SKA studies of in-situ synchrotron radiation from molecular clouds Clive Dickinson,1∗ R.Beck,2 R.Crocker3 R.M.Crutcher,4 R.D.Davies,1 K.Ferriere,5 G.Fuller,1 T.Jaffe,5 D.I.Jones,6 J.P.Leahy,1 E.J.Murphy,7 M.W.Peel,1 E.Orlando,8 5 T.Porter,8 R.J.Protheroe,9 T.Robishaw,10 A.W.Strong,11 R.A.Watson,1 1 0 F.Yusef-Zadeh12 2 1JodrellBankCentreforAstrophysics,TheUniversityofManchester,UK n 2MPIfR,Bonn,Germany a 3AustralianNationalUniversity,Canberra,Australia J 4DepartmentofAstronomy,UniversityofIllinois,USA 5 5IRAP,UniversitédeToulouse,CNRS,Toulouse,France ] 6DepartmentofAstrophysics,RadboudUniversity,Nijmegen,TheNetherlands A 7IPAC,Caltech,MC220-6,PasadenaCA,91125,USA G 8KavliInstitute,StanfordUniversity,Stanford,CA94305,USA . h 9DeptofPhysics,UniversityofAdelaide,SouthAustralia5005,Australia p 10DRAO,Penticton,BC,V2A6J9,Canada - o 11MPE,Garching,Germany r 12DepartmentofPhysicsandAstronomy/CIERA,NorthwesternUniversity,Evanston,IL.,USA t s a [ Observationsofthepropertiesofdensemolecularcloudsarecriticalinunderstandingtheprocess 1 v of star-formation. One of the most important, but least understood, is the role of the magnetic 4 fields. We discuss the possibility of using high-resolution, high-sensitivity radio observations 0 8 withtheSKAtomeasureforthefirsttimethein-situsynchrotronradiationfromthesemolecular 0 clouds. If the cosmic-ray (CR) particles penetrate clouds as expected, then we can measure the 0 . B-fieldstrengthdirectlyusingradiodata. Sofar,thissignaturehasneverbeendetectedfromthe 1 0 collapsingcloudsthemselvesandwouldbeauniqueprobeofthemagneticfield. Densecoresare 5 typically∼0.05pcinsize,correspondingto∼arcsecat∼kpcdistances,andfluxdensityestimates 1 : are∼mJyat1GHz. TheSKAshouldbeabletoreadilydetectdirectly, forthefirsttime, along v i lines-of-sightthatarenotcontaminatedbythermalemissionorcomplexforeground/background X synchrotronemission. Polarisedsynchrotronmayalso bedetectableprovidingadditional infor- r a mationabouttheregular/turbulentfields. AdvancingAstrophysicswiththeSquareKilometreArray June8-13,2014 GiardiniNaxos,Sicily,Italy ∗E-mail:[email protected] (cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/ SKAstudiesofsynchrotronradiationfrommolecularclouds 1. Introduction StarformationisoneofthekeyprocessesintheUniverseandisoffundamentalimportanceto astrophysics(McKee&Ostriker2007). Westilldonothaveafullunderstandingofstarformation and,inparticular,ontherolethatmagneticfieldsplay(Crutcher2012;Lietal.2014). CosmicRays (CRs)arethemainionisingand heatingagentindense, starless, molecularcloudcores(Padovani & Galli 2013). The role of magnetic fields has been debated for decades, but due to a lack of precise measurements it has still not been conclusively settled. There are two major classes of star-formationtheories: i)strongfieldmodelswheretheB-fieldcontrolsthemolecularcloud,with ambipolar diffusion driving the formation and collapse of dense cores, and ii) weak-field models whereturbulentflowstriggerstarformation. AlthoughB-fieldsareatleastcomparableinstrength totheturbulentpressureintheISM,thereisnodefinitiveevidenceforB-fieldsdominatinggravity orforambipolar-diffusion-drivenstarformation(Crutcher2012). It is very difficult to measure the detailed properties of dense molecular clouds, and perhaps the most difficult property is the magnetic field - its strength and geometry. The most common method is via Zeeman splitting of radio-frequency HI, OH, and CN lines or masers, which typi- cally gives only the magnetic field strength along the line-of-sight, B (Crutcher 2012)1. Other los methods are also difficult, which include using measurements of optical and near-infrared polar- isation (extinction along the line-of-sight) and sub-mm polarised thermal dust emission (difficult fromtheground)(Poidevinetal.2013). An alternative method of probing the magnetic field is via synchrotron radiation (Brown & Marscher 1977; Marscher & Brown 1978; Orlando & Strong 2013), which has rarely been men- tionedintheliteratureinrelationtomolecularclouds. Synchrotronradiationisproducedprimarily byrelativisticCRelectronswhendeceleratedbymagneticfields. Theintensityofsynchrotronra- diation depends only on the number and energy spectrum of CR electrons and the magnetic field strengthperpendiculartotheline-of-sight. WeknowthemeasuredenergyspectrumofCRs(Ack- ermann et al. 2012), at least on average at energies of relevance to radio synchrotron emission (∼GeV), and we know that the CR density varies slowly throughout the Galaxy and can pene- trate dense molecular clouds at these energies and above (Brown & Marscher 1977; Marscher & Brown 1978; Umebayashi & Nakano 1981).2 In molecular clouds, the magnetic field strength is much larger than in the ambient interstellar medium (Crutcher 1999). Therefore, in principle, the synchrotron intensity should give a detectable signature, which could be used as a probe of the magnetic field. This is a direct way of measuring the total magnetic field strength, including the irregular (turbulent) component, which most other indicators (Zeeman, optical polarisation) are notdirectlysensitiveto,sincetheymeasuretheregular(Zeeman)orordered(opticalpolarisation) field.3 Polarisedsynchrotronemissioncouldprovideadditionalinformation,includingtheordered 1ItispossibletogetthetotalB-fieldstrengthwhencompletelinesplittingisobserved,whichispossiblewithmasers (Crutcher1999);seealsoRobishaw(2014). 2TherehavebeenclaimsthatGeVCRscannotfullypenetratethedensestclouds,thussuppressingtheCRdiffusion coefficient(Jonesetal.2008;Protheroeetal.2008;Jonesetal.2011). 3Other indirect measures of the random component exist, such as the Chandrasekhar-Fermi method, which uses thedispersionofthemeasuredpolarisationanglestoprobethemagneticfieldintheplaneofthesky(Chandrasekhar &Fermi1953;Watsonetal.2001;Crutcheretal.2004). SeealsoHildebrandetal.(2009)andreferencesthereinfor furtherextensions. 2 SKAstudiesofsynchrotronradiationfrommolecularclouds versusanisotropicrandomcomponentandprojectedangleofthemagneticfieldonthesky. Itisthereforesomewhatsurprisingthatverylittleattentionhasbeengiventousingsynchrotron as a probe of molecular clouds. Jones et al. (2008) observed two nearby (3–4kpc) dense cold starless cores (G333.125–0.562 and IRAS15596–5301) with the Australian Telescope Compact Array (ATCA) at 1384 and 2368 MHz, to try to detect secondary leptons.4 They found upper limitsof∼0.5mJy/beamandconstrainedtheB-fieldstrengthtoB<500µG.However,thisisstill compatible with the scaling of |B| and n - more sensitivity is required. Protheroe et al. (2008); H Jones et al. (2011) only found upper limits from SgrB2 after subtraction of the dominant thermal emission. The only possible candidate so far is from the G0.13–0.13 molecular cloud detection, whichwasdetectedat74MHz,withanassociatedCOhotspot(Yusef-Zadehetal.2013). However, adisplacementbetweentheradiopositionandmolecularcoresuggestsitcouldbefromadifferent regionofspace. These non-detections can be partly understood due to the relatively weak (typically mJy or less) signal that is expected to come from in-situ synchrotron emission inside the cloud itself. This is due to the fact that on large scales (∼ 1–10pc), the magnetic fields in clouds appear to be relatively weak (∼10µG) while strong fields are on scales much smaller than this (∼0.05pc) resulting in a weak flux signal. Also, most of the collapsing clouds ("cores") are located at low latitudeswherethereissignificantconfusionfrombackgroundsynchrotronandfree-freeemission. Nevertheless,highresolutionandhighsensitivityobservationscouldallowmolecularcloudstobe mapped in some sight-lines. This may also shed light on CR penetration into the densest clouds, whichsometimesappearasaradiodarkcloud(RDC)(Yusef-Zadeh2012). Notethatrecenthigh- resolution5/20GHzJVLAobservationsoftheGalacticcentrecloudG0.216+0.016havedetected compact(<2.2arcsec,orsub-pc)non-thermalsources,whichmaybethesignatureofin-situsyn- chrotronradiationfromsecondaryCRelectrons(Jones2014). Inthischapterwebrieflyreviewthephysicsofsynchrotronradiationandmagneticfields,and therelationthatappearstoexistbetweentheminmolecularclouds. Wethengivetheprospectsof detectingthissignatureinmolecularcloudswiththeSKA. 2. Synchrotronradiation Synchrotronradiationisemittedprimarilybyrelativisticcosmic-rayelectronsspirallinginthe Galacticmagneticfield. Itisthisradiationthatoftendominatestheradioskyatfrequenciesbelow a few GHz. The theory of synchrotron radiation is well understood. For a power-law distribution ofelectronenergies, N(E)dE =N E−γdE , (2.1) 0 theemissivity, j ,ofsynchrotronradiationisgivenby ν j ∝N B(γ+1)/2ν(1−γ)/2 , (2.2) ν 0 4InthisarticlewefocusonprimaryCRelectrons,althoughtheconversionintosecondaryleptonscouldbesignif- icantinsomeclouds(Dogel’&Sharov1990;Protheroeetal.2008). ObservationswiththeSKAcouldalsoprovide usefulconstraintsonsecondaries. 3 SKAstudiesofsynchrotronradiationfrommolecularclouds where B is the magnetic field strength, N is the number density of electrons, γ is the power-law 0 index of electron energies, and ν is the observing frequency. Highly relativistic (GeV and above) CR electrons are expected to penetrate dense clouds freely (Umebayashi & Nakano 1981). If the electron energy spectrum is a power-law with slope −γ then the observed synchrotron radio emissionspectrumisalsoapower-lawwithslopeα =(1−γ)/2(fluxdensityS∝να). Thecosmic ray energy distribution at energies of order GeV can be approximated by a power-law with slope γ ≈+2.5–3.0(Ackermannetal.2012),whichcorrespondstoasynchrotronindexα ≈−0.8. This is indeed the typical spectral index observed at GHz frequencies (Reich & Reich 1988; Platania etal.1998). It can also be seen that the emissivity scales as B(γ+1)/2, which means it goes as approxi- mately B2. This is of relevance to molecular cloud collapse, since the magnetic field is expected to be significantly amplified during collapse, and thus could give a detectable signal from in-situ synchrotronradiation. 3. Magneticfieldsincollapsingclouds The role that magnetic fields play in the processes of molecular cloud and star formation has been debated for decades. Theoretical studies suggest that magnetic fields play an important if not crucial role in the evolution of interstellar clouds and the formation of stars. In summary, magneticfieldsprovidemagneticsupportagainstcloudcollapse. Therearevariousmodelsofstar formation and the details of the magnetic field are always important. For example, the core of a cloudcanbecomeunstableduetoambipolardiffusion,collapsingtoformstars,whiletheenvelope can remain in place. The connection between the core and the surrounding envelope by magnetic field lines can transfer angular momentum outward and make it possible for stars to form. Other starformationmodelshavethedissipationofmagnetisedturbulenceasacontrollingfactorinstar formation. Measuring the magnetic field is a key observation that allows us to infer i) whether supersonic motions are Alfvenic, and ii) the relative importance of the gravitational, kinetic and magnetic densities in dense clouds (Crutcher 1999). These observables thus allow us to test star formationmodelssuchasambipolardiffusionandturbulence(Crutcher2012;Lazarianetal.2012). Detailed measurements of the magnetic field strength and alignment are difficult. However, inrecentyears,directmeasurementsofthemagneticfieldstrengthhavebeenmade. Mostnotable are Zeeman splitting data (Crutcher 1999; Crutcher et al. 2010) and also sub-mm thermal dust emission (Poidevin et al. 2013). Detailed studies of Zeeman splitting from a sample of molecular clouds indicate that the thermal-to-magnetic pressure β ≈ 0.04, implying that magnetic fields p are important. Moreover, the measurements showed that magnetic field strengths scale with gas densities as B ∝ nκ ≈ 0.5—0.7, as shown in Fig. 1. This is close to the theoretical value κ = 0.47 predicted by models of ambipolar diffusion (Fiedler & Mouschovias 1993). The latest value appearstobeκ=0.65(Crutcher2012)butthereisconsiderablescatterinthemeasurement(Fig.1); our best-fitting value applied to detections above 3σ is κ =0.54±0.05, although there could be biases when neglecting non-detections (Crutcher et al. 2010). The large scatter may be related to the fact that Zeeman splitting is only sensitive to the regular (ordered and directional) magnetic field component along the line-of-sight; the B–n relation may be different for turbulent fields. H Furthermore, this trend only occurs above some density n ∼300cm−3, although this has still to 0 4 SKAstudiesofsynchrotronradiationfrommolecularclouds be determined precisely. Clearly more data, and complementary probes of the magnetic field, are neededtomakeprogressinthisarea. Figure 1: Zeeman splitting measurements of the magnetic field of a number of molecular clouds, plotted againstthevolumedensityofmoleculargas. Significant(>3σ)detectionsareshownasblackfilledcircles (datatakenfromCrutcheretal.2010). Thereisconsiderablescatter,yieldingvariousslopes(seeoverplotted lines)dependingontheexactmodelbeingfitted. Thepower-lawslopebetweenB anddensityn isinthe z H rangeκ≈0.5–0.7.Ourbestfit(solidline)fordetectionsgreaterthan3σ significanceyieldsκ=0.54±0.05 aboven =300cm−3. Preliminaryfigurereproducedfromaforthcomingpublication(Strongetal.2015). H 4. Predictionsforsynchrotronradiationfromcollapsingclouds Given that the synchrotron emissivity scales as ∼B2 and B scales as ∼n0.6, it is logical that H it should also scale roughly as the volume density i.e. j ∝ n . From this, one might expect ν H low frequency maps such as the Haslam et al. 408MHz map (Haslam et al. 1982) to be bright aroundgiantmolecularclouds(GMCs)andformolecularcloudstobeverybrightinhighresolution observations (e.g. VLA, ATCA). We will now use the observed scaling relation of B with n to H estimatethefluxdensityexpectedfortypicalmolecularclouds. WeassumethattheambientCRelectronspervademolecularcloudsunimpededandapower- lawdistributionofCRelectronenergieswithslopeγ andapower-lawslopebetweendensityn and H B-fieldstrengthBaboveavaluen =300cm−3. UsingtheCRfluxmodelofStrongetal.(2011), 0 thepredictedbrightnesstemperature(inmK)at408MHzcanbebeapproximatedby(Strongetal. 2015): (cid:18) (cid:19) (cid:18) (cid:19) T N (cid:16) n (cid:17)κ(γ+1)/2−1 pred =2.8×103 H H , (4.1) mK 1023cm−2 300cm−3 whereN isthecolumndensity(cm−2)andn thevolumedensity(cm−3). Thiscorrespondstoa H H predictedintegratedfluxdensity(inmJy)at1GHz,forasourcesubtendingasolidangleΩ , src 5 SKAstudiesofsynchrotronradiationfrommolecularclouds Table1: MolecularclouddatafromCrutcher(1999)withestimatesofpredictedintegratedsynchrotronflux densityS (mJy)at1GHzbasedonthestatisticalB–n scalinglaw(equation4.2)withκ=0.6. Theflux GHz H densityhasbeenscaledto1GHzassumingaspectralindexα=−0.8(γ=2.6). Thebrightnesstemperature T iswhatwouldbeobservedwitha51arcminbeam. GHz Name B n R D θ T S z H2 GHz GHz [µG] [cm−3] [pc] [kpc] [arcsec] [mK] [mJy] W3OH 3100 6.31×106 0.02 2.0 4.0 0.06 0.05 DR21OH1 710 2.00×106 0.05 1.8 11.2 0.30 0.27 SgrB2 480 2.51×103 22.0 7.9 1149 1200 1000 M17SW 450 3.16×104 1.0 1.8 236 31 27.0 W3(main) 400 3.16×105 0.12 2.0 24.3 0.49 0.43 S106 400 2.00×105 0.07 0.6 48.1 0.74 0.65 DR21OH2 360 1.00×106 0.05 1.8 11.2 0.14 0.13 OMC-1 360 7.94×105 0.05 0.4 50.3 2.3 2.0 NGC2024 87 1.00×105 0.2 0.4 196 14.6 13.0 W40 14 5.01×102 0.05 0.6 34.4 0.04 0.03 ρOph1 10 1.58×104 0.03 0.1 91.7 0.14 0.13 (cid:18) (cid:19) (cid:18) (cid:19)(cid:18) (cid:19) S Ω N (cid:16) n (cid:17)κ(γ+1)/2−1 pred =6.6×106 src H H . (4.2) mJy sr 1023cm−2 300cm−3 Table1listssomeexamplemolecularclouds,usingdatafromCrutcher(1999),withpredicted fluxdensitiesat1GHz. WehaveusedtheB−n relationabovewithκ=0.6,assumeasynchrotron H frequencyspectralindexα =1.0(γ =+3.0),andΩ =π/4×θ2. Densemolecularcloudshave src typicaldensitiesof105–106cm−3 inH andlinearsizesof∼0.05pc. Thisgivescolumndensities 2 of∼1023cm−2. Fortypicaldistancesofa∼kpc,thiscorrespondstoangularsizesof∼10arcsec. It can be seen that many of these sources have predicted flux densities of ∼mJy. It is interesting to see that a few sources have much larger predicted flux densities (e.g. SgrB2 at about 1Jy). However, one has to be careful since these are due to the large physical size assumed (22pc for SgrB2). Inpractice,thecollapsingcloudstendtobeverysmall,oftenclustered,inaparentcloud thatismuchlarger. Themagneticfieldmeasuredinthedensestregionsisunlikelytoapplytothe entire cloud. Thus it is easy to over-estimate the flux density in this way and this appears to be whydensemolecularcloudsarenotbrightinlowresolutionradiosurveyssuchastheHaslametal. (1982) 408MHz map. On the other hand, additional synchrotron from secondary leptons could boostthesynchrotronlevel(Protheroeetal.2008). Therefore,thepredictedfluxdensitiesshouldonlybeconsideredorder-of-magnitudeestimates at this pointsince the precise values depend very sensitively onthe choice ofκ andn and on the 0 observedinputparameters. Furthermore,thehugescatteraboutthisrelationobservedinFig.1al- readyindicatesthateitherthemeasurementsarenotrepresentativeofthemeanfield,or,thesimple B−n relationshipdoesnothold. Newobservationswillbecrucialfortestingthishypothesis. H 6 SKAstudiesofsynchrotronradiationfrommolecularclouds 5. ProspectsfortheSKA The high sensitivity and high resolution of the SKA should finally provide the first definitive detection of in-situ synchrotron radiation from molecular clouds themselves. The dense cores inside clouds are typically 0.05pc in size, corresponding to ∼ 1arcsec at a distance of ∼kpc. ThisiswellmatchedtothecoreofSKA1-MID,whichprovidesgoodu,vcoverage(andtherefore goodsurfacebrightnesssensitivity)upto∼5arcsec. EstimatedfluxdensitiesareofordermJyfor typicaldensemolecularcloudsatGHzfrequencies(seeTable1),whichisapproximatelythedepth to which current observations have been targeted (e.g. Jones et al. 2008). Optimal frequencies are in the range of a few hundred MHz (band 1–2 of SKA1-MID) to ∼2GHz (band 3 of SKA1- MID). The very lowest frequencies ((cid:46) 300MHz) will be affected by synchrotron and free-free self-absorption. Above a few GHz, the steep spectrum (α ≈−0.8) means the signal will be very weakandmoresusceptibletofree-freecontamination,whichhasaflatterspectralindex(α ≈−0.1 in the optically thin regime). The SKA is therefore the ideal instrument for studying this, yet untapped,areaofradioastronomy. Multi-frequency (matched u,v coverage) continuum mapping will be important to verify that theemissionisnon-thermal. Inprinciple,opticallythinthermalemissioncouldberemovedusing radio recombination lines, which could be detected using the wide-band spectral capability of the SKA. Mapping the polarisation of the synchrotron radiation provides further information on the magnetic field, such as the orientation and orderliness. The polarisation fraction could be up to ≈75%foraorderedfieldwithnodepolarisation. However, inpracticetheobservedlevelwillbe lower,makingthisamoredifficultmeasurementtomake. Lowerfrequencies(bands1/2)willalso likelybeaffectedbyFaradaydepolarisationalongtheline-of-sight. Diffuseclouds(e.g., Heiles1997;Myersetal.1995)willoftennotbeboundstructures, have modestmagneticfieldsandanin-situsynchrotronsignalwillnotbeobviousduetoconfusionfrom nearbyobjects. Giventheirlargesize(arcmintoseveraldegrees)anddiffusenature, theywillnot provide the cleanest signal and are not well-suited to SKA baselines. More well-defined clouds such as CO/sub-mm catalogues of molecular clouds (e.g., Roman-Duval et al. 2010; Planck Col- laboration et al. 2011) provide lists of hundreds of clouds but, due to the relatively low angular resolution,theyarerelativelylargeinsize(5–20pc)andmaynotcontainthedensestcloudsofin- teresthere. Proto-stellaroutflowsmayalsoemitappreciablesynchrotronradiation(Sokoloskietal. 2008) which will need to be carefully mapped. Nevertheless, with the large field-of-view of the SKA, it should be possible to find useful areas (Galactic plane and molecular clouds, particularly athighGalacticlatitudewherecleanersight-linesshouldexist)tosurveytolookforthesignature ofin-situsynchrotronemission. The best targets will be the most compact and dense clouds, where the magnetic fields are likely to be highest, and the signal will be the cleanest. Examples such as those in Table 1, with angularsizesof∼arcsecareidealfortheSKA.Anexamplewouldbetheringofdensemolecular cores in the W40 complex, where upper limits of ∼mJy exist but require deeper observations (Pirogov et al. 2013). The long baselines of SKA will in fact allow us to resolve the emission inside a dense cloud opening up the potential of studying the magnetic field structure within the cloud. Surveying a number of clouds in this way is likely to directly shed light on models of star formation through the alignment of the magnetic field. They should ideally have as little or no 7 SKAstudiesofsynchrotronradiationfrommolecularclouds free-free emission from ionised gas or nearby supernova remnants (SNRs).5 In many cases, such surveyscouldpiggy-backonotherSKAsurveysoftheGalaxy. The high density and quiescent nature of infrared dark clouds (e.g., Peretto & Fuller 2009), together with high resolution column density images available, will make them interesting targets for SKA observations to probe the magnetic field at early stages of formation. A good example wouldbethemassivestar-formingcoreswithintheinfrareddarkcloudSDC335.579-0.272(Peretto etal.2013). 6. Conclusionsandoutlook Observations of the properties of dense molecular clouds are critical in understanding the process of star-formation. One of the most important, but least understood, is the role of the magnetic fields. We propose to use high resolution, high sensitivity radio observations with the SKA to measure the in-situ synchrotron radiation from these molecular clouds, complementing othermethodssuchasZeemansplitting(Robishaw2014). IftheCRparticlespenetrateasexpected, thenwecanmeasuretheB-fieldstrengthdirectlyusingradiodata(theCRfluxisrelativelysmooth and varies slowly throughout the Galaxy with a scale-height of ≈1kpc; Orlando & Strong 2013; Stepanovetal.2014). Iftheycannot,thentheabilityoftheSKAtopickupextended,lowsurface- brightnessemissionmayallowustosearchforsynchrotronemissionfromtheouterregionsofsuch clouds. Collapsingcoresaretypicallyarcsecinsizeandfluxdensityestimatesare∼mJyat1GHz andthusshouldbereadilydetectablebytheSKA.Thelargefield-of-viewwillallowmanylines-of- sighttobeinvestigatedforagivenmolecularcloud. Multiplefrequenciesandradiorecombination lineswillallowseparationfromfree-freeemissionwhilepolariseddataopensupthepossibilityof mappingthefieldgeometrywithincollapsingclouds. CDacknowledgessupportfromanERCStarting(Consolidator)Grant(no.307209). 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