A lower limit of 50 microgauss for the magnetic field near the Galactic Centre
Roland M. Crocker, David Jones, Fulvio Melia, Jürgen Ott, Raymond J. Protheroe
AA lower limit of 50 microgauss for the magnetic field nearthe Galactic Centre
Roland M. Crocker, , David Jones, , , Fulvio Melia, J¨urgen Ott, , & Raymond J. Protheroe J.L. William Fellow, School of Physics, Monash University, Victoria, 3110, Australia Max-Planck-Institut f ¨ur Kernphsik, P.O. Box 103980 Heidelberg, Germany Department of Physics, School of Physics and Chemistry, University of Adelaide, North Terrace,South Australia, 5005, Australia Australia Telescope National Facility, Marsfield, 2122, N.S.W., Australia Physics Department, The Applied Math Program, and Steward Observatory, The University ofArizona, Tucson, Arizona, AZ 85721, USA Jansky Fellow, National Radio Astronomical Observatory, Charlottesville, P.O. Box O, 1003Lopezville Road, Socorro, New Mexico, NM 87801-0387, USA California Institute of Technology, 1200 E. California Blvd., Caltech Astronomy, 105-24,Pasadena, CA 91125, USA
The amplitude of the magnetic field near the Galactic Centre has been uncertain by twoorders of magnitude for several decades. On a scale of ∼
100 pc fields of ∼ µ G havebeen reported, implying a magnetic energy density more than 10,000 times stronger thantypical for the Galaxy. Alternatively, the assumption of pressure equilibrium between thevarious phases of the Galactic Centre interstellar medium (including turbulent molecular a r X i v : . [ a s t r o - ph . GA ] J a n as; the contested “very hot” plasma; and the magnetic field) suggests fields of ∼ µ Gover ∼
400 pc size scales . Finally, assuming equipartition, fields of only ∼ µ G have beeninferred from radio observations for 400 pc scales. Here we report a compilation of previousdata that reveals a down-break in the region’s non-thermal radio spectrum (attributable toa transition from bremsstrahlung to synchrotron cooling of the in situ cosmic-ray electronpopulation). We show that the spectral break requires that the Galactic Centre field be atleast ∼ µ G on 400 pc scales, lest the synchrotron-emitting electrons produce too much γ -ray emission given existing constraints . Other considerations support a field of 100 µ G,implying that > ∼
10% of the Galaxy’s magnetic energy is contained in only < ∼ Pinning down the large-scale magnetic field of the Galactic Centre (GC)– which provides ourclosest view of a galactic nucleus – within the greater than two orders of magnitude uncertaintyimplied by existing, rival analyses has long been of interest. A 1000 µ G field would have substan-tial ramifications for the region’s dynamics including limiting the diffusion distances of relativisticparticles (thereby excluding scenarios where the diffuse, ∼ TeV γ -ray glow of the GC is ultimatelyexplained as due to a single, astrophysical accelerator ). It would also generate a large enoughmagnetic drag to enhance the in-spiral rate of giant molecular gas clouds towards the GC , limitingthe lifetime of these to ∼
100 million years, implying ‘star-bursts’ on the same timescale. On theother hand, the formation of individual stars might be inhibited (or the stellar initial mass functionbiased) by a strong magnetic field’s tendency to support gas against gravitational collapse.2adio observations at 74 MHz and 330 MHz reveal a diffuse (but distinct) region of non-thermal radio emission covering the GC (out to ∼ ± ◦ or ∼ ± pc from the GC along theGalactic plane). Invoking the “equipartition” condition (minimizing the total energy in magneticfield and relativistic electrons), a field of only 6 µ G is inferred , typical for the Galaxy-at-large,climbing to 11 µ G over the inner ∼ ± . ◦ ( for extremal parameter values the field might reach100 µ G).To probe this radio structure at higher frequencies, we have assembled total-intensity, single-dish flux density data at 1.4, 2.4, 2.7, and 10 GHz . These data are polluted by line-of-sightsynchrotron emission in the Galactic plane both behind and in front of the GC and the flux densitycontributed by discrete sources; we used a combination of low-pass (spatial wavenumber) filteringto remove the latter and all-sky Galactic synchrotron background observations to remove the for-mer (see Supplementary Information). After this processing a distinct, non-thermal, radio structureis revealed in all radio maps up to 10 GHz (see Figure 1). However, the structure’s 74 MHz–10GHz spectrum is not described by a pure power-law: attempting to fit such to the cleaned data wefind a minimum χ of 4.9 per degree of freedom ( dof = 4 ), excluded at a confidence level of . σ (Figure 3).In fact, fitting separate power laws to, respectively, the background-subtracted lower threeand upper three radio data, we find that these extrapolations intersect at ∼ ∼ B , andbremsstrahlung a function of n H , the break frequency is a function of both parameters. Observationof a spectral break then determines acceptable pairs of B and n H for the synchrotron-emittingelectrons’ environment.We have modelled the cooled electron distribution and resulting synchrotron emission as afunction of B , n H , and spectral index at injection of the electron population. We have accountedfor losses due to inverse-Compton emission following collisions with ambient light , ionization,bremsstrahlung and synchrotron emission.The results of our fitting procedure are displayed in Figure 2 and Figure 3. A very importantconstraint is proffered by γ -ray data covering the non-thermal emission region: bremsstrahlung andinverse Compton emission will inescapably be generated by the electrons responsible for the ob-served radio emission (the energies of electrons synchrotron-radiating at ∼ GHz and bremsstrahlung-radiating at > ∼
100 MeV are very similar). This must not surpass the 300 MeV γ -ray flux from theregion measured by EGRET (see Figure 2b). This consideration rules out field amplitudes < ∼ µ G. In the context of diffuse, ∼ GeV emission around the GC, we eagerly await results from theFermi Gamma-ray Space Telescope whose sensitivity (more than an order of magnitude betterthan EGRET’s) promises, at worst, an increased lower-limit to the large-scale magnetic field or,more optimistically, a measurement of this field (in concert with radio observations). For now weemphasis that it is a novel analysis – not new data – that has allowed the new constraint on the GC4agnetic field.Figure 4 shows the energy densities of various GC ISM phases. For acceptable field am-plitudes, the cosmic ray electron population is considerably sub-equipartition with respect to theother GC ISM phases (even after accounting for filling factor effects), in particular, the magneticfield, explaining why the magnetic field estimate arrived at assuming equipartition is too low. Wehave also calculated the maximum possible energy density in the GC cosmic ray proton populationgiven the EGRET γ -ray constraints. Note that at ∼ µ G the magnetic field reaches equiparti-tion (at ∼
300 eV cm − ) with the putative ‘very hot’ ( ∼ . Moreover, theenergy density of the gas turbulence kinetic energy for the derived n H is within a factor of a fewof equipartition with these other phases up to magnetic field amplitudes of ∼ µ G.This situation – implying near pressure equilibrium between a number of GC interstellarmedium phases (including the plasma) – mirrors that of the Galactic disk, albeit at much higherpressure. Such considerations led to the prediction that the real GC magnetic field amplitude liesclose to ∼ µ G. Very recently, however, observations have shown that the X-ray emissionfrom a region around l = 0.08, b =1.42 (taken to be typical of the so-called X-ray Ridge) is due tounresolved point sources, implying the very hot X-ray plasma is illusory. This casts the pressureequilibrium argument into some doubt. Note, however, that our magnetic field lower limit holdsirrespective of whether the 8 keV plasma is real or not.There are a number of intriguing parallels above to the situation apparently pertaining within5he inner regions of starburst galaxies. These independently support the idea that the field is ac-tually close to 100 µ G. In starburst environments, it is contended , equipartition magnetic fieldvalues obtained from radio observations significantly underestimate the real field. Fields are actu-ally sufficiently high to be in hydrostatic equilibrium with the self-gravity of the gaseous disk of thestarburst. Such strong fields (together with high gas densities) guarantee that the relativistic parti-cle population dumps all its energy before being transported out of the system. This calorimetriclimit may explain why even very luminous star-bursting galaxies fall on the far-infrared–radio-continuum correlation . Circumstantial evidence also suggests that radio emission from starburstsis dominated by secondary electrons created in collisions between cosmic ray ions and gas (ratherthan directly-accelerated electrons).It is noteworthy, then, that the 60 micron and 1.4 GHz emission from the radio emissionregion place it within scatter of the far-infrared–radio-continuum correlation . Furthermore, inthe GC a ∼ µ G field is precisely in the range required to establish hydrostatic equilibrium(given the total and gaseous surface densities). A situation wherein a magnetic field providessignificant pressure support against gravity may lead to the development of the Parker instability and exactly this is suggested by mm-wave observations of molecular filaments of several hundredparsec length within ∼ ∼ µ Gfield. Finally, the diffuse, ∼ TeV γ -ray glow from the vicinity of the GC is most likely explained bycosmic ray impacts with gas. Unavoidably, such collisions would also produce copious secondaryelectrons which could then contribute significantly to the region’s synchrotron radio emission (forfields > ∼ µ G, 100% of the radio emission from the rectangular region shown in Figure 1 could6e attributed to secondary electrons). Taken altogether, these facts paint the Galactic centre as akinto a weak starburst with a magnetic field of ∼ µ G. References [1] Yusef-Zadeh, F. & Morris, M. G0.18-0.04 - Interaction of thermal and nonthermal radio struc-tures in the arc near the galactic center, Astron. J., 94, 1178-1184 (1987)[2] Morris, M., & Yusef-Zadeh, F. The thermal, arched filaments of the radio arc near the Galacticcenter - Magnetohydrodynamic-induced ionization?, Astrophys. J., 343, 703-712 (1989)[3] Morris, M. The Galactic Centre Magnetosphere, ArXiv Astrophysics e-prints, arXiv:astro-ph/0701050 (2007)[4] Revnivtsev, M., Sazonov, S., Churazov, E., Forman, W., Vikhlinin, A., & Sunyaev, R., Nature,458, 1142-1144 (2009)[5] Spergel, D. N., & Blitz, L. Extreme gas pressures in the Galactic bulge, Nature, 357, 665-667(1992)[6] LaRosa, T. N. et al. Evidence of a Weak Galactic Centre Magnetic Field from Diffuse Low-Frequency Nonthermal Radio Emission Astrophys. J., 626, L23-L27 (2005)[7] Hunter, S. D., et al. EGRET Observations of the Diffuse Gamma-Ray Emission from theGalactic Plane, Astrophys. J., 481, 205-240 (1997)78] Thompson, T. A., Quataert, E., Waxman, E., Murray, N., & Martin, C. L. Magnetic Fields inStarburst Galaxies and the Origin of the FIR-Radio Correlation, Astrophys. J., 645, 186-198(2006)[9] F. A. et al. Discovery of very-high-energy γ -rays from the Galactic Centre ridge, Nature, 439,695-698 (2006)[10] Wommer, E., Melia, F., & Fatuzzo, M. Diffuse TeV emission at the Galactic Centre,Mon. Not. Roy. Astron. Soc., 387, 987-997 (2008)[11] Reich, W., Reich, P., Fuerst, E. The Effelsberg 21 CM radio continuum survey of the Galacticplane between L = 357 deg and L = 95.5 deg, Astron. Astrophys., Suppl.,83, 539-568 (1990)[12] Reich, W., Fuerst. E., Steffen, P., Reif, K., Haslam, C.G.T. A radio continuum survey of theGalactic Plane at 11 CM wavelength. I - The area L = 357.4 to 76 deg, B = -1.5 to +1.5 deg,Astron. Astrophys., Suppl., 58, 197-248 (1984)[13] Duncan, A.R., et al. A deep radio continuum survey of the southern Galactic plane at 2.4GHz, Mon. Not. Roy. Astron. Soc., 277, 36-52 (1995)[14] Handa, T. et al. A radio continuum survey of the Galactic plane at 10 GHz, Proc. Astron. Soc.Jap., 39, 709-753 (1987)[15] Porter, T. A., Moskalenko, I. V., & Strong, A. W. Inverse Compton Emission from GalacticSupernova Remnants: Effect of the Interstellar Radiation Field, Astrophys. J., 648, L29-L32(2006) 816] Atwood, W. B., et al. The Large Area Telescope on the Fermi Gamma-Ray Space TelescopeMission, Astrophys. J., 697, 1071-1102 (2009)[17] Koyama, K., Awaki, H., Kunieda, H., Takano, S., & Tawara, Y. Intense 6.7-keV iron lineemission from the Galactic Centre, Nature, 339, 603-605 (1989)[18] Morris, M., & Serabyn, E. The Galactic Centre Environment, Ann. Rev. Astron. Astrophys.,34, 645-702 (1996)[19] Ferri`ere, K., Gillard, W., & Jean, P. Spatial distribution of interstellar gas in the innermost 3kpc of our galaxy, Astron. Astrophys., 467, 611-627 (2007)[20] Paglione, T. A. D., Jackson, J. M., Bolatto, A. D., & Heyer, M. H. Interpreting the HCN/COIntensity Ratio in the Galactic Centre, Astrophys. J., 493, 680-693 (1998)[21] Voelk, H. J. The correlation between radio and far infrared emission for disk galaxies: acalorimeter theory, Astron. Astrophys., 218, 67-70 (1989)[22] Yun, M. S., Reddy, N. A., & Condon, J. J. Radio Properties of Infrared-selected Galaxies inthe IRAS 2 Jy Sample, Astrophys. J., 554, 803-822 (2001)[23] Parker, E. N. The Dynamical State of the Interstellar Gas and Field, ApJ, 145, 811-833 (1966)[24] Fukui, Y., et al. Molecular Loops in the Galactic Center: Evidence for Magnetic Flotation,Science, 314, 106-109 (2006)[25] Yamauchi, S et al. Optically thin hot plasma near the Galactic center - Mapping observationsof the 6.7 keV iron line, Astrophys. J., 365, 532-538 (1990)926] G¨usten, R., & Philipp, S. D. Galactic Centre Molecular Clouds, The Dense InterstellarMedium in Galaxies, Proceedings of the 4th Cologne-Bonn-Zermatt Symposium, Zermatt,Switzerland, 22-26 September 2003. Edited by S.Pfalzner, C. Kramer, C. Staubmeier, and A.Heithausen. Springer proceedings in physics, Vol. 91. Berlin, Heidelberg: Springer, 253-263(2004) (astro-ph/0402019)[27] Crocker, R. M., et al. The Cosmic Ray Distribution in Sagittarius B Astrophys. J., 666, 934-948 (2007) Supplementary Information
Acknowledgements
RMC thanks Troy Porter for useful conversations about the GC interstellar radiationfield. DIJ thanks Monash University for hospitality. RMC and DIJ thank John Dickey for advice about radiodata analysis.
Author Contributions
R.M.C. led the work and performed the main analysis. D.I.J. performed the anal-ysis of radio data including development of the Fourier-based technique for background and foregroundremoval, was responsible for original radio observations, and provided critical scientific discussion. F.M.provided input on theoretical and statistical problems and critical discussion of scientific interpretation. J.O.supervised the analysis of archival radio data and the taking of original radio data and provided input onstatistics. R.J.P. provided input on thermal and relevant non-thermal processes and critical discussion of sci-entific interpretation. R.J.P. and R.M.C. provided supervision of D.I.J. as doctoral candidate. All the authorsdiscussed the results and commented on the manuscript.
Author Information
The authors declare that they have no competing financial interests. Correspon- ence and requests for materials should be addressed to R.M.C. (email: [email protected]).Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. alactic Longitude G a l a c t i c La t i t ude
2° 0° -2° -1.0°-0.5° +0.5°+1.0°
Figure 1
Figure 1 | Total intensity image of the region at 10 GHz . Radio map convolved to a resolutionof 1 ◦ . × ◦ .2 with contours at 10, 20, 40, 80, 160 and 240 Jy/beam. (Native resolution andconvolved images at ν ≥ ). The smallrectangle delineates the region from which the HESS collaboration determines a diffuse ∼ TeV γ -ray intensity . 12 (cid:230) (cid:230) (cid:230) (cid:230) (cid:230) (cid:64) Ν (cid:144) Hz (cid:68) Log (cid:64) Ν F Ν (cid:144) H zJy (cid:68) (a) (cid:45) (cid:64) E Γ (cid:144) eV (cid:68) Log (cid:64) E Γ F (cid:144) e V c m (cid:45) s (cid:45) s r (cid:45) (cid:68) (b) Figure 2:
Figure 2
Figure 2 | Spectrum of the region: data and models. ( a ) Radio . Points:– circles : flux den-sity of the radio structure after removal of flux from sources with sizes < ◦ . . The verticalline divides the 74 MHz datum (that receives no contribution from Galactic plane synchrotronbackground/foreground) from all the other data (that do) – hence the discontinuity in the modelcurves at 100 MHz; squares : discrete source flux densities (measured directly by the VLA at330 MHz[ ] and otherwise obtained through the Fourier technique described in the Supplemen-tary Information); arrows : discrete source lower limits obtained from ATCA. The error bars show68% confidence intervals. Curves : the best-fit flux density due to synchrotron emission from acooled electron distribution plus Galactic plane synchrotron for the case of ( solid ) 100 µ G and( dotted ) 30 µ G fields (the break in the radio synchrotron curve mirrors a corresponding break in13he distribution of radiating electrons; we assume the electrons are injected with a power law inmomentum ); dashed : the best-fit null hypothesis case of a pure power-law signal plus Galacticplane synchrotron. ( b ) Broadband.
Models for magnetic field of 100 µ G (solid) and 30 µ G (dot-ted). The injected electron spectrum is assumed to follow a power law up to ∼ TeV. (Notethat the cut-off energy is only weakly constrained. Our conclusions are not sensitive to the exactcut-off energy, however, given that radio observations guarantee the existence of ∼ GeV electronswhose bremsstrahlung emission we can compare against the EGRET intensity datum.) The lowerenergy curves are synchrotron emission. The higher are bremsstrahlung plus inverse Comptonemission from the same electrons responsible for the synchrotron (a figure showing the individualbremsstrahlung and inverse Compton contributions is shown in the Supplementary Information).The upper limits are due to observations by EGRET at 300 MeV and HESS at TeV.14 Σ Σ Σ Σ Σ Σ Σ Σ NullNullCooledCooled1.0 1.5 2.0 2.5 3.0 3.51.10.5.50.2.20.3.30.1.515.7.70. log (cid:64) B (cid:144) Μ G (cid:68) Χ (cid:144) do f Figure 3
Figure 3 | Plot of the χ per degree of freedom as a function of magnetic field amplitude. Curves: (as labelled) model for a cooled primary electron model (with 3 degrees of freedom) andthe null hypothesis of a pure power law electron distribution (with 4 dof ). The solid curves areconstrained by the requirement that the γ -ray emission from the synchrotron-radiating electronpopulation be less than the upper limit obtained from EGRET data (dotted curves not so con-strained). The horizontal dashed lines mark the 1,2,3,4 σ confidence limits for a model with 3 dof and do not apply to the null hypothesis (which only achieves a best fit acceptable at the ∼ . σ level). 15 lasmaplasmamagmagturbturb ppee (cid:45) (cid:64) B (cid:144) Μ G (cid:68) l og (cid:64) U X (cid:144) e V c m (cid:45) (cid:68) Figure 4
Figure 4 | Energy density in various phases of the GC interstellar medium as a functionof magnetic field.
Bands show 68% confidence limits (except for p ). Labels denote:– ‘mag’:magnetic field; ‘plasma’: the disputed X-ray emitting 8 keV plasma (with a density − ); ‘turb’: turbulent motions of the local gas (assuming it is at the best-fit n H and has a veloc-ity dispersion in the range 15–30 km/s typical for GC molecular clouds – note that, as the n H required to fit the radio spectrum increases with B , the turbulent energy density is an increasingfunction of B ); ‘ e ’: cosmic ray electrons; and ‘ p ’: a conservative upper bound on the cosmic rayproton energy density inferred from the 300 MeV γ -ray upper limit (a putative proton population,16olliding with ambient gas at the best-fit n H , would produce γ -rays mostly via neutral meson de-cay). The electron and proton energy densities take into account the filling factor of gas at thegiven value of n H [ ] (we take the filling factor of gas within σ around the best-fit n H value);this is a correction upwards to the values of U e and U p by an amount 100–1000. Without thiscorrection, one would have U e (cid:39) U B at ∼ µ G in agreement with the estimate derived assumingequipartion . A factor of 2 uncertainty19