Dissecting The Core Of The Tarantula Nebula With MUSE - ESO

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Astronomical ScienceDOI: 10.18727/0722-6691/5053Dissecting the Core of the Tarantula Nebula withMUSEPaul A. Crowther1Norberto Castro2Christopher J. Evans 3Jorick S. Vink4Jorge Melnick5Fernando Selman 51 Department of Physics & Astronomy,University of Sheffield, United Kingdom2 Department of Astronomy, University ofMichigan, Ann Arbor, USA3 UK Astronomy Technology Centre,Royal Observatory, Edinburgh, UnitedKingdom4 Armagh Observatory, United Kingdom5 ESOWe provide an overview of Science Verification MUSE observations ofNGC 2070, the central region of theTarantula Nebula in the Large MagellanicCloud. Integral-field spectroscopy ofthe central 2 2 arcminute region provides the first complete spectroscopiccensus of its massive star content, nebular conditions and kinematics. Thestar formation surface density ofNGC 2070 is reminiscent of the intensestar-forming knots of high-redshift galaxies, with nebular conditions similar tolow-redshift Green Pea galaxies, someof which are Lyman continuum leakers.Uniquely, MUSE permits the star formation history of NGC 2070 to be studiedwith both spatially resolved and integratedlight spectroscopy.Tarantula NebulaThe Tarantula Nebula (30 Doradus) inthe Large Magellanic Cloud (LMC) isintrinsically the brightest star-formingregion in the Local Group and has beenthe subject of numerous studies acrossthe electromagnetic spectrum. Its low(half-solar) metallicity and high star formation intensity are more typical of knotsin high-redshift star-forming galaxies thanlocal systems thanks to its very rich stellar content (Doran et al., 2013). Indeed,30 Doradus has nebular conditions thatare reminiscent of the galaxies knownas Green Peas. These are local extremeemission-line galaxies that are analoguesof high-redshift, intensely star-forminggalaxies, some of which have been con-40The Messenger 170 – December 2017firmed as Lyman continuum leakers (forexample, Micheva et al., 2017).The Tarantula Nebula is host to hundredsof massive stars that power the strongHa nebular emission, comprising mainsequence OB stars, evolved blue supergiants, red supergiants, luminous bluevariables and Wolf-Rayet (WR) stars. Theproximity of the LMC (50 kpc) permitsindividual massive stars to be observedunder natural seeing conditions (Evanset al., 2011). The exception is R136, thedense star cluster at the LMC’s core thatnecessitates the use of adaptive opticsor the Hubble Space Telescope (HST; see K horrami et al., 2017, Crowther et al.,2016). R136 has received particular attention since it hosts very massive stars( 100 M ; Crowther et al., 2016) that arethe potential progenitors of pair-instabilitysupernovae and/or merging black holeswhose gravitational wave signatures haverecently been discovered with LIGO.Star formation in the Tarantula Nebulabegan at least 15–30 Myr ago, as witnessed by the cluster Hodge 301, whosestellar content is dominated by red supergiants. There was an upturn in its rateof star formation within the last 5–10 Myr,which peaked a couple of Myr ago inNGC 2070, the central ionised region thathosts R136. Star formation is still ongoing, as witnessed by the presence ofmassive young stellar objects and clumpsof molecular gas observed with ALMA(Indebetouw et al., 2013). The interplaybetween massive stars and the inter stellar medium also permits the investi gation of stellar feedback at high spatialand spectral resolutions (for example, Pellegrini, Baldwin & Ferland, 2011).MUSE observations of NGC 2070NGC 2070, the central region of theTarantula Nebula, was observed with theMulti Unit Spectroscopic Explorer (MUSE)as part of its original Science Verificationprogramme at the Very Large Telescope(VLT) in August 2014. MUSE is a widefield, integral-field spectrograph, providing intermediate-resolution (R 3000at Ha) spectroscopy from 4600–9350 Åover one square arcminute with a pixelscale of 0.2 arcseconds. Four overlappingMUSE pointings provided a 2 2 arcmin-ute mosaic which encompasses boththe R136 star cluster and R140 (an aggregate of WR stars to the north). See Figure 1 for a colour-composite of the central 200 160 pc of the Tarantula Nebulaobtained with the Advanced Camerafor Surveys (ACS) and the Wide FieldCamera 3 (WFC3) aboard the HST. Theresulting image resolution spanned 0.7to 1.1 arcseconds, corresponding to aspatial resolution of 0.22 0.04 pc, providing a satisfactory extraction of sourcesaside from R136. Four exposures of600 s each for each pointing provided ayellow continuum signal-to-noise (S/N) 50 for 600 sources. A total of 2255sources were extracted using SExtractor,while shorter 10- and 60- second exposures avoided saturation of strong nebular lines. Absolute flux calibration wasachieved using V-band photometry fromSelman et al. (1999). An overview of thedataset, together with stellar and nebularkine matics, is provided by Castro et al.(submitted to A&A).Spatially resolved nebular propertiesWe present colour-composite imagesextracted from the MUSE datacubes inFigure 2, highlighting the stellar contentand ionised gas, respectively. Figure 2asamples 6640, 5710 and 4690 Å, suchthat most stars appear white except forcool supergiants (orange; for example,Melnick 9 in the upper left) and WR stars,which appear blue owing to strong He II4686 Å emission. Examples of the WRstars include R134 to the right of the central R136 star cluster and the R140complex at the top, which hosts WNand WC stars, subsets of WR stars thathave dominant lines of ionised nitrogenand ionised carbon respectively. In contrast, Figure 2b highlights the distributionof low-ionisation gas ([S II] 6717 Å, red),high-ionisation gas ([O III] 5007 Å, blue)and hydrogen (Hα, green). Green pointsources generally arise from broad Hαemission from WR stars and relatedobjects.Owing to the presence of ionised gasthroughout NGC 2070, our MUSE datasets enable the determination of nebularproperties. Adopting a standard Milky Wayextinction law, there is a wide variationin extinction throughout the region with

NASA, ESA, D. Lennon et al.Figure 1. MUSE 2 2arcminute mosaic (whitesquare) superimposedon a colour- c ompositeimage of the TarantulaNebula (correspondingto 200 160 parsecs),obtained with the ACSand WFC3 instrumentsaboard HST.1Figure 2. (a) VLT/MUSE colour-composite image ofNGC 2070 (2 2 arcminutes) sampling 6640 Å (red),5710 Å (green), and 4690 Å (blue). Blue sources areWR stars with prominent He II 4686 Å emission,b)–69 05ಿ20ೀ–69 05ಿ20ೀ40ೀ40ೀDeclination (J2000)Declination (J2000)a)while orange sources are predominantly red supergiants. (b) VLT/MUSE colour-composite image ofNGC 2070 (2 2 arcminutes) sampling [S II] 6717 Å(red), Hα (green), and [O III] 5007 Å ��40ೀ30ೀ52s48s4440Right ascension (J2000)ss36s5 h38 m32 s07ಿ00ೀ30ೀ52 s48 s44 s40 sRight ascension (J2000)36 s5 h38 m32 sThe Messenger 170 – December 201741

Astronomical ScienceCrowther P. A. et al., Dissecting the Core of the Tarantula Nebula with MUSE130001000900–69 05ಿ20ೀ12500–69 ��11000Te (K)06ಿ00ೀn e (cm –3)600Declination (J2000)Declination 30ೀMassive stars in NGC 2070MUSE permits the first complete spectroscopic census of massive stars withinNGC 2070. Previous surveys have beenrestricted to multi-object spectroscopyusing slitlets or fibres (Bosch et al., 1999;Evans et al., 2011). Spectral lines in theblue are usually employed in the classification of OB stars, so the 4600 Å bluelimit to MUSE has required the development of green and yellow diagnostics.Representative OB spectra from MUSEare presented in Figure 4 with classifications from blue spectroscopy using theFibre Large Array Multi Element Spectro-42The Messenger 170 – December 20172.01.50.546004800He IHe IN IIIC IIIHe IIHe I1.0500048 s44 s40 s36 sRight ascension (J2000)5 h38 m32 sMUSE 1433VFTS 599 O3 III(f*)Teff 57000 (K)MUSE 1008VFTS 511 O5 V((n))((fc))zTeff 43000 (K)MUSE 276VFTS 491 O6 V((fc))Teff 41000 (K)MUSE 2193VFTS 585 O7 V(n)Teff 36000 (K)MUSE 1870VFTS 611 O8 V(n)Teff 35000 (K)MUSE 1334VFTS 635 O9.5 IVTeff 31000 (K)MUSE 1689VFTS 420 B0.5 IaTeff 24000 (K)5200Wavelength (Å)graph (FLAMES) on the VLT (Walborn etal., 2014). A spectroscopic analysis of270 sources with He II 5412 Å absorptionis now underway using the non-localthermodynamic equilibrium atmosphericcode FASTWIND (Puls et al., 2005), yielding temperatures and luminosities fromHe I 4921 Å and He II 5412 Å. Preliminaryfits to the illustrative spectra are alsoshown in Figure 4.Ultimately we will determine the propertiesof all of the massive stars in NGC 2070in order to fully characterise its recentstar-formation history, substituting resultsfrom long-slit HST spectroscopy usingthe Space Telescope Imaging Spectrograph (STIS) for the central parsec ofR136 (Crowther et al., 2016). Quantitativeanalysis of the MUSE data should alsoprovide useful insights to incorporate into540056009000C IV52 sSi III07ಿ00ೀ2.5Normalised fluxcoefficients spanning 0.15 c(Hb) 1.2.On average, we found c(Hb) 0.55 mag,which is in excellent agreement withlong-slit results from Pellegrini, Baldwin& Ferland (2011). Nebular lines also permit the determination of electron densities and temperatures from [S II] and [S III]diagnostics as illustrated in Figure 3. Thedust properties towards the TarantulaNebula are known to be non-standard,with an average c(Hb) 0.6 obtainedfrom the law presented by Maíz-Apellánizet al. (2014) and R 4.4, although thishas little bearing on the nebular conditionsdetermined here owing to the use of redspectral diagnostics.0O IIIFigure 3. (Above) Distribution of gas density (left)and temperature (right) within the MUSE field of view,based on [S II] 6717/6731 Å and [S III] 6312/9069 Ådiagnostics.5 h38 m32 sHe II44 s40 s36 sRight ascension (J2000)He I[O III]48 sHβ52 sC III30ೀ07ಿ00ೀ5800Figure 4. Blue to yellow spectroscopy of representative OB stars in NGC 2070 observed with VLT/MUSE(black solid lines), including spectral types from theVLT-FLAMES Tarantula survey, and temperaturesfrom FASTWIND model fits (dashed red lines) to He I4921 Å and He II 5412 Å.stellar evolution theory. For instance,Castro et al. (2014) suggested empiricalboundaries for the zero- and terminal-agemain sequences from their analysis of alarge sample of OB stars. The MUSEdata will enable a homogeneous analysisof a larger stellar sample, spanning abroad range of evolutionary stages (forexample, main sequence, blue and redsupergiants, and WR stars).Of course, it is well known that massivestars prefer company, so it is likelythat many of the MUSE point sourcesare multiple. Fortunately, the majority of

81e–106.56.07Figure 5. Integrated MUSE spectrum of NGC 2070,revealing a striking emission line spectrum, withcharacteristics reminiscent of Green Pea galaxies,plus WR bumps in the blue (upper inset, He II 4686 Åarising from WN stars) and yellow (lower inset, C IV5801–12 Å due to WC stars).1e–12Flux (erg s –1 cm –2 Å 2.82.75700543210500046505750470058005500Wavelength (Å) assive stars in NGC 2070 have previmously been monitored spectroscopicallywith VLT/FLAMES, revealing many short- period systems. In addition, 30 Doradushas been the target of the Chandra X-rayVisionary Programme (T-ReX) using theAdvanced CCD Imaging Spectrometer(ACIS-I). This programme monitored X-rayemission from the Tarantula Nebulaover 630 days, permitting longer-periodsystems to be identified. For example, Melnick 34, the blue emission-line star tothe left of R136 in Figure 2a, has beenrevealed as an eccentric colliding-windbinary by its T-ReX variability (Pollock etal., 2017).Integrated spectrum of NGC 2070In addition to spectra of the spatially- resolved stars and gas in NGC 2070, it ispossible to sum the MUSE observationsto arrive at the integrated spectrum ofthe region. NGC 2070 would subtend0.6 arcseconds if it were located at a distance of 10 Mpc, so MUSE offers theunique opportunity to study both the spatially-resolved properties of an intensivelystar-forming region and its aggregatecharacteristics. The integrated spectrumof NGC 2070 is presented in Figure 5. Inaddition to strong nebular lines, the highthroughput of MUSE and the proximity ofNGC 2070 allow a plethora of weakerfeatures to be revealed in the integratedspectrum, including the non-standarddensity diagnostic Cl III 5517/5537 Å. Figure 5 also highlights broad blue (He II4686 Å) and yellow (C IV 5801–12 Å) WRfeatures in the integrated spectrum, withno evidence for a nebular contribution475058506000480059006500to the former. These are often observedin the integrated light of extragalacticstar-forming regions.Figure 6a compares the strong-line nebular characteristics of NGC 2070with Sloan Digital Sky Survey (SDSS)star-forming galaxies and indicates similar high-excitation properties to thoseof Green Pea galaxies (Micheva et al.,2017). Analysis of the integrated spectrum reveals c(Hb) 0.57 for a standardextinction law, such that the dereddenedHα luminosity is 1.5 1039 erg s –1, corresponding to one-eighth of the entireTarantula Nebula (Doran et al., 2013).The current star formation rate (SFR) forNGC 2070 is 0.008 M yr –1. This hasbeen obtained by adopting a standardKennicutt (1998) relation between Hαluminosity and SFR, modified for a K roupa (2002) initial mass function bydividing by a factor of 1.5, and inferringa high star formation surface densityof ΣSFR 10 M yr –1 kpc –2. Conditionsare similar to clumps of intensively starforming galaxies at high redshifts, asdemonstrated in Figure 6b (adapted fromJohnson et al., 2017).Properties inferred from the integratedlight of NGC 2070The inferred age of the region from theequivalent width of Hα is 4 Myr, implying a mass of 105 M for an instantaneous burst of star formation. This is doublethe mass estimated for the central R136cluster. In reality, there is an age spreadof 0–10 Myr for massive stars within theentire Tarantula Nebula (Schneider et al.,2017), although the peak of star formationwas inferred to be 4.5 Myr ago, excluding R136 (which has an age of 1.5 Myr;Crowther et al., 2016). The Hα-derivedionising output is 1051 photons s –1 forNGC 2070, equivalent to 100 O7 Vstars. This corresponds to 300 O starsfor the nebular derived age (Schaerer &Vacca, 1998), in good agreement with thenumber of MUSE sources that displayHe II 5412 Å absorption, albeit neglectingthe (significant) contri bution of the WRstars to the cumulative ionising output.We derive log (O/H) 12 8.25 forNGC 2070, adopting N and S temperatures for singly- and doubly-ionised oxygen, respectively. However, the blueMUSE cutoff excludes the use of thestronger [O III] 4363 Å line. Direct determinations for the entire 30 Doradusregion indicate a somewhat higher oxygen content (for example, log (O/H) 12 8.33; Tsamis et al., 2003). Since WRline luminosities are metallicity dependent(Crowther & Hadfield, 2006) one wouldinfer 20 mid-WN and five early WC starsin NGC 2070, or N(WR)/N(O) 0.08, byadopting LMC templates. This is in reasonable agreement with the resolved WRcontent of the MUSE field, namely 10 WNstars, 6 Of/WN stars and 2 WC stars.The rich star cluster R136 hosts four ofthe most massive WN5h stars in theregion, but only contributes one third ofthe cumulative He II 4686 Å emission. Incontrast, the less prominent R140 complex, which hosts two WN6 stars and oneWC star, contributes another third of theHe II 4686 Å emission and dominates theintegrated C IV 5808 Å and C III 4650 Åflux. This arises from the relatively weakwind strengths of main sequence WN5hstars, as opposed to the significantlystronger emission from classical WN stars.Strong-line calibrations are widelyemployed to infer the metallicity of extragalactic H II regions because of the faintness of auroral lines. Application ofthe commonly used calibrations fromPettini & Pagel (2004), using the N2 andThe Messenger 170 – December 201743

O3N2 indices, would imply a SmallMagellanic Cloud (SMC)-like oxygen content of log (O/H) 12 8.0, significantlylower than our direct determination. If onehad to rely on strong-line diagnostics forNGC 2070, the use of SMC-metallicityWR templates from Crowther & Hadfield(2006) would suggest an unrealisticallyhigh number of mid-WN stars, and, inturn, N(WR)/N(O) 0.3. This would represent a severe challenge to current single/binary population synthesis models for astarburst region with 0.2 Z , in stark contrast with the N(WR)/N(O) 0.07 and0.4 Z that has been obtained from ourspatially resolved spectroscopy of theregion.Crowther P. A. et al., Dissecting the Core of the Tarantula Nebula with MUSE1.5a)1.0Extreme GPGPGP LeakersNGC 2070log10([O III]) 5007/Hβ)Astronomical �1.0–0.5log10([N II]) 6584/Hα)0.00.5b)z 51z 1.3–3.4z 1.3z 1.7z 0.10log10(SFR, M /yr)Baldwin, J. A., Phillips, M. M. & Terlevich, R. 1981,PASP, 93, 5Bosch, G. et al. 1999, A&AS, 137, 21Castro, N. et al. 2014, A&A, 570, 13Crowther, P. A. et al. 2016, MNRAS, 458, 624Doran, E. et al. 2013, A&A, 558, A134Evans, C. J. et al. 2011, A&A, 530, A108Kennicutt, R. C. 1998, ARA&A, 36, 189Khorrami, Z. et al. 2017, A&A, 602, A56Kroupa, P. 2002, Science, 295, 82Indebetouw, R. et al. 2013, ApJ, 774, 73Johnson, T. L. et al. 2017, ApJ, 843, L21Maíz-Apellániz, J. et al. 2014, A&A, 564, A63Micheva, G. et al. 2017, ApJ, 845, 165Pellegrini, E. W., Baldwin, J. A. & Ferland, G. J. 2011,ApJ, 738, 34Pettini, M. & Pagel, B. E. J. 2004, MNRAS, 348, L59Puls, J. et al. 2005, A&A, 435, 669Pollock, A. M. T. et al. 2017, MNRAS, in pressSchaerer, D. & Vacca, W. D. 1998, ApJ, 497, 618Selman, F. et al. 1999, A&A, 341, 98Schneider, F. et al. 2017, Science, in pressTsamis, Y. et al. 2003, MNRAS, 338, 687Walborn, N. et al. 2014, A&A, 564, A40z 1.5–2.2–1z 1–1.5z 2.5–2NGC 2070Links1 ubble News 206–3–41.044The Messenger 170 – December 20171.52.02.5log10(radius, pc)3.03.5Figure 6. (a) BPT diagram (Baldwin, Phillips & Terlevich, 1981) illustrating the similarity in integrated strengthsbetween NGC 2070/Tarantula (filled/open redsquare), Green Pea(green circles), extremeGreen Pea (blue diamonds), and Lymancontinuum emittingGreen Pea (pink triangles) galaxies, plusSDSS star-forming galaxies (black dots);updated from Figure 2of Micheva et al. (2017).(b) Comparison betweenthe integrated star- formation rate ofNGC 2070/Tarantula(filled/open red square)and star-forming knotsfrom galaxies, spanninga range of redshifts(adapted from Figure 2of Johnson et al., 2017).

using slitlets or fibres (Bosch et al., 1999; Evans et al., 2011). Spectral lines in the blue are usually employed in the classifi-cation of OB stars, so the 4600 Å blue limit to MUSE has required the develop-ment of green and yellow diagnostics. Representative OB spectra from MUSE are presented in Figure 4 with classifica-

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