGPS 21 cm cont. 1.420 132 - - 1 mJy beam − MIPS/
Spitzer µ m 6 - - 0.01 MJy sr − PACS/
Herschel µ m 12 - - 0.08 MJy sr − SPIRE/
Herschel µ m 37 - - 1.2 MJy sr − Moreover, the parent RCW 106 MCC is also the siteof ongoing star formation. It is centered on l ∼ ◦ , b ∼ − . ◦ V LSR ∼ −
50 km s − (Bains et al. 2006).Its substructureshave been subject to a variety of spectral line and con-tinuum studies, progressively unravelling the star forma-tion and evolutionary history of the region. Specifically,(Lynga 1964) and (Urquhart et al. 2007) measured theimpact of UV emitted by various nearby OB stars thationize the surrounding environment. 1.2 mm continuummaps revealed 95 clumps with masses ranging from 40 to10 M (cid:12) , some of which have infrared (IR) counterpartssuggesting embedded star formation (Mookerjea et al.2004). Higher density tracer such as CO , CS, HCO + ,HCN, and HNC emission lines show prominent velocityfeatures centered on the V LSR of RCW 106 that coincidewith 1.2 mm dust clumps and are also sites of massivestar formation (Bains et al. 2006; Wong et al. 2008; Loet al. 2009). A recent NH survey toward dense clumps ofthe RCW 106 complex revealed a large sample of cold col-lapsing clumps that are potentially forming stars (Loweet al. 2014).The two densest and most massive sites of ongoingstar formation core are MMS5 and MMS68 (Mookerjeaet al. 2004). They are associated with the ultracom-pact H II (UCH II ) regions G333.6–0.2 (Fujiyoshi et al.2005, 2006) and IRAS 16164-5096, respectively. Highresolution adaptive optics observations with 0. (cid:48)(cid:48) ∼ ∼ −
100 pc), which is the largest scale of molecu-lar cloud in the Galaxy but the smallest scale that canbe probed in nearby galaxies. On this scale, RCW 106is a massive filamentary molecular cloud complex withan aspect ratio larger than 2: see Figure 5. Our goalis to study the global properties of the molecular cloudssurrounding RCW 106 and establish a relationship be-tween the large scale structure of these clouds and theirenhanced star formation.The various data sets used in this paper are describedin Section 2. We present the two star-forming MCCs identified using mainly the CO emission in Section 3.The distances of these two cloud complexes are investi-gated in Section 4. Physical properties highlighting massand mass surface density are discussed in Section 5. Thedynamical nature of the clouds are investigated in Sec-tion 6. In Section 7, we discuss the method of predctingthe SFRs and the evidence that the RCW 106 complexcan be quanlified as a ministarburst.Finally, we summarize our findings in Section 8. DATA
To investigate the properties of the region we used var-ious continuum and spectral line tracers, summarized inTable 1 and discussed below. CO molecular line from the ThrUMMS survey
The ThrUMMS survey (Barnes et al. 2013) covers theGalactic plane between 300 ◦ ≤ l ≤ ◦ and − ◦ ≤ b ≤ ◦ in the CO (1–0), CO (1–0), C O (1–0), and CNspectral lines at frequencies of 115.27, 110.20, 109.78, and113.60 GHz, respectively. xv The survey was conductedusing the new Very Fast Mapping (VFM) technique de-veloped for use with the 22 m Mopra telescope. xvi
Half-Nyquist sampling rates were obtained along the scanningdirection and between scan rows, yielding a 72 (cid:48)(cid:48) resolu-tion. Compared to the traditional on-the-fly mappingtechnique, the VFM technique is capable of observinga larger area in the same amount of time if Nyquist-sampled maps are not needed. As well as being moreefficient, it provides a nearly uniform sensitivity per unitarea. The data were converted from antenna tempera-ture T ant to main-beam temperature, T mb , by dividing T ant by the main-beam efficiencies η mb of 0.42 for CO(1–0), and of 0.43 for both CO (1–0) and C O (1–0).
21 cm continuum and H I line emission from theSGPS survey From the Southern Galactic Plane Survey (SGPS;McClure-Griffiths et al. 2005) we extracted the H I atomic line and 21 cm continuum data over the range of330 ◦ ≤ l ≤ ◦ and b = ± ◦ . The survey was conductedusing the Australia Telescope Compact Array (ATCA) xv The data available to date for this study covered the range − . ◦ ≤ b ≤ . ◦ xvi Operation of the Mopra radio telescope is made possible byfunding from the National Astronomical Observatory of Japan, theUniversity of New South Wales, the University of Adelaide, and theCommonwealth of Australia through CSIRO. olecular cloud and star formation in the surrounding of the giant H II region RCW 106 3 Figure 1.
21 cm continuum image (color scale) from the SGPS. Contour levels (white) are 0.2, 0.6, 1, and 2 Jy beam − . Within thisregion, we analysed continuum and molecular line data listed in Table 1 in 31 distinct boxes (green). The linear ruler (top of diagram)assumes a distance of 3.6 kpc. OH masers (Caswell et al. 1980) are plotted as red crosses, H II regions (Jones & Dickey 2012) as magentaasterisks, and prominent OB clusters as blue stars. Further details are given in Appendix A and Table 3. interferometer and was complemented with the Parkes64 m telescope (FWHM = 15 (cid:48) ) for short spacings. Theobservations were performed simultaneously in a spectralline mode with 1024 channels across a 4 MHz bandwidthcentered at 1420 MHz and in a continuum mode with32 channels across a 128 MHz bandwidth centered at1384 MHz (McClure-Griffiths et al. 2001, 2005). For ourstudy, we used data which have a FWHM 2 . (cid:48)
2, line rms1.6 K km s − , and continuum rms < − (seeTable 1). Infrared data from Herschel and Spitzer
We used 160 and 250 µ m images from the HerschelSpace Observatory to examine more deeply the proper-ties of the cold dust near RCW 106. The fields were ob-served as part of the
Herschel
Infrared Galactic PlaneSurvey (Hi-GAL; Molinari et al. 2010) at 70 / µ mwith the Photodetector Array Camera and Spectrome-ter (PACS; Poglitsch et al. 2010) and at 250 / / µ mwith the Spectral and Photometric Imaging REceiver(SPIRE; Griffin et al. 2010). The Hi-GAL data weretaken in parallel mode with a fast scanning speed of 60 (cid:48)(cid:48) s − . The raw (level-0) data of each individual scan fromboth PACS and SPIRE were calibrated and deglitchedusing HIPE xvii version 11.0. The SPIRE and PACS level-1 data were then fed to version 18 of the Scanamorphos software package xviii (Roussel 2013) to regrid and createthe final maps. For our region, three Hi-Gal fields of2 . ◦ × . ◦ µ mimages have angular resolutions of 12 (cid:48)(cid:48) and 37 (cid:48)(cid:48) , and 1 σ rms of 0.08 and 1.0 MJy sr − .To trace the warm dust emission associated with high-mass star formation, we used the 24 µ m image fromthe Multi-band Imaging Photometer for Spitzer
Galactic xvii
HIPE is a joint development software by the
Herschel
Sci-ence Ground Segment Consortium, consisting of ESA, the NASA
Herschel
Science Centre, and the HIFI, PACS, and SPIRE consor-tia. xviii
Plane Survey (
MIPSGAL ; Carey et al. 2009). SPATIAL AND KINEMATIC STRUCTURE
21 cm continuum: defining the major structures
In the 21 cm continuum image (Fig. 1) the giant H II region RCW 106 (Rodgers et al. 1960) is the largest(1 . ◦ × ◦ ) and brightest ( ∼
287 Jy) area. RCW 106is an elongated structure in 332 . ◦ < l < . ◦ − . ◦ < b < . ◦
1, and angled roughly 60 ◦ with respect toGalactic aast within the l, b plane (hereafter referred toas the “aastern” part). In the region 330 . ◦ < l < . ◦ − . ◦ < b < . ◦ l ∼ ◦ . Projected in this gap is an OB associa-tion R103B (332 . ◦ − . ◦
08) (Mel’Nik & Efremov 1995)and also another further to the south, R103A (331 . ◦ − . ◦ ∼ . ∼ σ noiselevel above the local background. Most of the sources areH II regions whose properties are classified according totheir morphologies in Appendix A. Additionally, thereare four confirmed and two candidate supernova rem-nants (SNRs) (Green 2009). For full details of each sub-region refer to Appendix A and the summary providedin Table 3.In Fig. 1 we also mark the positions of OH masers andH II regions found in large scale surveys (Caswell et al.1980; Jones & Dickey 2012). The positions of masersand H II regions detected from radio recombination linescoincide with the bright radio continuum sources helpto distinguish substructures and to quantify the cloudproperties (see Sect. 4). H. Nguyen, Q. Nguyen Luong, P. G. Martin, et al. −140 −120 −100 −80 −60 −40 −20 0 V LSR (km s −1 ) A v e r a g e < T m b > ( K ) V LSR =-91.4km s −1 FWHM=8.4km s −1 V LSR =-63.5km s −1 FWHM=6.4km s −1 V LSR =-47.6km s −1 FWHM=7.8km s −1 EastWestFull
Figure 2.
The CO spectrum averaged over the entire region( l = 330 ◦ − ◦ , solid black), eastern part ( l = 332 . ◦ − ◦ ,dotted green), and western part ( l = 330 . ◦ − . ◦
5, dashed blue)as defined in Fig. 1.
CO and H I line emission To investigate the relative contributions of differentclouds superimposed along the line of sight, we first plotthe average CO spectrum of the entire region alongwith the distinctive spectra of the two main structures,eastern and western, defined in Section 3.1 (Fig. 2).In the average spectrum there are three distinct peakswithin the velocity range −
140 to 10 km s − . The spec-tra for both main structures exhibit significant peaksat roughly the same velocities, in the ranges [ − − − − − −
35] km s − centered on V LSR = − −
63, and −
48 km s − , respectively. Theseranges are highlighted as green, blue, and red sectors inFig. 2. For the western region, the three spectral max-ima are roughly the same, on average ∼ V LSR ∼ −
55 km s − and theother at ∼ −
45 km s − (see Fig. 2). The peak at ∼ − − coincides with the CO peak that Bains et al.(2006) attributed to the cloud surrounding the RCW 106cluster.In Figs. 3a-c and d-f we display maps of the CO andH I emission integrated over three velocity ranges: [ − − − − − −
40] km s − . Additionallyin Fig. 3, the positions of OH masers and H II regionsmostly coincide both spatially and spectrally with themolecular gas peaks.Fig. 3a (corresponding to the green velocity range inFig. 2) shows extended CO emission spanning the entirelongitude range. Its main peak is between 331 ◦ and 332 ◦ in longitude and extends just 0 . ◦ l = 333 . ◦ −
80 to −
60 km s − (themiddle white and blue velocity ranges in Fig. 2), shownin Fig. 3b, is dominated by the strongest emission fromthe Western region, but is otherwise relatively free fromemission from the central plane (i.e., b ∼ ◦ ). On theother hand, the map shown in Fig. 3c, integrated from −
60 to −
40 km s − (the middle white and red velocity ranges in Fig. 2) is much brighter near the RCW 106H II region and has a diffuse filamentary component inthe western region.The H I emission in Figs. 3d-f does not appear to cor-relate very well with either CO or 21 cm continuumemission, indicating that star formation activity does notcorrelate with this extended phase of the gas. However,in Figures 3d-e, H I seems to form in the outer envelopeof the MCCs.We have integrated the CO , CO , and H I datacubes in Galactic latitude (from − . ◦ . ◦
5) to pro-duce position-velocity ( (cid:96) − V ) diagrams (Fig. 4). Themain emission in the (cid:96) − V maps is from −
72 km s − to −
35 km s − as in the averaged spectra (Fig. 2). Anotherfeature of strong though less prominent emission is from −
112 km s − to −
80 km s − at low (cid:96) (western end). Thisis detached from the lower velocity structure supportingour division of the emission into two different complexes:in the East is the RCW 106 MCC spanning the velocityrange from −
72 to −
40 km s − , the West is the complexMCC 331.0+0.0 ( V LSR = −
90 km s − ) (hereafter referredto with the informative name MCC331–90 ) spanning thevelocity range from −
112 to −
80 km s − . Insight from thermal dust emission
Figure 5 shows images of the thermal dust emissionobtained in the mid-IR by
Spitzer and in the far-IR andsubmillimeter by
Herschel (Sect. 2.3). xix
These provideinsight into the dust content, cloud morphology, and ra-diation field. In particular, data at 24 µ m (Fig. 5a) allowus to trace local dust heating from the UV photons fromyoung OB stars. Far-IR and submillimeter data probecooler dust as well, both in the cloud and outside, andaway from the influence of OB stars (Figures 5b-c).The morphology of far-IR emission is generally similarto the CO emission (Fig. 3) suggesting that the gas anddust coexist within the MCCs. We also detect infrareddark clouds (IRDCs), seen as absorption in the mid-IRdata but as emission in the far-IR data. These IRDCsare potential sites of massive star formation (Hennebelleet al. 2001; Rathborne et al. 2006; Simon et al. 2006;Peretto et al. 2010; Nguyen Luong et al. 2011a). Inthe dust emission at all wavelengths we see two mainconcentrations of dust at 331 ◦ and 333 ◦ whose detailedmorphology matches well with that in the angled struc-tures seen in the radio continuum and in CO . Thiscorrelation indicates that both the 21 cm continuum andthe dust heating are generated by the massive stars in theRCW 106 and MCC331–90 MCCs. The
Herschel far-IRdata also reveal finger-like protrusions at the bottom ofthe structure linked to MCC331–90 . These features areplausibly formed by radiation pressure from numerousOB stars in the local interstellar medium (e.g., Krumholz& Matzner 2009; Gritschneder et al. 2010; Tremblin et al.2013).In summary, the dust emission data from 8–500 µ mconfirm that this region has two distinct MCCs that areboth extremely active in forming massive stars, includinghosting a young stellar cluster. xix Emission from Polycyclic Aromatic Hydrocarbons (PAHs)at 8 µ m (not shown here) is also prominent in massive star formingregions (Peeters et al. 2004). olecular cloud and star formation in the surrounding of the giant H II region RCW 106 5 Figure 3. Upper – CO integrated map for (a): −
112 to −
80 km s − ; (b): −
80 to −
60 km s − ; (c): −
60 to −
40 km s − . The blackcontours are from the 21 cm continuum emission in Fig. 1 with levels of 0.2, 0.6, 1, and 2 Jy beam − . As in Fig. 1 positions of catalogedOH masers and H II regions are marked, but now only in the map with the appropriate velocity range. OB clusters are marked on allpanels. Lower – H I integrated maps ((d), (e), and (f)) for the same velocity ranges. The black contours are the CO emission from thecorresponding panels above with levels in increments of 20 K km s − from 10 to 90 K km s − for (d) and (e) and 10 to 150 K km s − for (f). H. Nguyen, Q. Nguyen Luong, P. G. Martin, et al.
Figure 4. H I (color) , CO (black contour, upper panel) , and CO (blue contour, lower panel) position-velocity diagramsintegrated from b = − . ◦ b = 0 . ◦
5. As in previous figures, red crosses represent OH masers and magenta asterisks represent H II regions.The brightest 21 cm continuum sources in this region are at l = 331 . ◦ l = 333 . ◦ I absorption against thesesources results in vertical bands of relatively low (green) net H I emission cutting through higher intensity (yellow, red) regions for theentire velocity range corresponding to foreground gas. olecular cloud and star formation in the surrounding of the giant H II region RCW 106 7 Figure 5.
Dust emission (colour) from
Spitzer µ m (Top), Herschel µ m (Middle), and Herschel µ m (Bottom), overlaid with21 cm continuum emission contours (black). Blue stars mark the locations of bright OB clusters. H. Nguyen, Q. Nguyen Luong, P. G. Martin, et al. DISTANCES AND LOCATION OF THE OF RCW 106 ANDMCC331–90 COMPLEXES
As shown in Fig. 2, the RCW 106 molecular cloud com-plex has two peaks centered at −
48 and −
63 km s − ,whereas the MCC331–90 molecular cloud complex hasonly one V LSR peak at ∼ −
90 km s − . We calculate thekinematic distances using the Galactic rotation curve ofReid et al. (2009), following the approach of Roman-Duval et al. (2009). For RCW 106 and MCC331–90 ,we obtain near distances of 3.3–4.1 kpc and 4.6–6.0 kpc,respectively, in contrast to far distances 10.2–12 kpc and8.5–10 kpc, respectively. The far-distance solutions arevery unlikely. First, at a distance of 11 kpc the MCCdimension would be ∼
200 pc,tripled the actual size cal-culated from the near solution. Second, the mass ofRCW 106 would be a ∼ − M (cid:12) , as massive asthe CMZ. Furthermore, at a Galactic latitude of -1 ◦ itwould imply that these MCCs are ∼
300 pc off the Galac-tic mid-plane, which seems implausible even for the thickdisc. The near kinematic distance to the RCW 106 com-plex is also consistent with the photometric distance es-timates to the RCW 106 OB cluster of 3.6 kpc (Mois´eset al. 2011).The 11 OH maser emission found by Caswell et al.(1980) lie in the spatial and velocity range of either theRCW 106 or MCC331–90 complex (see Figs. 1, 3 and 4),except for one source that lies in the spatial range ofRCW 106 with an offset of ∼
40 km s − in V LSR . Theaverage near kinematic distances of the OH masers alsocoincide with those of these two MCC, therefore con-firm our argument for near kinematic distance. (Jones& Dickey 2012) resolved the distance ambiguities for theRCW 106 and MCC331–90 complexes using H I absorp-tion and found that more than 75% have near distances.. Among these, 15 are assigned the near distances andfour are uncertain.To be consistent with the measurements from othermethods in the literature (i.e., Bains et al. 2006; Mois´eset al. 2011), we adopt 3.6 kpc as the distance toRCW 106. We also conclude that separation into twomain molecular structures is physically meaningful, withboth connected to signposts of massive star formationsuch as OH masers or H II regions. Hereafter, we adopta distance of 3.6 kpc to the RCW 106 complex and 5 kpcto the MCC331–90 complex.With our calculated distances for these MCCs, we caninfer their locations in the Milky Way (Fig. 6). Basedon models of the Galaxy (Georgelin & Georgelin 1976;Rodriguez-Fernandez & Combes 2008; Reid et al. 2009),RCW 106 can be placed in the nearby Scutum–Centaurusarm, the major arm of the Galaxy, while MCC331–90 isin the less prominent Norma arm.The Scutum–Centaurus arm, a counterpart to thePerseus arm (Fig. 6), is characterized by a high frac-tion of dense gas (Sakamoto et al. 1997; Russeil et al.2005) which may be the reason why the most massiveYoung Massive Clusters (YMCs) such as Westerlund 1,RSGC 1, RSGC 3 (Portegies Zwart et al. 2010), andthe ministarburst W43 (Nguyen Luong et al. 2011b) arefound therein. The (cid:96) − V diagram of the CO and H II emission shows a gradient toward the RCW 106 H II re-gion (see upper dashed white line in Fig. 4) that mightreflect a velocity gradient along the Scutum–Centaurus Figure 6.
Artist’s rendition of the Milky Way seen face-on byRobert Hurt of the
Spitzer
Science Centre with advice from RobertBenjamin from the University of Wisconsin–Whitewater. Coloreddots mark the positions of GMCs, except for the black dot whichmarks the Galactic center. The RCW 106 complex is at l = 333 . ◦ l = 331 . ◦ arm toward the RCW 106 complex.Although the Norma arm, a counterpart to the Sagit-tarius arm (Fig. 6), has less active star formation overall,MCC331–90 is known to host the luminous and efficientmassive star forming cloud G331.5–0.1 (Merello et al.2013a,b). MASSES AND MASS SURFACE DENSITIES OF THERCW 106 AND MCC331–90 COMPLEXES
We obtain the mass from W ( CO ), the CO spectrum integrated over the velocity range − −
40 km s − for RCW 106 and −
112 to −
80 kms − forMCC331–90 . The molecular hydrogen column densityis from N H = X × W ( CO ), with X = 1 . × cm − K − km s − (Dame et al. 2001). This X factor isclose to the value of 1 . × cm − K − km s − derivedfor the Perseus molecular arm from the diffuse gamma-ray emission (Abdo et al. 2010) and is lower than thevalue of 2 . × cm − K − km s − used for W43 andCygnus X in Schneider et al. (2006) and Nguyen Luonget al. (2011b). Although X is uncertain because of theoptical depth, metallicity, and excitation conditions,the mass estimates are probably accurate to a factorof two. The mass M total in an area A cloud would thenbe M total = N H × A cloud × µ H m H where µ H = 2 . H molecular weight and m H is the H atomicmass.We use the CO integrated intensity maps to definethe extent of each cloud and adopt a rectangular shapethat covers the main extent of the cloud (see Fig. 3)which is then used to calculate A using the assumed kine-matic distance. The areas of the two clouds are 2 . × pc and 2 . × pc , which yield effective diameters of183 and 137 pc, respectively. However, we integrate onlywithin the contour N H = 5 × cm − (correspondingto W ( CO ) = 277 K km s − ) to minimize the effectsof the foreground and background emission. This yieldsolecular cloud and star formation in the surrounding of the giant H II region RCW 106 9 Table 2
Properties of the two molecular cloud complexes in comparison with other star-forming complexes
Complex A clouda D b Vel. range c d d M e Σ gasf σ g M virh α viri Σ SFRj (pc ) (pc) ( km s − ) (kpc) (M (cid:12) ) (M (cid:12) pc − ) ( km s − ) (M (cid:12) ) (M (cid:12) yr − kpc − )RCW 106 2.6 ×
183 -80 to -40 3.6 5.9 ± . × × k MCC331–90 2.2 ×
167 -112 to -80 5.0 2.8 ± . × . × < Gould Belt > l l l - 0.27 l . × l - -W43 m × ∼
140 80 to 100 6.0 7.1 ×
473 9.3 2.4 × m k Cygnus X m,n × ∼
160 -10 to 20 1.7 5.0 ×
250 4.2 5.5 × m,o × ∼
350 -225 to 225 8.5 3.0 × a Surface areas calculated at adopted distances. b Equivalent diameter D = (cid:112) A cloud /π . c Main velocity range of the structure. d Adopted distance. e Mass calculated from the CO emission using an X of 1 . × cm − K − km s − except for Gould Beltclouds being calculated from dust extinction map and for CMZ from thermal dust emission. f Mass surface density Σ gas = M/A cloud . g Velocity dispersion σ = ∆ V FWHM / √ h Virial mass. i
3D virial parameter described by α vir = M vir /M total . j Star formationrate (SFR) density. k First numbers are the past SFR density and bold numbers in brackets are the future SFR density (see Sect. 7). l Average of 20 large molecular clouds from
Spitzer cores to disks and Gould Belt surveys (Heiderman et al. 2010). m From NguyenLuong et al. (2011b) with X factor scaled to 1 . × cm − K − km s − . n From Schneider et al. (2006) using CO. o FromDahmen et al. (1998) using C O. masses 5 . × M (cid:12) and 2 . × M (cid:12) and approximatesurface densities 220 and 130 M (cid:12) pc − for the RCW 106and MCC331–90 complexes, respectively. We estimatean uncertainty of 30% because of the background andforeground confusion as well as optical depth. The prop-erties are collected in Table 2.The total clump mass deduced from measurementsof CO (Bains et al. 2006), mm continuum (Mooker-jea et al. 2004), and far-IR (Karnik et al. 2001) towardRCW 106 is only 10–30% of our estimate, for two rea-sons. First, the previous measurements estimate onlythe clump mass whereas we calculate the total gas masswhich includes in addition the more diffuse large-scalestructure. xx Second, while these three studies focus inon the ∼ ∼ M (cid:12) , area 32 pc , and surface den-sity of 80 M (cid:12) pc − (Table 2). Compared to these, thetwo complexes studied here are ∼
100 times larger inarea and yet because the masses are so much higher themass surface densities are also higher. In reference to atypical massive star forming region such as Cygnus X, themass surface density of RCW 106 is comparable, whileMCC331–90 is about half. But compared to the masssurface densities of the extreme massive star forming re-gions such as W43 or CMZ molecular cloud, the valueshere are lower still (Table 2). DYNAMICS OF RCW 106 AND MCC331–90 COMPLEXES
We fit a single Gaussian profile to the spectra ofthe RCW 106 and MCC331–90 MCCs and obtainsFWHM widths of 10.6 and 8.7 km s − correspondingto a one-dimensional (1D) velocity dispersion σ =∆ V FWHM / √ − , respectively.This value is within the typical range for MCCs foundin other observations (W43, Nguyen Luong et al. 2011a;Cygnus X, Schneider et al. 2006) or in simulations ofmolecular clouds in spiral arms (Dobbs & Pringle 2013).First, to compare with other complexes, we estimatethe gravitational instability criterium for the RCW 106 xx The ratio of total clump mass to the total gas mass canalso be considered as the clump formation efficiency, as elaboratedfurther in Section 7. complex using the spherical approximation and assumingthat its effective radius R sph (half the diameter D inTable 2) as: α vir = 5 × R sph σ GM = M vir M total . (1)Both complexes have low spherical virial parameters (0.35 for RCW 106 and 0.5 for MCC331–90 ; see Table 2).However, since the global structure of RCW 106 com-plex is filamentary with an aspect ratio r = l w =
180 pc50 pc =3 .
6, we use the gravitational instability criterium of aninfinite-length isothermal gas cylinder (Ostriker 1964; In-utsuka & Miyama 1992; see also Dibai 1958; Ozernoi1964). This model defines a critical line mass (massper unit length, M line , crit solely on the sound speed c s ,above which the filamentary cloud is unstable againstcollapsing. For a large molecular cloud complex, the in-ternal motion depends also on turbulence in addition toits thermal motion, therefore we assume that the appar-ent ”sound speed c s ” equals the observed line width ∆ V (similar as in Dobashi et al. 2014). We calculate the filamentary virial parameter α filvir , defined as: α filvir = M line , crit M line , gas = 2 c s /GM/l = 465∆ VM/l [M (cid:12) pc − ][M (cid:12) pc − ] . (2)The total gas mass M is integrated over the RCW 106and MCC331–90 filamentary clouds with length l andwidth w . We obtain low α filvir of 0.15 for RCW 106 and0.26 for MCC331–90 , which confirm that these two com-plexes are indeed gravitationally unstable.While the gravitationally unstable states of theseclouds are certain, a more detailed investigation is neededto justify their origin. They might be caused by strongdynamical affects related to their locations spread alongthe respective spiral arms (see Section 4) and/or feed-back from the vigorous burst of star formation within(see Section 7). A BURST OF STAR FORMATION IN THE RCW 106COMPLEX Ly α continuum photons from young stars create H II regions quantifiable by free–free emission. We find 30H II regions hosting O- or B-type stars in the RCW 106complex (Table 3). We checked the CO and H I V LSR of0 H. Nguyen, Q. Nguyen Luong, P. G. Martin, et al.gas surrounding these H II regions and confirmed thatthey are in the velocity range of the RCW 106 complex.We estimated the radio continuum flux density S ν of eachH II region by aperture photometry on the the 21 cm mapwithin the contour defined by the 3 σ noise level (see Ap-pendix A). An average background level of ∼ .
09 Jybeam − was derived by averaging the intensity withina surrounding empty region. Following Mezger & Hen-derson (1967), the number of Ly α continuum photonspowering an ionization-bounded region can be computedusing N Ly α . × s − = S ν Jy (cid:20) ν GHz (cid:21) . (cid:20) T e K (cid:21) − . (cid:20) d kpc (cid:21) . (3)where T e = 8000 K (Wilson et al. 2012) is the electrontemperature, ν = 1 .
42 GHz is the observing frequency,and d is the distance to the H II region. The distance is3.6 kpc for the RCW 106 complex (see Sect. 4).If we assume that a single main-sequence star domi-nates the ionization, xxi we can then use N Ly α to obtainthe spectral type (Thompson 1984; Martins et al. 2008).The estimated types are O7V or earlier.Given that an O7 star with mass ∼
25 M (cid:12) emits N Ly α = 5 × s − (Martins et al. 2005), we can es-timate an upper limit on the number of stars that havespectral type O7 or earlier in each H II region of theRCW 106 cloud complex (Table 3); in total, there are upto 54 O7 stars currently formed. Assuming that the stel-lar masses in the RCW 106 complex follow the SalpeterInitial Mass Function (IMF) dN/dm = A × m − . (Salpeter 1955) and have a minimum cut-off mass of0.08 M (cid:12) and maximum cut-off mass of 50 M (cid:12) , the to-tal stellar mass of RCW 106 will then be M ∗ = A (cid:90) m − α dm = A × m − . − m − . − . . (4)The normalization factor A can be calculated from thetotal number of O7 stars calculated above by the approx-imation that this is the total number of all stars withmasses in the range of 25 to 50 M (cid:12) so that N m u m l = A × m − . − m − . − . . (5)To gauge the uncertainty, we varied the upper masslimit to 100 M (cid:12) and allowed for a dispersion of ± ∼ +2753 − for the factor A and a totalstellar mass of ∼ +17 − × M (cid:12) . Replacing the SalpeterIMF by the Kroupa IMF (Kroupa 2001) lowers the totalstellar mass but it is within the calculated uncertainty. The immediate past
Because an O7V star with a mass of 25 M (cid:12) has a life-time of ∼ × yr starting from its first appearanceon the main-sequence diagram, we can calculate thestar formation efficiency (cid:15) pastcloud , the star formation rate xxi Of course, it is possible that the ionization is produced bya combination of several stars of somewhat later spectral type;however, usually only the brightest star dominates the output ofionizing photons.
SF R pastcloud , and the SFR density Σ pastSFR , cloud of the entireRCW 106 cloud complex for this “past” time interval as (cid:15) pastcloud = M ∗ M cloud + M ∗ = 0 . +0 . − . , (6) SF R pastcloud = M ∗ × yr = 0 . +0 . − . M (cid:12) yr − , (7)Σ pastSFR , cloud = SF RA cloud = 9 . +3 . − . M (cid:12) yr − kpc − . (8)with the total stellar mass M ∗ = 4 . × M (cid:12) , thetotal gas mass M cloud = 5 . × M (cid:12) calculated over anarea of A cloud = 2 . × pc .The estimated (cid:15) pastcloud is in the middle of the range forGMCs, 0.002–0.2, as derived from the ionizing flux ofyoung stars (Murray 2011). However, the SFR den-sity of RCW 106 is high and for its surface density of220 M (cid:12) pc − (Sect. 5) it is well above the trend in theSchmidt–Kennicutt relation (see Fig. 7). This shows thatRCW 106 has been very active in forming massive starsduring the last 2 × yr. The immediate future
The large amount of gas remaining in this complexsuggests that the burst of star formation activity in theRCW 106 molecular cloud complex might not yet be fin-ished. We investigated this by estimating the future star-formation activity using the dense clump and core pop-ulations.First, we make an assumption that the dense clumpmass of 2 . − . × M (cid:12) derived from CO line emis-sion (Bains et al. 2006) or 1.2 mm continuum emission(Wong et al. 2008) represents the clump mass of the en-tire RCW 106 complex. This assumption is justified sincemost of the dense gas concentrates in the region consid-ered by Bains et al. (2006) and Wong et al. (2008) (seefor example Figure 5). We estimate the clump forma-tion efficiency, (cid:15) cloud → clump , xxii as the ratio of the totalclump mass to the total cloud mass (Eden et al. 2012,2013; Louvet et al. 2014): (cid:15) cloud → clump = M clump M cloud ∼ . ± . . (9)The estimated (cid:15) cloud → clump of the RCW 106 complex isabout 10 times higher than that of the famous massivestar-forming region Cygnus X and almost equal to thatof W43 (Nguyen Luong et al. 2011b). Thus the RCW 106complex is forming massive clumps almost as efficientlyas the W43 star forming region despite the mass surfacedensity of RCW 106 being four times lower.Secondly, a cloud’s efficiency at converting mass intodense clumps subsequently affects the star formation ef-ficiency (Eden et al. 2012; Louvet et al. 2014). If thedense clumps ( r ∼ r ∼ . (cid:15) clump → core , and dense cores will form protostars with amass transfer efficiency (cid:15) core →∗ , the future star formationefficiency of the entire cloud will be (cid:15) cloud →∗ = (cid:15) cloud → clump × (cid:15) clump → core × (cid:15) core →∗ . (10) xxii This parameter is also known as the compactness of a molec-ular cloud (Nguyen Luong et al. 2011b). olecular cloud and star formation in the surrounding of the giant H II region RCW 106 11 Figure 7.
The Schmidt-Kennicutt relation between the star formation rate (SFR) density and gas surface density extending from normalspiral galaxies to starbursts (Kennicutt 1998). Also plotted are values for the average Milky Way, the immediate past and future of theRCW 106 complex, the past and future SFR density of the W43 complex, the past and future SFR density of the Cygnus X complex, andthe past SFR density of the CMZ complex.
If we adopt typical values (cid:15) clump → core ∼ . − . (cid:15) core →∗ ∼ . − . (cid:15) cloud →∗ ∼ . ± . ∼ × yr (Motte et al. 2007; Russeil et al. 2012),similar or OB stars, we calculate the “future 2 × yr”(“future”) SFR and SFR density as SF R futurecloud = (cid:15) cloud →∗ × yr M cloud = 0 . ± . (cid:12) yr − (11)andΣ futureSFR , cloud = SF R futurecloud A cloud = 4 . ± . (cid:12) yr − kpc − . (12)This SFR density and the surface density also combineto place the RCW 106 complex in the starburst regimeof the Schmidt–Kennicutt relation (Kennicutt 1998; seeFig. 7). It should be possible to refine and confirm thefuture SFR density of the RCW 106 complex by count-ing the massive dense cores identified using the Herschel data. This will be the topic of a subsequent paper.These SFR densities are similar to the future SFR den-sity of W43 and higher than that of Cygnus X, reinforcingthe conclusion that RCW 106 is undergoing a ministar-burst at present. The similarity between past and futureSFR densities implies that this ministarburst event is notyet finished but is in the declining phase. It is also muchhigher than the SFR density of CMZ, the region of the galactic plane within a few degrees of the Galactic centre,well-known for its inefficiency in star formation despitebeing 100 times more massive than RCW 106 complex(Immer et al. 2012; Kruijssen et al. 2014).While this is consistent with the large mass reservoirstill available in the RCW 106 complex, there are stillquestions outstanding including what the final conver-sion efficiency will be and its relationship to the virialparameter (Section 6).
Uncertainties
The calculations in Sections 7.1 and 7.2 should involvethe general uncertainties from the assumptions of theIMF, the timescales, and the mass transfer efficiencies.Several authors suggest that the IMF of the extreme star-formation environment is top-heavy (Stolte et al. 2002,2005; Chabrier et al. 2014). However, the validity ofthis statement is still largely debated (Kim et al. 2006;Brandner et al. 2008). If we use an IMF with a steeperslope, the SFR likely increases by about 20%. We as-sume a burst period timescale of 2 × yr, which mighthave variations within a factor of two; but this is themost appropriate assumption since this is the measuredtimescale of an OB star (Martins et al. 2005) and anMDC (Motte et al. 2007). The third uncertainty is themass transfer efficiencies from clouds to clumps, to coresand to stars. Our assumptions of these quantities are theaverage results of the current state-of-the-art theoreticaland observational studies, therefore the true values mightvary at most 10%–30%. We already take these three un-certainties into account in our calculations.The uncertainties from measurements of Ly α photons2 H. Nguyen, Q. Nguyen Luong, P. G. Martin, et al.and the clump mass are low since we focus only on thebrightest regions in the radio continuum map (in caseof estimating the Ly α photons) and in millimeter con-tinuum or CO maps (in case of estimating the clumpmass). For the later case, the clump mass is verified withan independent measurement from NH emission (Loweet al. 2014). CONCLUSIONS
Using a combination of data sets, we have defined andcharacterized two MCCs in the general direction of theRCW 106 OB cluster, and we have labelled them theRCW 106 and MCC331–90 complexes. Their propertiesare summarized as follows • The V LSR for gas in the RCW 106 complex rangesbetween −
40 and −
80 km s − and the V LSR forMCC331–90 is between −
80 and −
112 km s − . Bothcomplexes have velocity dispersions comparable to thatof Cygnus X ( σ =4.2 km s − ) but smaller in comparisonto W43 ( σ =9.3 km s − ), which is right in the Galacticbar and a known turbulent area. • OH masers and H II region studies confirmed the nearkinematic distances of 3.6 kpc for RCW 106 and 5 kpc forMCC331–90 . This places the RCW 106 molecular cloudcomplex in the Scutum–Centaurus arm and MCC331–90in the Norma arm. • The RCW 106 complex has a mass of 5 . × M (cid:12) and a surface density of ∼
220 M (cid:12) pc − , whereas theMCC331–90 complex has a mass of 2 . × M (cid:12) andsurface density of ∼
130 M (cid:12) pc − . These surface den-sities are higher compared to the average Gould Beltcloud ( ∼
70 M (cid:12) pc − ) and the surface areas are 10 timeslarger. • These two complexes are of comparable size( d ∼
180 pc) but have less mass in comparison to otherlarge GMCs such as W43 and Cygnus X. Hence, the sur-face densities are smaller. • The virial parameters are greater than unity, indicat-ing that they are gravitationally unbound. • Using the 21 cm continuum, we separated the regioninto 31 subregions: 25 containing H II regions and 6 con-taining SNRs. We estimated that there are about 50young O7V stars currently in the RCW 106 complex. • For the RCW 106 molecular cloud complex, we derivea past global star formation efficiency, SFR, and SFRdensity and estimate values to quantify star formation inthe near future. These values suggest that the RCW 106complex is undergoing a ministarburst event.
Acknowledgements : H.N. is grateful for a CITA sum-mer student internship. P.G.M. acknowledges supportfrom the Canadian Space Agency and the Natural Sci-ences and Engineering Research Council of Canada. Na-dia Lo’s postdoctoral fellowship is supported by CON-ICYT/FONDECYT postdoctorado under project No.3130540.
REFERENCESAbdo, A. A., Ackermann, M., Ajello, M., et al. 2010, ApJ, 710,133 Alves, J., Lombardi, M., & Lada, C. J. 2007, A&A, 462, L17Bains, I., Wong, T., Cunningham, M., et al. 2006, MNRAS, 367,1609Barnes, P., Muller, E., Inderm´’uhle, B., et al. 2013, ApJBrandner, W., Clark, J. S., Stolte, A., et al. 2008, A&A, 478, 137Bronfman, L., Alvarez, H., Cohen, R. S., & Thaddeus, P. 1989,ApJS, 71, 481Carey, S. J., Noriega-Crespo, A., Mizuno, D. R., et al. 2009,PASP, 121, 76Caswell, J. L. & Haynes, R. F. 1975, MNRAS, 173, 649Caswell, J. L., Haynes, R. F., & Goss, W. M. 1980, AustralianJournal of Physics, 33, 639Chabrier, G., Hennebelle, P., & Charlot, S. 2014, ApJ, 796, 75Dahmen, G., Huttemeister, S., Wilson, T. L., & Mauersberger, R.1998, A&A, 331, 959Dame, T. M., Hartmann, D., & Thaddeus, P. 2001, ApJ, 547, 792Dibai, E. A. 1958, Soviet Ast., 2, 226Dobashi, K., Matsumoto, T., Shimoikura, T., et al. 2014, ApJ,797, 58Dobbs, C. L. & Pringle, J. E. 2013, MNRAS, 432, 653Dutra, C. M., Bica, E., Soares, J., & Barbuy, B. 2003, A&A, 400,533Eden, D. J., Moore, T. J. T., Morgan, L. K., Thompson, M. A.,& Urquhart, J. S. 2013, MNRAS, 431, 1587Eden, D. J., Moore, T. J. T., Plume, R., & Morgan, L. K. 2012,MNRAS, 422, 3178Fujiyoshi, T., Smith, C. H., Caswell, J. L., et al. 2006, MNRAS,368, 1843Fujiyoshi, T., Smith, C. H., Moore, T. J. T., et al. 2005, MNRAS,356, 801Georgelin, Y. M. & Georgelin, Y. P. 1976, A&A, 49, 57Grave, J. M. C., Kumar, M. S. N., Ojha, D. K., Teixeira,G. D. C., & Pace, G. 2014, A&A, 563, A123Green, D. A. 2009, Bulletin of the Astronomical Society of India,37, 45Griffin, M. J., Abergel, A., Abreu, A., et al. 2010, A&A, 518, L3+Gritschneder, M., Burkert, A., Naab, T., & Walch, S. 2010, ApJ,723, 971Heiderman, A., Evans, II, N. J., Allen, L. E., Huard, T., & Heyer,M. 2010, ApJ, 723, 1019Hennebelle, P., P´erault, M., Teyssier, D., & Ganesh, S. 2001,A&A, 365, 598Immer, K., Menten, K. M., Schuller, F., & Lis, D. C. 2012, A&A,548, A120Inutsuka, S. & Miyama, S. M. 1992, ApJ, 388, 392Jackson, J. M., Rathborne, J. M., Shah, R. Y., et al. 2006, ApJS,163, 145Jones, C. & Dickey, J. M. 2012, ApJ, 753, 62Jones, P. A., Burton, M. G., Cunningham, M. R., et al. 2012,MNRAS, 419, 2961Karnik, A. D., Ghosh, S. K., Rengarajan, T. N., & Verma, R. P.2001, MNRAS, 326, 293Kennicutt, Jr., R. C. 1998, ApJ, 498, 541Kim, S. S., Figer, D. F., Kudritzki, R. P., & Najarro, F. 2006,ApJ, 653, L113Kroupa, P. 2001, MNRAS, 322, 231Kruijssen, J. M. D., Longmore, S. N., Elmegreen, B. G., et al.2014, MNRAS, 440, 3370Krumholz, M. R. & Matzner, C. D. 2009, ApJ, 703, 1352Kuchar, T. A. & Clark, F. O. 1997, ApJ, 488, 224Kumar, M. S. N. 2013, A&A, 558, A119Lo, N., Cunningham, M., Bains, I., Burton, M. G., & Garay, G.2007, MNRAS, 381, L30Lo, N., Cunningham, M. R., Jones, P. A., et al. 2009, MNRAS,395, 1021Lo, N., Redman, M. P., Jones, P. A., et al. 2011, MNRAS, 415,525Lockman, F. J. 1979, ApJ, 232, 761Louvet, F., Motte, F., Hennebelle, P., et al. 2014, A&A, 570, A15Lowe, V., Cunningham, M. R., Urquhart, J. S., et al. 2014,MNRAS, 441, 256Lynga, G. 1964, Meddelanden fran Lunds AstronomiskaObservatorium Serie II, 141, 1Martins, F., Hillier, D. J., Paumard, T., et al. 2008, A&A, 478,219Martins, F., Schaerer, D., & Hillier, D. J. 2005, A&A, 436, 1049 olecular cloud and star formation in the surrounding of the giant H II region RCW 106 13 McClure-Griffiths, N. M., Dickey, J. M., Gaensler, B. M., et al.2005, ApJS, 158, 178McClure-Griffiths, N. M., Green, A. J., Dickey, J. M., et al. 2001,ApJ, 551, 394Mel’Nik, A. M. & Efremov, Y. N. 1995, Astronomy Letters, 21, 10Mercer, E. P., Clemens, D. P., Meade, M. R., et al. 2005, ApJ,635, 560Merello, M., Bronfman, L., Garay, G., et al. 2013a, ApJ, 774, L7Merello, M., Bronfman, L., Garay, G., et al. 2013b, ApJ, 774, 38Mezger, P. G. & Henderson, A. P. 1967, ApJ, 147, 471Miyazaki, A. & Tsuboi, M. 2000, ApJ, 536, 357Mois´es, A. P., Damineli, A., Figuerˆedo, E., et al. 2011, MNRAS,411, 705Molinari, S., Swinyard, B., Bally, J., et al. 2010, A&A, 518,L100+Mookerjea, B., Kramer, C., Nielbock, M., & Nyman, L.-˚A. 2004,A&A, 426, 119Motte, F., Bontemps, S., Schilke, P., et al. 2007, A&A, 476, 1243Motte, F., Nguyˆen Luong, Q., Schneider, N., et al. 2014, A&A,571, A32Murray, N. 2011, ApJ, 729, 133Myers, P. C. 2014, ApJ, 781, 33Nguyen Luong, Q., Motte, F., Hennemann, M., et al. 2011a,A&A, 535, A76Nguyen Luong, Q., Motte, F., Schuller, F., et al. 2011b, A&A,529, A41+Ostriker, J. 1964, ApJ, 140, 1056Ozernoi, L. M. 1964, Soviet Ast., 8, 137Parmentier, G. & Pfalzner, S. 2013, A&A, 549, A132Peeters, E., Spoon, H. W. W., & Tielens, A. G. G. M. 2004, ApJ,613, 986Peretto, N., Fuller, G. A., Plume, R., et al. 2010, A&A, 518, L98+Poglitsch, A., Waelkens, C., Geis, N., et al. 2010, A&A, 518, L2+Portegies Zwart, S. F., McMillan, S. L. W., & Gieles, M. 2010,ARA&A, 48, 431 Rathborne, J. M., Jackson, J. M., & Simon, R. 2006, ApJ, 641,389Reid, M. J., Menten, K. M., Zheng, X. W., et al. 2009, ApJ, 700,137Robitaille, T. P., Meade, M. R., Babler, B. L., et al. 2008, AJ,136, 2413Rodgers, A. W., Campbell, C. T., & Whiteoak, J. B. 1960,MNRAS, 121, 103Rodriguez-Fernandez, N. J. & Combes, F. 2008, A&A, 489, 115Roman-Duval, J., Jackson, J. M., Heyer, M., et al. 2009, ApJ,699, 1153Roussel, H. 2013, PASP, 125, 1126Russeil, D., Adami, C., Amram, P., et al. 2005, A&A, 429, 497Russeil, D., Zavagno, A., Adami, C., et al. 2012, A&A, 538, A142Sakamoto, S., Hasegawa, T., Handa, T., Hayashi, M., & Oka, T.1997, ApJ, 486, 276Salpeter, E. E. 1955, ApJ, 121, 161Schneider, N., Bontemps, S., Simon, R., et al. 2006, A&A, 458,855Simon, R., Rathborne, J. M., Shah, R. Y., Jackson, J. M., &Chambers, E. T. 2006, ApJ, 653, 1325Stolte, A., Brandner, W., Grebel, E. K., Lenzen, R., & Lagrange,A.-M. 2005, ApJ, 628, L113Stolte, A., Grebel, E. K., Brandner, W., & Figer, D. F. 2002,A&A, 394, 459Thompson, R. I. 1984, ApJ, 283, 165Tremblin, P., Minier, V., Schneider, N., et al. 2013, A&A, 560,A19Urquhart, J. S., Busfield, A. L., Hoare, M. G., et al. 2007, A&A,461, 11Walsh, A. J., Hyland, A. R., Robinson, G., & Burton, M. G.1997, MNRAS, 291, 261Wilson, T. L., Casassus, S., & Keating, K. M. 2012, ApJ, 744, 161Wong, T., Ladd, E. F., Brisbin, D., et al. 2008, MNRAS, 386,1069APPENDIX A.
21 CM CONTINUUM SOURCES
In Fig. 1, we selected 31 rectangular subregions in the 21 cm continuum data in which there is significant emission.These are the “boxes” whose details are captured in Table 3.Across the continuum image the background is rather similar, about ∼ .
09 Jy beam − , and contours are givenstarting at the 3 σ noise level above the local background. We integrated the background-subtracted 21 cm intensitywithin the area defined by the 3 σ noise contour. The flux densities range from 0.1 to 12 Jy and the sizes range from afew arcminutes to several hundred arcminutes. There is no apparent correlation of size and flux density. From theseparameters and the distance, we have derived and tabulated the characteristics of the star formation in Table 3. Starformation in the RCW 106 complex is discussed in Section 7.Because of the massive star formation across the region there are many H II regions. These can be classified accordingto their shapes: circular or irregular. The H II region around RCW 106 is classified separately as a giant H II regionand the components are displayed in Figure 8. There are also several SNRs seen in projection on the region. Thesestructures can be seen in magnified views of these “boxes” in Figs. 9–11 in the subsections below. Within each figure,the boxes are shown on the same angular scale, as labeled, with the box number at the lower left of each sub-image.The spatial locations and relationships of the boxes can be seen in Fig. 1. In all figures, except Fig. 11 for SNRs, alinear scale is also given, assuming a distance of 3.6 kpc, although the structures associated with MCC331–90 (Table 3)are at 5 kpc.4 H. Nguyen, Q. Nguyen Luong, P. G. Martin, et al. Table 3
Details of boxes and subregions as selected from the 21 cm continuum map No (cid:96) b Type V LSR
Cloud Ang. size Area S ν (21 cm) SFR density Mass Σ gas log N Ly α M tot O7V( ◦ ) ( ◦ ) ( km s − ) ( (cid:48) ) (pc ) (Jy) (M (cid:12) yr − kpc − ) (M (cid:12) ) (M (cid:12) pc − ) (s − ) (M (cid:12) )1 334.7 0 CH II -60 to -30 RCW 106 2.6 - 0.48 0.73 - - 47.68 0 02 334.7 -0.6 CH II -60 to -30 RCW 106 2.9 - 0.96 1.3 - - 48 0 03 334.6 -0.1 CH II -60 to -30 RCW 106 5.5 - 1.8 0.78 - - 48.31 0 04 334.6 0.4 CH II -60 to -30 RCW 106 1.6 - 0.12 0.53 - - 47.21 0 05 334.5 0.8 CH II -60 to -30 RCW 106 3.7 - 0.29 0.14 - - 47.76 0 06 334.2 0.1 SNR - - 27.7 - 6.4 0.24 - - 48.88 - -7 333.6 -0.2 GH II -97 to -78 MCC331–90 31.5 2109.35 - - 1.7 × × × × × × × × × × × × × × × × × × × × × × × × × × × × × A.1.
RCW 106: a giant H II Region structure
The H II region complex RCW 106 spans approximately 1 . ◦ ◦ ≤ l ≤ . ◦ II regions, star clusters, and SNR G333.6–00.2which is located at the brightest peak (see Fig. 8a).Box 9 (RCW106b). An extended structure of ∼
17 pc in length (see Fig. 8b). It has an irregular shape withprominent protrusions. The main intensity peak is crescent shaped.Box 10 (RCW106c). An irregular H II region with a fairly large main peak and size of almost 9 pc (see Fig. 8c).Box 11 (RCW106d). An irregular H II region with a fairly large main peak and a tail (see Fig. 8d).Box 13 (RCW106e). An irregular structure with three main peaks loosely connected and a size of about 17.5 pc(Fig. 8e).Box 18 (RCW106f). Diffuse H II region with four dominant peaks (Fig. 8f). A.2.
Circularly shaped H II regions A number of the smaller H II regions in Fig. 1 appear to be fairly “circular” in structure. These are illustrated inFig. 9 and described below.Box 1. The compact H II region contains the single IRAS source 16226–4900 at its center.olecular cloud and star formation in the surrounding of the giant H II region RCW 106 15 Figure 8.
Components of the giant H II structure RCW 106. The distance scales use both near and far kinematic distances. Figure 9.
Circularly shaped H II regions. The distance scales use both near and far kinematic distances. Box 2. An ultracompact H II region coincides with an IR star cluster [DBS 2003] 170. xxiii As discussed by Dutraet al. (2003), the composition is a bit ambiguous but there are a few bright stars. The H II region GAL334.71–00.67found in this box is 17.4 kpc away as determined by its positive velocity (Russeil et al. 2005).Box 3. The compact H II region that we detect is roughly 5.8 pc in size and probably coincides with the H II regionGAL 334.68–00.11. It has a nearby cluster of IR sources (Robitaille et al. 2008).Box 4. This compact H II region is roughly 1.6 pc in size, which almost makes it an ultracompact candidate.Box 5. There is a YSO on the border of the compact H II region. The star cluster [MCM 2005b] 79 (Mercer et al.2005) is also nearby. This source has a faint protrusion in the lowest contour, not so significant that we describe it asirregular.Box 8. This compact H II region has a few YSOs nearby.Box 12. This compact H II region contains the IR star cluster [DBS 2003] 102. It also coincides with the known H II regions GAL 332.98+00.79 and [WHR97] 16112–4943. The latter is classified as UCHII (Walsh et al. 1997).Box 14. This compact grouping encompasses the known H II regions [KC 97c] G332.5–00.1, GRS 332.54–00.11, andGAL 332.54–00.11 (Kuchar & Clark 1997) and the star clusters [DBS 2003] 160 and 161. This clump is a potentialOB cluster as seen from the numerous OB stars within the region. There are also some YSOs and outflow regionswithin the clump. xxiii [ ref. ] http://simbad.u-strasbg.fr . Figure 10.
Irregularly shaped H II regions. The distance scales use both near and far kinematic distances. Box 17. This is the compact H II region IRA
S 16119-5048 driving an outflow.Box 19. An H II region hosting a star cluster [MCM 2005b] 74. It has a cometary shape and is significantly brighterwith an off-center peak in its circular structure. It is located near the supernova remnant, SNR G332.4-0.4.Box 23. Most likely the compact H II region GAL 331.36+00.51, the structure is elliptical with a slight protrusionalong the longitudinal axis and is 8.6 pc in size.Box 25. A possible compact H II region. It is ∼ II regions [KC 97c] G331.3–00.2, GAL331.28–00.19, and GAL 331.26–00.19 within its peak.Box 29. A compact H II region, 2.9 pc in size and associated with the IR cluster [DBS 2003] 153. It also hosts theknow H II region IRAS II region probably powered by GAL 330.04–00.05. A.3.
Irregularly shaped H II regions The remaining H II regions as seen in Fig. 10 are “irregular” in outline as compared those in Fig. 9. Descriptions ofthese follow.Box 22. An extended H II region of ∼
10 pc in size. It has numerous IR star clusters and known H II regions. Itdisplays a comet-like structure with its brightest and largest feature at 331 . ◦
33. There are two smaller peaks trailingoff from the tail. They are diffuse and seemingly attached to the main structure.Box 24. A compact H II region with the IR star cluster, [DBS 2003] 158. It is elongated and has double peaks.The main peak is off-center in one of the protrusions where the known H II regions are located. It is also near thesupernova candidate MSC331.8+0.0 whose structure may be related.Box 26. The structure is elongated with three main compact peaks that may all be connected. Together they forman elongated structure that is ∼
11 pc in length.Box 27. Both GAL 331.03–00.15 and PMN J1610–5150 coincides with this H II region. There are numerous OBassociations spread around the main intensity peak.Box 28. This has been subdivided into three regions 28A, 28B, and 28C from largest to smallest with sizes ∼
12, 4,and 3 pc, respectively. The H II region 28A hosts numerous known H II regions and the IR star cluster [DBS 2003] 155while 28B appears as a compact H II region with IR star cluster [DBS 2003] 154. 28C is as yet undefined. Individually,28A has a comet-like shape with two tails while 28B and 28C are circular in nature. A.4.
Supernova remnants and candidates