ALMA reveals a cloud-cloud collision that triggers star formation in N66N of the Small Magellanic Cloud
Naslim Neelamkodan, Kazuki Tokuda, Susmita Barman, Hiroshi Kondo, Hidetoshi Sano, Toshikazu Onishi
DDraft version January 25, 2021
Typeset using L A TEX twocolumn style in AASTeX61
ALMA REVEALS A CLOUD–CLOUD COLLISION THAT TRIGGERS STAR FORMATION IN N66N OF THESMALL MAGELLANIC CLOUD
Naslim N, Kazuki Tokuda,
2, 3
Susmita Barman, Hiroshi Kondo, Hidetoshi Sano, and Toshikazu Onishi Department of Physics, United Arab Emirates University, Al-Ain, UAE, 15551. Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan National Astronomical Observatory of Japan, National Institutes of Natural Science, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan School of Physics, University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Telangana, Hyderabad, 500046, India
ABSTRACTWe present the results of Atacama Large Millimeter/submillimeter Array (ALMA) observation in CO(1–0) emissionat 0.58 × resolution toward the brightest H ii region N66 of the Small Magellanic Cloud (SMC). The CO(1–0)emission toward the north of N66 reveals the clumpy filaments with multiple velocity components. Our analysis showsthat a blueshifted filament at a velocity range 154.4–158.6 km s − interacts with a redshifted filament at a velocity158.0–161.8 km s − . A third velocity component in a velocity range 161–165.0 km s − constitutes hub-filaments.An intermediate-mass young stellar object (YSO) and a young pre-main sequence star cluster have hitherto beenreported in the intersection of these filaments. We find a V-shape distribution in the position-velocity diagram at theintersection of two filaments. This indicates the physical association of those filaments due to a cloud-cloud collision.We determine the collision timescale ∼ ∼ − ) and displacement ( ∼ ∼ ⊙ galaxy, the SMC, with similar kinematics as inN159W-South and N159E of the Large Magellanic Cloud. Keywords: galaxies: individual (SMC) — stars: formation
Naslim et al. INTRODUCTIONThe massive stars significantly influence the dynamicsand the physical conditions of the interstellar mediumand play a key role in galaxy evolution. It is, therefore,essential to understand the fundamental physical pro-cesses of the parent molecular cloud. These stars arerelatively rare because they are short-lived and evolvequickly compared to their low-mass counterparts. Theyare often found in clusters, and their early formationphase is highly complex due to the influence on the localenvironment by gravitational collapse and high radiationpressure (Zinnecker & Yorke 2007; Motte et al. 2018).The triggered star formation at the shock compressedlayers of two colliding clouds is suggested to be a possibleway to induce the formation of massive stars in a clusterenvironment (Inoue & Fukui 2013). Recent advances inmolecular line observations have revealed several shredsof evidence for the cloud–cloud collision that triggers theformation of massive cloud cores and stars in the Galac-tic clouds (e.g., Dobashi et al. 2014; Torii et al. 2015,2017; Fukui et al. 2016, 2018a,b). Observations withAtacama Large Millimeter Array (ALMA) have allowedus to extend these findings to the extragalactic clouds.These include N159W-South and N159E in the LargeMagellanic Cloud (LMC; Fukui et al. 2015; Tokuda et al.2019; Fukui et al. 2019), and some evident star-formingregions in M33 (Tokuda et al. 2020; Muraoka et al. 2020;Sano et al. 2020).The Small Magellanic Cloud (SMC) galaxy is an ex-cellent laboratory to study the high-mass star-formingregions in the low-metallicity at a sub-parsec resolutiondue to its close proximity (61 kpc; Hilditch et al. 2005)and low metallicity (0.1–0.2 Z ⊙ ; Russell & Dopita 1992;Rolleston et al. 2003; Lee et al. 2005). The star-formingregion N66 is the largest and the most luminous H ii region in the SMC, which comprises a variety of stel-lar population, including young stellar objects (YSO),pre-main sequence (PMS) and main-sequence OB starassociation. The region hosts nearly 33 OB stars, thatis about half of the entire SMC hot star population(Massey et al. 1989; Walborn et al. 2000; Evans et al.2006). The majority of this massive star population islocated in the central bar of the nebula that appears asa well-defined arc structure extending from south-eastto north-west in an H α map (Figure 1). These starsare the major sources of photo-ionization in the centralbar of N66 that can effectively trigger star formationvia stellar wind and shock (Elmegreen & Lada 1977).A study of gas and dust content of the N66 by Rubioet al. (2000) shows a tight correlation of H emissionwith CO and infrared aromatic emission. The CO hasbeen largely photo-dissociated by far-ultraviolet (FUV) radiation from nearby massive stars in both central barand northern filament. Similar characteristic of photo-dissociation regions are reported in many Galactic andextragalactic clouds (Tielens et al. 1993; Roussel et al.2007; Naslim et al. 2015). N66 is, therefore, the mostappropriate target to investigate the high-mass star for-mation mechanism in the SMC.The deep imaging survey with the Hubble Space Tele-scope shows that the region harbors at least five clustersof low-mass PMS stars with a significant age difference(Hennekemper et al. 2008). These include the stellarpopulation of age ≤ ≤ ≤ CO(1–0) toward the northern filamentsof N66 (N66N) in the SMC. OBSERVATIONS/DATAWe used the ALMA archival data (P.I., Erik Muller, CO( J = 1–0) line with afrequency resolution of 61 kHz and a channel number of3840. We performed the imaging process using the Com-mon Astronomy Software Application (CASA) package(McMullin et al. 2007) version 5.4.1. The weightingscheme of the tclean was “Briggs” with a robust param-eter of 0.5. The auto-multithresh procedure (Kepleyet al. 2020) automatically selected the emission maskin the dirty and residual images in tclean . We con-tinued the deconvolution process until the intensity ofthe residual image attains the ∼ σ noise level. Theresultant beam size and rms noise level are 2 . ′′ × . ′′ × ) and 0.022 Jy beam − (=0.59 K) at a velocity resolution of 0.2 km s − , respec-tively. olecular cloud N66 CO( J = 1–0) toward the N66 region. We appliedthe same flow described in the previous paragraph tothe data reduction (imaging) process. We converted the12 m array data to be the same resolution as the 7 marray data, and then produced the integrated intensityratio map in the pixels at more than 3 σ detection. Sincethe flux ratio between the two data set is almost 1, weconclude that the 12 m array data does not suffer fromthe significant missing flux. We used the 12 m arrayalone data throughout this manuscript. RESULTS3.1. CO(1–0) spatial distributions and filaments
Figure 2 shows the velocity integrated intensity distri-bution of CO(1–0) emission, which reveals the filamen-tary and clumpy structures of N66N. Two well-orderedfilaments (A and B) appear as elongated structures ex-tending from the dense clumps toward the north-eastof N66 (Figure 2). There is a third filamentary struc-ture which constitutes multiple small filaments, similarto the structures indicated as hub-filaments in manyother high-mass star-forming regions (Peretto et al.2013; Motte et al. 2018; Tokuda et al. 2019; Kumaret al. 2020). In Figure 1, we show the distribution of CO(1–0) emission in N66N on an H α map (Smith &MCELS Team 1998), and the Spitzer 8.0 µ m map (Gor-don et al. 2011). For comparison, we show the locationof 4 YSOs closer to the N66N and 41 OB stars (Duftonet al. 2019) that distribute over the entire N66. The ion-ized gas traced by H α emission shows a giant H ii regionin the center of N66 that appears as a bar. Toward theN66N filamentary complex, the H α emission appears tobe more diffuse. At the south-west of the CO(1–0)filaments, there exists a compact H α emission complex.The distribution of 8.0 µ m emission in N66N shows mul-tiple filaments, and the morphology resembles the spa-tial distribution of CO(1–0) emission.To estimate the filament’s mass and size, we identifythe filament structures using the python package as-trodendro (Rosolowsky et al. 2008).
Astrodendro characterizes the molecular gas as a structure tree withleaves, branches, and trunks in a three-dimensional datacube. The trunk represents the parent cloud, whichconstitutes the brightest structures as leaves, and thelow-density connecting structures as branches. We iden-tify the filaments as the parent structures that are thetrunks in the dendrogram.
Astrodendro provides ba-sic parameters such as size, velocity dispersion, and flux of molecular structures. We obtain the length andwidth of the filaments, A and B, from the respectivemajor and minor axes of the trunks ( σ x , σ y ). We de-rive the apparent velocity width of each filament usingthe velocity dispersion, σ v , assuming a Gaussian dis-tribution (∆ V =2 √ ln σ v ). The derived velocity dis-persion of filament A is 0.7 km s − that correspondsto a velocity width of 1.7 km s − . The length andwidth of filament A is 13 pc, and 3.8 pc respectively.Assuming a CO-to-H conversion factor 7.5 × cm − (K km s − ) − for the SMC (Muraoka et al. 2017), we findan H column density of 3.2 × cm − for the filamentA that translates to an average mass ∼ × M ⊙ .The filament B shows a velocity dispersion of 0.88 kms − (∆ V =2.1 km s − ) with a length ∼
21 pc and width ∼ column density of filament B is esti-mated to be 5.8 × cm − with a mass ∼ × M ⊙ .3.2. Velocity structures
The complexity of velocity structures is visible in aseries of channel maps (Figure 3). The molecular cloudhas an elongated filamentary structure that spread overa velocity range 154.4–165.2 km s − . Filament A shows avelocity range 154.4–158.6 km s − and filament B showsa velocity range 158.0–161.8 km s − . We find a third ve-locity component (Filament C) in a velocity range 161–165.0 km s − .Figure 4a shows the first-moment intensity weightedvelocity (moment 1) map of CO(1–0) emission inN66N. The moment 1 map reveals the blueshifted andredshifted velocity components (A and B) relative tothe systemic velocity ∼
158 km s − . In Figure 4b, weshow the three velocity components in CO(1–0) inte-grated intensity contours. The blue contour representsthe blueshifted filament A with the peak emission ata velocity of 157.4 km s − and the green contour rep-resents the redshifted filament B with a velocity peakat 160 km s − . The red represents the redshifted thirdcomponent C at a peak velocity of 163 km s − .The position-velocity (PV) diagram at the intersectionof filaments A and B shows a V-shape gas distribution(Figure 5), indicating the physical connection of two fil-aments. This V-shape gas distribution at the filamentintersection in the PV diagram is a key observationalsignature of the cloud–cloud collision (discussion in sec-tion 5). CO CORES IN FILAMENTS AND ASSOCIATEDYOUNG STELLAR POPULATIONThe velocity integrated CO(1–0) intensity map ofN66N shows localized emission peaks along the fila-ments. We use astrodendro to identify the molecular olecular cloud N66 ii region between the fil-ament intersection and the central bar. We do not haveenough parametric information such as the mass andluminosity of YSO 548. Sewi lo et al. (2013) provide an8.0 µ m magnitude of [10.69] for YSO 548, indicating anintermediate-mass (mass range 5-10 M ⊙ ) object accord-ing to the selection criterion suggested by Chen et al.(2009). The YSO 548 is associated with a CO coreof virial mass 156 M ⊙ . It is also possible that a mas-sive YSO remains to be formed in the densest core atthe intersection. Sewi lo et al. (2013) report a mass of ∼ ⊙ and a luminosity of 3.24 × L ⊙ for YSO 544.The CO core associated with YSO 544 shows a virialmass of ∼ ⊙ , although there is no extended COemission.The CO(1–0) filaments in N66N have similar mor-phology with the 8.0 µ m emission, indicating the pho-toelectric heating of polycyclic aromatic hydrocarbonby FUV radiation, possibly from OB stars in the cen-tral bar. The extended low-density H α emission and CO(1–0) filaments in N66N show a vertical orientationwith the H ii region in the central bar. Ye et al. (1991)suggest a high-velocity ’Champagne flow’ from the ex-citing OB stars in the central bar toward the north-east.The H ii region in the central bar are photo-dissociatingthe natal molecular clouds, but some of the clouds arestill surviving at N66N where the cloud collision seemsto be taking place, resulting in a formation of at leastan intermediate-mass star at the intersection of two fil-aments. In addition to the YSO 548, a PMS star clusteris located in the filament intersection. This PMS starcluster is a relatively younger population (0.25–2.5 Myr)than those in the central bar (0.5–10 Myr; Hennekemperet al. 2008) and located 24 pc away from the central barin the filament intersection. We expect this low-massPMS cluster has been formed prior to the cloud–cloudcollision as it is found to be slightly older than the es-timated collision timescale ∼ ii region blown by the supernova remnantSNR B0057-724 (Ye et al. 1991; Naz´e et al. 2002) (Fig-ure 1), which is in the east of the filament intersection ata projected distance of ∼
21 pc. Although the redshiftedfilament C has an overlap with an edge of the X-rayemission (Naz´e et al. 2002), we do not find a continuousvelocity gradient with an expanding shell of moleculargas that is blown from SNR B0057-724 in the directionof filament intersection. We finally remark on star formation in the SMC-likelow-metallicity environment (0.1–0.2 Z ⊙ ) and the futureprospect of this study. The numerical simulations byRicotti et al. (1997); Ricotti & Ferrara (2002) show thatthe kinetic energy dissipation of the cloud collision de-creases in a lower-metallicity condition due to the longercooling time-scale of the shocked gas than the charac-teristic collision time; hence a significant effect on thestar formation is expected. Our finding indicates thatcloud–cloud collision does occur and work as a trigger ofstar formation in low-metallicity environment. However,the N66N region hosts several YSOs, such as YSO 544,associated with a compact/isolated CO clump, whichis not evident in more than two velocity componentsat the current angular resolution. The subsequent stepis to understand how common the cloud–cloud collisionthroughout the N66 region and the SMC, and how theproperties of parental molecular clouds affect the massand luminosity of embedded YSO. A more comprehen-sive analysis extending to the other YSOs using the cur-rent data set in N66 will be presented in a future paper. CONCLUSIONWe report clumpy filaments with multiple velocitycomponents toward N66N in the SMC by ALMA. Ourresults are concluded as follow:1. The ALMA observation of CO(1–0) emissionin N66N shows a blueshifted velocity component(A) in a velocity range 154.4–158.6 km s − anda redshifted component (B) in a velocity 158.0–161.8 km s − . A third redshifted component (C)in a velocity 161-165.0 km s − shows hub-filamentdistribution.2. An intermediate-mass YSO has been found at theintersection of filaments A and B. We find a V-shape gas distribution in the PV diagram taken atthe intersection of filaments A and B, indicatingtheir physical association. These filament char-acteristics are similar to the cloud–cloud collisionreported in N159E of the LMC.3. We find the H column densities of two inter-acting filaments using a CO-to-H conversionfactor 7.5 × cm − (K km s − ) − . The red-shifted component has an H column density of5.8 × cm − and mass ∼ × M ⊙ , while theblueshifted component shows a column density3.2 × cm − and mass ∼ × M ⊙ .4. We estimate the timescale of collision ∼ ∼ − and dis-placement ∼ Naslim et al.
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