Giant magnetized outflows from the centre of the Milky Way

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Giant magnetized outflows from the centre of the Milky Way

  1. 1. LETTER doi:10.1038/nature11734Giant magnetized outflows from the centre of theMilky WayEttore Carretti1, Roland M. Crocker2,3, Lister Staveley-Smith4,5, Marijke Haverkorn6,7, Cormac Purcell8, B. M. Gaensler8,Gianni Bernardi9, Michael J. Kesteven10 & Sergio Poppi11The nucleus of the Milky Way is known to harbour regions of from radio continuum data as the Galactic Centre spur10, appears tointense star formation activity as well as a supermassive black hole1. connect back to the Galactic Centre; we label the other substructuresRecent observations have revealed regions of c-ray emission reach- the northern and southern ridges. The ridges’ magnetic field directionsing far above and below the Galactic Centre (relative to the Galactic (Fig. 3) curve, following their structures. The Galactic Centre spur andplane), the so-called ‘Fermi bubbles’2. It is uncertain whether these southern ridges also seem to have GeV c-ray counterparts (Fig. 2; alsowere generated by nuclear star formation or by quasar-like out- compare ref. 3). The two limb brightening spurs at the biconical lobebursts of the central black hole3–6 and no information on the struc- base are also visible in the WMAP map, where they appear to con-tures’ magnetic field has been reported. Here we report observations nect back to the Galactic Centre area. A possible third spur developsof two giant, linearly polarized radio lobes, containing three ridge- northeast from the Galactic Centre. These limb brightening spurs arelike substructures, emanating from the Galactic Centre. The lobes also obvious in the Stokes U map as an X-shaped structure centred ateach extend about 60 degrees in the Galactic bulge, closely corres- the Galactic Centre (Supplementary Fig. 3).ponding to the Fermi bubbles, and are permeated by strong mag- Such coincident, non-thermal radio, microwave and c-ray emissionnetic fields of up to 15 microgauss. We conclude that the radio lobes indicates the presence of a non-thermal electron population coveringoriginate in a biconical, star-formation-driven (rather than black- at least the energy range 1–100 GeV (Fig. 4) that is simultaneouslyhole-driven) outflow from the Galaxy’s central 200 parsecs that synchrotron-radiating at radio and microwave frequencies andtransports a huge amount of magnetic energy, about 1055 ergs, into upscattering ambient radiation into c-rays by the inverse Comptonthe Galactic halo. The ridges wind around this outflow and, we process. The widths of the ridges are remarkably constant at ,300 pcsuggest, constitute a ‘phonographic’ record of nuclear star forma- over their lengths. The ridges have polarization fractions of 25–31%tion activity over at least ten million years. (see Supplementary Information), similar to the average over the lobes. We use the images of the recently concluded S-band Polarization All Given this emission and the stated polarization fractions, we inferSky Survey (S-PASS) that has mapped the polarized radio emission of magnetic field intensities of 6–12 mG for the lobes and 13–15 mG forthe entire southern sky. The survey used the Parkes Radio Telescope at the ridges (see Figs 2 and 3, and Supplementary Information).a frequency of 2,307 MHz, with 184 MHz bandwidth, and 99 angular An important question about the Fermi bubbles is whether they areresolution7. ultimately powered by star formation or by activity of the Galaxy’s The lobes we report here exhibit diffuse polarized emission (Fig. 1), central, supermassive black hole. Despite their very large extent, thean integrated total intensity flux of 21 kJy, and a high polarization c-ray bubbles and the X-shaped polarized microwave and X-ray struc-fraction of 25%. They trace the Fermi bubbles excepting the top western tures tracing their limb-brightened base11 have a narrow waist of(that is, right) corners where they extend beyond the region covered by only 100–200 pc diameter at the Galactic Centre. This matches thethe c-ray emission structure. Depolarization by H II regions establishes extent of the star-forming molecular gas ring (of ,3 3 107 solarthat the lobes are almost certainly associated with the Galactic Centre masses) recently demonstrated to occupy the region12. With 5–10%(Fig. 2 and Supplementary Information), implying that their height is of the Galaxy’s molecular gas content1, star-formation activity in this,8 kpc. Archival data of WMAP8 reveal the same structures at a ‘central molecular zone’ is intense, accelerating a distinct cosmic raymicrowave frequency of 23 GHz (Fig. 3). The 2.3–23 GHz spectral population13,14 and driving an outflow11,15 of hot, thermal plasma,index a (with flux density S at frequency n modelled as Sn / na) of cosmic rays and ‘frozen-in’ magnetic field lines6,14,16.linearly polarized emission interior to the lobes spans the range 21.0 to One consequence of the region’s outflow is that the cosmic ray21.2, generally steepening with projected distance from the Galactic electrons accelerated there (dominantly energized by supernovae) areplane (see Supplementary Information). Along with the high polari- advected away before they lose much energy radiatively in situ14,16,17.zation fraction, this phenomenology indicates that the lobes are due This is revealed by the fact that the radio continuum flux on scales upto cosmic-ray electrons, transported from the plane, synchrotron- to 800 pc around the Galactic Centre is in anomalous deficit withradiating in a partly ordered magnetic field. respect to the expectation afforded by the empirical far-infrared/radio Three distinct emission ridges that all curve towards Galactic west continuum correlation18. The total 2.3 GHz radio continuum fluxwith increasing Galactic latitude are visible within the lobes (Fig. 1); from the lobes of ,21 kJy, however, saturates this correlation as nor-two other substructures proceeding roughly northwest and southwest malized to the 60 mm flux (2 MJy) of the inner ,160 pc diameterfrom around the Galactic Centre hint at limb brightening in the bico- region (ref. 19). Together with the morphological evidence, thisnical base of the lobes. These substructures all have counterparts in strongly indicates that the lobes are illuminated by cosmic ray elec-WMAP polarization maps (Fig. 3), and one of them9, already known trons accelerated in association with star formation within this region1 CSIRO Astronomy and Space Science, PO Box 276, Parkes, New South Wales 2870, Australia. 2Max-Planck-Institut fur Kernphysik, PO Box 103980, 69029 Heidelberg, Germany. 3Research School of ¨Astronomy and Astrophysics, Australian National University, Weston Creek, Australian Capital Territory 2611, Australia. 4International Centre for Radio Astronomy Research, M468, University of Western 5Australia, Crawley, Western Australia 6009, Australia. ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), M468, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia6009, Australia. 6Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands. 7Leiden Observatory, Leiden University, PO Box 9513, 2300 RALeiden, The Netherlands. 8Sydney Institute for Astronomy, School of Physics, The University of Sydney, New South Wales 2006, Australia. 9Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,Cambridge, Massachusetts 02138, USA. 10CSIRO Astronomy and Space Science, PO Box 76, Epping, New South Wales 1710, Australia. 11INAF Osservatorio Astronomico di Cagliari, Strada 54 Localita `Poggio dei Pini, I-09012 Capoterra (CA), Italy.6 6 | N AT U R E | VO L 4 9 3 | 3 J A N U A RY 2 0 1 3 ©2013 Macmillan Publishers Limited. All rights reserved
  2. 2. LETTER RESEARCH Northern ridge Galactic Centre spur Limb brightening spurs Southern ridge0 0.017 0.034 0.051 0.068 0.085 0.1 0.12 0.14 0.15 0.17 P (Jy per beam)Figure 1 | Linearly polarized intensity P at 2.3 GHz from S-PASS. The thick ridges appear to be the front side of a continuous winding of collimateddashed lines delineate the radio lobes reported in this Letter, while the thin structures around the general biconical outflow of the lobes (see text). Thedashed lines delimit the c-ray Fermi bubbles2. The map is in Galactic Galactic Centre spur is nearly vertical at low latitude, possibly explained by acoordinates, centred at the Galactic Centre with Galactic east to the left and projection effect if it is mostly at the front of the northern lobe. At its higherGalactic north up; the Galactic plane runs horizontally across the centre of the latitudes, the Galactic Centre spur becomes roughly parallel with the northernmap. The linearly polarized intensity flux density P (a function of the Stokes pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ridge (above), which itself exhibits little curvature; this is consistent with theparameters Q and U,P: Q2 zU 2 ) is indicated by the colour scale, and given overall outflows becoming cylindrical above 4–5 kpc as previously suggested11.in units of Jy per beam with a beam size of 10.759 (1 Jy ; 10226 W m22 Hz21). In such a geometry, synchrotron emission from the rear side of each cone isThe lobes’ edges follow the c-ray border up to Galactic latitude b < | 30 | u, from attenuated by a factor >2 with respect to the front side, rendering it difficult towhich the radio emission extends. The three polarized radio ridges discussed in detect the former against the foreground of the latter and of the Galactic planethe text are also indicated, along with the two limb brightening spurs. The (see Supplementary Information).(see Supplementary Information), and that the lobes are not a result of Given the calculated fields and the speed of the outflow, the totalblack hole activity. magnetic energy for each of the ridges, (4–9) 3 1052 erg (see Sup- The ridges appear to be continuous windings of individual, colli- plementary Information), is injected at a rate of ,1039 erg s21 over amated structures around a general biconical outflow out of the Galactic few million years; this is very close to the rate at which independentCentre. The sense of Galactic rotation (clockwise as seen from Galactic modelling6 suggests Galactic Centre star formation is injecting mag-north) and angular-momentum conservation mean that the ridges netic energy into the region’s outflow. On the basis of the ridges’ indi-get ‘wound up’20 in the outflow with increasing distance from the vidual energetics, geometry, outflow velocity, timescales and plasmaplane, explaining the projected curvature of the visible, front-side content (see Supplementary Information), we suggest that their foot-of the ridges towards Galactic west. Polarized, rear-side emission is points are energized by and rotate with the super-stellar clusters inha-attenuated, rendering it difficult to detect against the stronger emission biting1 the inner ,100 pc (in radius) of the Galaxy. In fact, we suggestfrom the lobes’ front-side and the Galactic plane (Fig. 1 and Sup- that the ridges constitute ‘phonographic’ recordings of the pastplementary Information). ,10 Myr of Galactic Centre star formation. Given its morphology, For cosmic ray electrons synchrotron-emitting at 2.3 GHz to be able the Galactic Centre spur probably still has an active footprint. In con-to ascend to the top of the northern ridge at ,7 kpc in the time it takes trast, the northern and southern ridges seem not to connect to thethem to cool (mostly via synchrotron emission itself) requires vertical plane at 2.3 GHz. This may indicate their footpoints are no longertransport speeds of .500 km s21 (for a field of 15 mG; see Fig. 4). Given active, though the southern ridge may be connected to the plane bythe geometry of the Galactic Centre spur, the outflowing plasma is a c-ray counterpart (see Fig. 2). Unfortunately, present data do notmoving at 1,000–1,100 km s21 (Fig. 4 and Supplementary Infor- allow us to trace the Galactic Centre spur all the way down to themation), somewhat faster than the ,900 km s21 gravitational escape plane: but a connection is plausible between this structure and onevelocity from the Galactic Centre region21, implying that 2.3-GHz- (or some combination) of the ,1u-scale radio continuum spurs15,22radiating electrons can, indeed, be advected to the top of the ridges emanating north of the star-forming giant molecular cloud complexesbefore they lose all their energy. Sagittarius B and C; a connection is also plausible with the bright, 3 J A N U A RY 2 0 1 3 | VO L 4 9 3 | N AT U R E | 6 7 ©2013 Macmillan Publishers Limited. All rights reserved
  3. 3. RESEARCH LETTER Figure 2 | Lobes’ polarized intensity and c-ray spurs. Schematic rendering of the edges of two c-ray substructures evident in the 2–5 GeV Fermi data as displayed in figure 2 of ref. 2, which seem to be counterparts of the Galactic γ-ray spur Centre spur and the southern ridge. The map is in Galactic coordinates, with Galactic east to the left and Galactic north up; the Galactic plane runs horizontally across the centre of the map approximately. The linearly polarized intensity flux density P is indicated by the colour scale, and given in units of Jy per beam with a beam size of 10.759. The latter appears to be connected to the Galactic Centre by its c-ray counterpart. With the flux densities and polarization fraction quoted in the text, we can infer equipartition26 magnetic field intensities of Beq < 6 mG (1 mG ; 10210 T) if the synchrotron-emitting electrons occupy the entire volume of the lobes, or ,12 mG if they occupy only a 300-pc-thick skin (the width of the ridges). For the southern ridge, Beq < 13 mG; for the Galactic Centre spur, Beq < 15 mG; and, for the northern ridge, Beq < 14 mG. Note the large area of depolarization and small-angular-scale signal modulation visible across the Galactic plane extending up to | b | < 10u Depolarization/ on either side of the Galactic Centre (thin dashed line). This depolarization is modulation area due to Faraday rotation by a number of shells that match Ha emission regions27, most of them lying in the Sagittarius arm at distances from the Sun up to 2.5 kpc, and some in the Scutum-Centaurus arm at ,3.5 kpc. The small-scale modulation is associated with weaker Ha emission encompassing the same H II regions and most probably associated with the same spiral arms. Thus γ-ray spur 2.5 kpc constitutes a lower limit to the lobes’ near-side distance and places the far side beyond 5.5 kpc from the Sun (compare ref. 9). Along with their direction in the sky, this suggests that the lobes are associated with the Galactic bulge and/or Centre. recent activity of the supermassive black hole, perhaps occurring in concert with enhanced nuclear star-formation activity4. Our data indicate that the process of gas accretion onto the Galactic0 0.017 0.034 0.051 0.068 0.085 0.1 0.12 0.14 0.15 0.17 nucleus inescapably involves star formation which, in turn, energizes P (Jy per beam) an outflow. This carries away low-angular-momentum gas, cosmic rays and magnetic field lines, and has a number of important conse-non-thermal ‘radio arc’1 (itself longitudinally coincident with the quences. First, the dynamo activity in the Galactic Centre24, probably,4-Myr-old Quintuplet23 stellar cluster). required to generate its strong17 in situ field, requires the continual The magnetic energy content of both lobes is much larger than the expulsion of small-scale helical fields to prevent dynamo saturation25;ridges, (1–3) 3 1055 erg. This suggests the magnetic fields of the lobes the presence of the ridges high in the halo may attest to this process.are the result of the accumulation of a number of star formation Second, the lobes and ridges reveal how the very active star formationepisodes. Alternatively, if the lobes’ field structure were formed over in the Galactic Centre generates and sustains a strong, large-scalethe same timescale as the ridges, it would have to be associated with magnetic field structure in the Galactic halo. The effect of this on Galactic Centre spur Northern ridge NW limb brightening NE limb brightening SW limb brightening Southern ridge 0.00012 0.0 0.00010 Brightness temperature (K)Figure 3 | Polarized intensity and magnetic angles at 23 GHz from centred at the Galactic Centre. Grid lines are spaced by 15u. The emissionWMAP8. The magnetic angle is orthogonal to the emission polarization angle intensity is plotted as brightness temperature, in K. The vector line length isand traces the magnetic field direction projected on to the plane of the sky proportional to the polarized brightness temperature (the scale is shown by the(headless vector lines). The three ridges are obvious while traces of the radio line in the bottom-left corner, in K). Data have been binned in 1u 3 1u pixels tolobes are visible (2.3 GHz edges shown by the black solid line). The magnetic improve the signal-to-noise ratio. From a combined analysis of microwave andfield is aligned with the ridges and curves following their shape. Two spurs c-ray data (see also Supplementary Information) we can derive the followingmatch the lobe edges northwest and southwest of Galactic Centre and could be magnetic field limits (complementary to the equipartition limits reported in thelimb brightening of the lobes. A third limb brightening spur candidate is also text and Fig. 2): for the overall lobes/bubbles, B . 9 mG; and for the Galacticvisible northeast of the Galactic Centre. The map is in Galactic coordinates, Centre spur, 11 mG , B , 18 mG.6 8 | N AT U R E | VO L 4 9 3 | 3 J A N U A RY 2 0 1 3 ©2013 Macmillan Publishers Limited. All rights reserved
  4. 4. LETTER RESEARCH 6 9. Jones, D. I., Crocker, R. M., Reich, W., Ott, J. & Aharonian, F. A. Magnetic substructure in the northern Fermi bubble revealed by polarized microwave emission. 6 μG Astrophys. J. 747, L12–L15 (2012). 10. Sofue, Y., Reich, W. & Reich, P. The Galactic center spur — A jet from the nucleus? 15 μG Astrophys. J. 341, L47–L49 (1989). 5 11. Bland-Hawthorn, J. & Cohen, M. The large-scale bipolar wind in the Galactic Center. Astrophys. J. 582, 246–256 (2003). 12. Molinari, S. et al. A 100 pc elliptical and twisted ring of cold and dense molecular log[Range (pc)] clouds revealed by Herschel around the Galactic Center. Astrophys. J. 735, 4 L33–L39 (2011). 13. Aharonian, F. A. et al. Discovery of very-high-energy c-rays from the Galactic Centre 2.3GHz, 15 μG ridge. Nature 439, 695–698 (2006). 23GHz, 15 μG 14. Crocker, R. M. et al. Wild at heart: the particle astrophysics of the Galactic Centre. 50 GeV (IC) 1 GeV (IC) 3 Mon. Not. R. Astron. Soc. 413, 763–788 (2011). 15. Law, C. J. A multiwavelength view of a mass outflow from the Galactic Center. Astrophys. J. 708, 474–484 (2010). 16. Crocker, R. M. et al. c-rays and the far-infrared-radio continuum correlation reveal a powerful Galactic Centre wind. Mon. Not. R. Astron. Soc. 411, L11–L15 (2011). 2 17. Crocker, R. M., Jones, D. I., Melia, F., Ott, J. & Protheroe, R. J. A lower limit of 50 8 9 10 11 12 log[Ee (eV)] microgauss for the magnetic field near the Galactic Centre. Nature 463, 65–67 (2010). 18. Yun, M. S., Reddy, N. A. & Condon, J. J. Radio properties of infrared-selectedFigure 4 | The vertical range of cosmic ray electrons as a function of their galaxies in the IRAS 2 Jy sample. Astrophys. J. 554, 803–822 (2001).kinetic energy, Ee. Two cases are reported, for field amplitudes of 15 and 6 mG 19. Launhardt, R., Zylka, R. & Mezger, P. G. The nuclear bulge of the Galaxy III. Large(blue and red curves, respectively). Owing to geometrical uncertainties, adiabatic scale physical characteristics of stars and interstellar matter. Astron. Astrophys.losses cannot be determined so the plotted range (y axis) actually constitutes an 384, 112–139 (2002).upper limit. Electrons are taken to be transported with a speed given by the sum 20. Heesen, V., Beck, R., Krause, M. & Dettmar, R.-J. Cosmic rays and the magnetic field in the nearby starburst galaxy NGC 253 III. Helical magnetic fields in the nuclearof the inferred vertical wind speed (1,100 km s21) and the vertical component outflow. Astron. Astrophys. 535, A79 (2011).of the Alfven velocity in the magnetic field. The former is inferred from the 21. Muno, M. P. et al. Diffuse X-ray emission in a deep Chandra image of the Galacticgeometry of the northern ridge: if its footpoint has executed roughly half an Center. Astrophys. J. 613, 326–342 (2004).orbit in the time the Galactic Centre spur has ascended to its total height of 22. Pohl, M., Reich, W. & Schlickeiser, R. Synchrotron modelling of the 400 pc spur at,4 kpc, its upward velocity must be close to9 1,000 km s21 3 (r/100 pc)21 3 the galactic center. Astron. Astrophys. 262, 441–454 (1992). 23. Hußmann, B., Stolte, A., Brandner, W. & Gennaro, M. The present-day massvrot/(80 km s21), where we have normalized to a footpoint rotation speed of function of the Quintuplet cluster. Astron. Astrophys. 540, A57 (2012).80 km s21 at a radius of 100 pc from the Galactic Centre12 (detailed analysis 24. ` Ferriere, K. Interstellar magnetic fields in the Galactic center region. Astron.gives 1,100 km s21: see Supplementary Information). In a strong, regular Astrophys. 505, 1183–1198 (2009).magnetic field, the electrons are expected to stream ahead of the gas at the 25. Brandenburg, A. & Subramanian, K. Astrophysical magnetic fields and nonlinearAlfven velocity28 in either the ridges (B<15 mG, vA <300 km s{1 ; this is a vert dynamo theory. Phys. Rep. 417, 1–209 (2005).lower limit given that nH =0:008 cm{3 on the basis of the ROSAT data29) or in 26. Beck, R. & Krause, M. Revised equipartition and minimum energy formula for magnetic field strength estimates from radio synchrotron observations. Astron.the large-scale field of the lobes (B<6 mG, vA >100 km s{1 for vert Nachr. 326, 414–427 (2005).nH =0:004 cm{3 in the lobes’ interior as again implied by the data). Also 27. Gaustad, J. E., McCullough, P. R., Rosing, W. & Van Buren, D. A robotic wide-angleplotted as the vertical dashed lines are the characteristic energies of electrons Ha survey of the southern sky. Publ. Astron. Soc. Pacif. 113, 1326–1348 (2001).synchrotron radiating at 2.3 and 23 GHz (for a 15 mG field) and into 1-GeV and 28. Kulsrud, R. & Pearce, W. P. The effect of wave-particle interactions on the50-GeV c-rays via inverse Compton (‘IC’) upscattering of a photon propagation of cosmic rays. Astrophys. J. 156, 445–469 (1969). 29. Almy, R. C. et al. Distance limits on the bright X-ray emission toward the Galacticbackground with characteristic photon energy 1 eV; and the approximate 7 kpc Center: evidence for a very hot interstellar medium in the galactic X-ray bulge.distance of the top of the northern ridge from the Galactic plane. Astrophys. J. 545, 290–300 (2000). Supplementary Information is available in the online version of the paper.the propagation of high-energy cosmic rays in the Galactic halo should Acknowledgements This work has been carried out in the framework of the S-bandbe considered. Third, the process of gas expulsion in the outflow may Polarization All Sky Survey collaboration (S-PASS). We thank the Parkes Telescope staffexplain how the Milky Way’s supermassive black hole is kept relatively for support, both while setting up the non-standard observing mode and during thequiescent1, despite sustained, inward movement of gas. observing runs. R.M.C. thanks F. Aharonian, R. Beck, G. Bicknell, D. Jones, C. Law, ¨lk M. Morris, C. Pfrommer, W. Reich, A. Stolte, T. Porter and H. Vo for discussions, and the ¨ Max-Planck-Institut fur Kernphysik for supporting his research. R.M.C. alsoReceived 8 August; accepted 26 October 2012. acknowledges the support of a Future Fellowship from the Australian Research Council through grant FT110100108. B.M.G. and C.P. acknowledge the support of an Australian1. Morris, M. & Serabyn, E. The Galactic Centre environment. Annu. Rev. Astron. Laureate Fellowship from the Australian Research Council through grant Astrophys. 34, 645–701 (1996). FL100100114. M.H. acknowledges the support of research programme 639.042.915,2. Su, M., Slatyer, T. R. & Finkbeiner, D. P. Giant gamma-ray bubbles from Fermi-LAT: which is partly financed by the Netherlands Organisation for Scientific Research (NWO). active galactic nucleus activity or bipolar galactic wind? Astrophys. J. 724, The Parkes Radio Telescope is part of the Australia Telescope National Facility, which is 1044–1082 (2010). funded by the Commonwealth of Australia for operation as a National Facility managed3. Su, M. & Finkbeiner, D. P. Evidence for gamma-ray jets in the Milky Way. Astrophys. by CSIRO. We acknowledge the use of WMAP data and the HEALPix software package. J. 753, 61 (2012).4. Zubovas, K., King, A. R. & Nayakshin, S. The Milky Way’s Fermi bubbles: echoes of Author Contributions E.C. performed the S-PASS observations, was the leader of the the last quasar outburst? Mon. Not. R. Astron. Soc. 415, L21–L25 (2011). project, developed and performed the data reduction package, and did the main5. Crocker, R. M. & Aharonian, F. Fermi bubbles: giant, multibillion-year-old analysis and interpretation. R.M.C. provided theoretical analysis and interpretation. reservoirs of Galactic Center cosmic rays. Phys. Rev. Lett. 106, 101102 (2011). L.S.-S., M.H. and S.P. performed the S-PASS observations. M.J.K. performed the telescope special set-up that allowed the survey execution. L.S.-S., M.H., B.M.G., G.B.,6. Crocker, R. M. Non-thermal insights on mass and energy flows through the M.J.K. and S.P. were co-proposers and contributed to the definition of the project. C.P. Galactic Centre and into the Fermi bubbles. Mon. Not. R. Astron. Soc. 423, performed the estimate of the Ha depolarizing region distance. E.C. and R.M.C. wrote 3512–3539 (2012). the paper together. All the authors discussed the results and commented on the7. Carretti, E. in The Dynamic ISM: A Celebration of the Canadian Galactic Plane Survey manuscript. (eds Kothes, R., Landecker, T. L. & Willis, A. G.) 276–287 (ASP Conf. Ser. CS-438, Astronomical Society of the Pacific, 2011). Author Information Reprints and permissions information is available at8. Hinshaw, G. et al. Five-year Wilkinson Microwave Anisotropy Probe observations: www.nature.com/reprints. The authors declare no competing financial interests. data processing, sky maps, and basic results. Astrophys. J. 180 (suppl.), 225–245 Readers are welcome to comment on the online version of the paper. Correspondence (2009). and requests for materials should be addressed to E.C. (Ettore.Carretti@csiro.au). 3 J A N U A RY 2 0 1 3 | VO L 4 9 3 | N AT U R E | 6 9 ©2013 Macmillan Publishers Limited. All rights reserved
  5. 5. Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands 8 Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia 9 Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138, USA 10 CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia 11 INAF Osservatorio Astronomico di Cagliari, St. 54 Loc. Poggio dei Pini, I-09012 Capoterra (CA), ItalySUPPLEMENTARY INFORMATION doi:10.1038/nature11734 1 Data We use the images of the recently concluded S-band Polarization All Sky Survey (S-PASS) that has mapped the polarized radio emission of the entire southern sky with the Parkes Radio Telescope at a frequency of 2307 MHz, with 184 MHz bandwidth, and 9’ angular resolution7 . See Table S1 for a description of the flux densities from the various objects we identify in the main text. For historical interest, Parkes telescope investigations of the Galactic Centre (GC) – and even the discovery of a potential outflow from this region – date back some half a century30 . 1.1 Inference of total intensity flux densities at 2.3 GHz Confusion with Galactic foregrounds, especially free–free and HII regions, means that we cannot directly measure the total intensity flux density of the Lobes for |b| < 15◦ . We follow the following procedure to ∼ circumvent this problem and estimate the total intensity flux density of the whole Lobes: 1. We measure the integrated polarized intensity of both (whole) Lobes (emission within the edges). 1 structure solid angular 2.3 GHz pol. 2.3 GHz name angle width pol. flux frac. total flux [deg2 ] [deg] density [Jy] density [Jy] S-PASS Lobes north 1751 2610 ± 100 0.25 ±0.02 10440 ± 450 south 2009 2780 ± 110 0.26±0.02 10690 ± 450 total 3760 5390 ± 150 0.26±0.02 21130 ± 720 Northern Ridge 51.8 2.75 174 ± 14 0.31 ± 0.06 560 ± 110 GC Spur 50.5 1.9 236 ± 8 0.25 ± 0.03 960 ± 132 Southern Ridge 117.6 3.0 373 ± 35 0.31 ± 0.04 1215 ± 130 Table S1: Observed quantities 2. We measure the integrated total and polarized intensity from both Lobes at all latitudes |b| > 15◦ where the free-free emission is marginal compared to the synchrotron at this frequency. 3. On the basis of these measurements we infer the intrinsic polarization fraction of the synchrotron emission from the Lobes. This is done on the area mentioned above (|b| > 15◦ ), that is about 63% of the whole solid angle covered by the Lobes. 4. Assuming the same intrinsic polarization fraction, we infer the integrated, total intensity flux density from the remaining 37% of the Lobes that we cannot measure directly. 1.2 Minimum distance to Lobes from (de)polarization phenomenology Neither the γ-ray nor the microwave data1 allow us to infer the distance to the Lobes, but the 2.3 GHz W W W. N A T U R E . C O M / N A T U R E | 1 S-PASS data show depolarization at low latitudes around the Galactic Plane that is very informative in this
  6. 6. emission from the Lobes. This is done on the area mentioned above (|b| > 15 ), that is about 63% of the whole solid angle covered by the Lobes. pol. flux name angle width frac. total flux [deg 2] [deg] density [Jy] density [Jy] RESEARCH SUPPLEMENTARY INFORMATION polarization fraction, we infer the integrated, total intensity flux density 4. Assuming the same intrinsic S-PASS Lobes from the remaining 37% of the Lobes that we cannot100 0.25 ±0.02 10440 ± 450 north 1751 2610 ± measure directly. south 2009 2780 ± 110 0.26±0.02 10690 ± 450 total 3760 5390 ± 150 0.26±0.02 21130 ± 720 Northern Ridge to Lobes from (de)polarization phenomenology ± 110 1.2 Minimum distance 51.8 2.75 174 ± 14 0.31 ± 0.06 560 GC Spur 50.5 1.9 236 ± 8 0.25 ± 0.03 960 ± 132 Southern Ridge 117.6 3.0 373 ± 35 0.31 ± 0.04 1215 ± 130 Neither the γ-ray nor the microwave data1 allow us to infer the distance to the Lobes, but the 2.3 GHz S-PASS data show depolarization at low latitudes around the Galactic Plane that is very informative in this Table S1: Observed quantities context. S-PASS linear polarized emission and Stokes Q and U images reveal two strong depolarization areas on either side of the Galactic Centre encompassed by a border of small scale modulated signal that extends up to some |b|integratedBoth are generated by Faraday Rotation Lobes at all latitudes |b| > 15◦ 2. We measure the ≃ 10◦ . total and polarized intensity from both effects generating depolarization (the former) the free-free emission is marginal compared to the synchrotron at this frequency. and U without where and polarization angle modulation that generates small scale mixing of Stokes Q significant depolarization (the latter). By pinning down the objects responsible for this depolarization we 3. On the basis of these measurements we infer the intrinsic polarization fraction of the synchrotron can infer a lower limit on the distance to the Lobes. emission from the Lobes. This is done on the area mentioned above (|b| > 15◦ ), that is about 63% of the whole solid angle covered by the Lobes. Figure S1 shows a comparison between S-PASS Stokes Q (left panel) and the H-α emission as seen in the SHASSA28 mapsame intrinsic The S-PASS fraction, we infer the integrated, total intensity flux density 4. Assuming the (right panel). polarization image reveals a number of circular, arc, and bow features in the depolarization regions37% of the Lobes that we cannot measurein SHASSA maps well. We investigated from the remaining that match the H-alpha emission regions directly. these associations to identify the individual H-α regions and found that most belong to the Sagittarius arm. Moreover, some regions of the more distant Scutum-Centaurus arm are evident emerging behind the Sagit- tarius arm, like the Scutum super–shell, generating depolarization as well. A few examples of these regions 1.2 Minimum distance to Lobes from (de)polarization phenomenology are:Evidence that a distinct, extended, thermal X-ray G355.05+0.04, G347.70+1.90, thermal energy – seen in thethe H-α 1 G4.28+0.55, G6.09-1.29, G17.4–4.55, source – containing close to 1056 erg G345.00+1.70. Thus, direction regions responsible for the depolarization are not objectspreviously beenarm but29 . located at least 1.5–2.5 kpc of the Galactic Centre is actually located in its physical vicinity has in the local claimed are Neither the γ-ray nor the microwave data1 allow us to infer the distance tokpc from us. but the 2.3 GHz from us. Depolarization against the Scutum-Centaurus arm occurs at 3.0-4.0 the Lobes, S-PASS data show depolarization at low latitudes around the Galactic Plane that is very informative in this 2 context. S-PASS linear polarized emission and Stokes Q and U images reveal two corresponds to weaker The Faraday modulated region surrounding these two areas of depolarization strong depolarization areas enclosing the same group of H-α regions and must be associated of small same spiral arms.signal that H-α on either side of the Galactic Centre encompassed by a border with the scale modulated extends up to some |b| ≃ 10◦ . Both are generated by Faraday Rotation effects generating depolarization (the former) and polarization angle modulation that generates small scale mixing of Stokes Q and U without The large scale emission must come from the background of the depolarizing objects. The Lobes’ significant depolarization (the latter). By pinning down the objects responsible for this depolarization we front sides, then, must sit at least at 2.5 kpc from us if we conservatively only account for depolarization can infer a lower limit on the distance to the Lobes. by the Sagittarius arm objects. The transverse dimension of the lobes is some 50◦ , so that, assuming a cylindrical geometry, its centre has to be at least at 4.0 kpc from us and its far side at 5.5 kpc. This is already in the Figureregion. a comparison between S-PASS Stokes Q (left panel) and the H-α emission as seen in bulge S1 shows the SHASSA28 map (right panel). The S-PASS image reveals a number of circular, arc, and bow features in the depolarization regions that match the H-alpha emission regions in SHASSA maps well. We investigated In summary, the S-PASS data implies that the lobes must be located at least as distant as the rim of the these associations to identify the individual H-α regions and found that most belong to the Sagittarius arm. Galactic Bulge, in the direction of the Galactic Centre, and huge (extending at least 4.0-5.0 kpc both north Moreover, some regions of the more distant Scutum-Centaurus arm are evident emerging behind the Sagit- and south of the Galactic Plane, implying a minimum total vertical extent of 8.0-10.0 kpc). tarius arm, like the Scutum super–shell, generating depolarization as well. A few examples of these regions 1 Evidence that a distinct, extended, thermal X-ray source – containing close to 1056 erg thermal energy – seen in the direction of the Galactic Centre is actually located in its physical vicinity has previously been claimed29 . 2 Equipartition magnetic field calculation 2 2.1 Equipartition magnetic field calculation for entire structures We assume that the average path length through the Lobes is 5 kpc and examine two limiting cases: i) synchrotron emission occurs through the entire volume of the Lobes in which case their 21 kJy flux density implies a 6 µG equipartition11 field as reported; ii) the synchrotron emission occurs in an ensheathing region of 300 pc thickness (the approximate width of the Ridges) with a highly but not perfectly regular field structure. Such a layer would have an equipartition field of 12 µG. Note that, given the rather high polarization fraction at 2.3 GHz, limiting case i) is likely to be somewhat in tension with the inference31, 32 that – given a lack of observed polarized microwave emission coincident with the WMAP ‘haze’33 (see §3) – the interior, volume-filling field structure is highly turbulent (though see ref. 34 ). Calculated equipartition magnetic field amplitudes (and the lower and upper magnetic field amplitude limits determined below) for2 | W W W. N A theRLobes and T U R the Ridges are reported systematically in Table S2 (and consequent magnetic energy T U E . C O M / N A for E densities, total energies, inferred magnetic energy injection rates are presented in Table S3); the Ridges are
  7. 7. In summary, the S-PASS data implies that the lobes must be located at least as distant as the rim of the Galactic Bulge, in the direction of the Galactic Centre, and huge (extending at least 4.0-5.0 kpc both north and south of the Galactic Plane, implying a minimum total vertical extentSUPPLEMENTARY INFORMATION RESEARCH of 8.0-10.0 kpc). 2 Equipartition magnetic field calculation 2.1 Equipartition magnetic field calculation for entire structures We assume that the average path length through the Lobes is 5 kpc and examine two limiting cases: i) synchrotron emission occurs through the entire volume of the Lobes in which case their 21 kJy flux density implies a 6 µG equipartition11 field as reported; ii) the synchrotron emission occurs in an ensheathing region of 300 pc thickness (the approximate width of the Ridges) with a highly but not perfectly regular field structure. Such a layer would have an equipartition field of 12 µG. Note that, given the rather high polarization fraction at 2.3 GHz, limiting case i) is likely to be somewhat in tension with the inference31, 32 that – given a lack of observed polarized microwave emission coincident with the WMAP ‘haze’33 (see §3) – the interior, volume-filling field structure is highly turbulent (though see ref. 34 ). Calculated equipartition magnetic field amplitudes (and the lower and upper magnetic field amplitude limits determined below) for the Lobes and for the Ridges are reported systematically in Table S2 (and consequent magnetic energy densities, total energies, inferred magnetic energy injection rates are presented in Table S3); the Ridges are remarkably similar in their gross characteristics despite their different ages. For the magnetic field energy of the entire structures (in the volume filling field scenario) we assume a total volume of 2 × 1067 cm3 ; the modelled, 300 pc-thick sheath has volume 1 × 1066 cm3 . structure assumed physical height vertical assumed l.o.s. Beq [⋆] BBBnd [†] Bmax [‡] name distance width top extent pathlength [µG] [µG] [µG] [pc] [pc] [pc] [pc] [pc] S-PASS Lobes volume 5 000 6 3 >9 vol B|| <25 -filling [α23 2.3 = 1.15] [α33 23 = −0.7] vol < 43[§] Btot shell only 300 12 >9 [α23 = −1.15] 2.3 [α33 23 = −0.7] Northern Ridge 6 000 290 7000 2 100 290 14 [α23 = −1.15] 2.3 GC Spur 8 000 210 4 000 4 000 270 15 [α23 = −1.01] 2.3 11 − 18 [α33 = −1.25] 23 Southern Ridge 6 000 320 7000 4 900 320 13 [α23 = −1.05] 2.3 Table S2: Derived quantities I: ⋆ Equipartition11 magnetic field. Relative statistical error is 1% from uncertainty in total, 2.3 GHz flux and 6% from uncertainty in 2.3 to 23 GHz spectral index. † Broadband limits on the allowed magnetic field amplitude determined from the consideration that electrons synchrotron radiating at microwave frequencies (and therefore contributing to the WMAP haze emission) will also inverse-Compton radiate into ∼GeV γ-rays (and therefore contribute to the Fermi Bubbles’ intensity). Note that the WMAP haze is significantly less extensive in b than the Bubbles or Lobes and that the limit only applies to the solid angle over which it is observed, roughly b < 30◦ . ‡ Derived from tentative detection of polarized, 2.3 GHz emission ∼ from rear surface of the outflow and consequent requirement that the change in the polarization angle due to differential Faraday rotation satisfy ∆θ < π over the 184 MHz bandwidth of the Parkes 2.3 GHz observations. Derived from B|| <25 assuming a turbulent magnetic field. § vol4 W W W. N A T U R E . C O M / N A T U R E | 3
  8. 8. RESEARCH SUPPLEMENTARY INFORMATION 2.1.1 Statistical uncertainties Formally, the sources of statistical uncertainty in the determination of the equipartition field in the Lobes originate in errors in the measurement of the total flux density from these structures and the 2.3 to 23 GHz spectral index (see Table S1). The 3% relative error on the total flux density from the Lobes implies a partial contribution of 1% to the relative statistical error on Beq . The 9% relative error on the spectral index implies a partial contribution of 6% to the relative statistical error on Beq . 2.1.2 Systematic uncertainties The systematic error on Beq is dominated by the uncertainty in K0 , the proton-to-electron number den- sity ratio. We choose the theoretically-motivated35 and observationally-suggested (from local cosmic ray measurements36 ) value of 100 for this parameter. It is to be admitted, however, that, in the unusual environ- ment of the Lobes and Ridges, we cannot be certain this value holds. Still, the dependence of Beq on K0 is rather weak so that even variation of this parameter by fully an order of magnitude leads to only a ∼ 60% change in Beq (e.g., we have, for the volume-filling field Beq = {4, 6, 10} for K0 = {10, 100, 1000}). Moreover, given the timescale requisite to transport the cosmic rays from the plane and the much longer cooling times5 of cosmic ray ions than electrons in the environment of the Lobes, we expect that – if any- thing – K0 = 100 is likely to be underestimate, implying that, conservatively, the equipartition magnetic field we estimate is likely to be lower than the real field. Finally, on the question of whether the physical circumstances in the Lobes and Ridges are such that equipartition actually holds or is, at least, a reason- able approximation, we explain immediately below how an analysis of the broadband data covering the Lobes and the GC Spur implies lower limits to the real magnetic fields in these structures approaching the equipartition magnetic field values we obtain. 3 Broadband phenomenology At lower Galactic latitudes the Fermi Bubbles – and the Lobes – are coincident with a non-thermal mi- crowave ‘haze’ found in total intensity WMAP 20-60 GHz data34, 37 of luminosity (1 − 5) × 1036 erg/s (cf. the 1-100 GeV luminosity of the Bubbles of 2 × 1037 erg/s[3 ]) and their edges are coincident with an hourglass-shaped X-ray structure seen at lower Galactic latitudes in ROSAT data29 (and attributed to an outflow driven by Galactic centre star formation12 and also clearly evident in the Stokes U parameter map at 23 GHz (see Figure S3). There are intriguing similarities and differences between emission seen in different wavebands. We find regions of emission coincident with the 2.3 GHz map not only in the microwaves but also in X-rays (Figure S2) and in γ-rays (Figure 2, main text). In the south west, a spur of X-ray emission appears to wrap around the edge of the southern Fermi Bubble, paralleling but not exactly coincident with the Southern Ridge; this indicates this feature is not simply a limb-brightening in the cone of outflowing plasma (a γ-ray 54 | W W W. N A T U R E . C O M / N A T U R E
  9. 9. SUPPLEMENTARY INFORMATION RESEARCHfeature coincident with the Southern Ridge and appearing to ‘wrap’ in the same fashion is also evident:see Figure 2 of the main text). Coincident, non-thermal emission in 2.3 GHz and 23 GHz polarization and∼GeV γ-rays is evident in the GC Spur, the Southern Ridge, and, indeed, over almost the entire extent ofthe Bubbles. This indicates a non-thermal electron population covering at least the energy range ∼(1-100)GeV (Figure 2, main text) that is simultaneously synchrotron radiating into radio/microwave frequenciesand up-scattering ambient light into γ-rays via the inverse Compton process2 . We find, however, that the broadband data cannot be explained with a single power-law electronpopulation: the spectrum between 2.3 and 23 GHz is considerably steeper (α < − 1.0 for Fν ∝ ν α ) than ∼the very hard spectrum (−0.4 > α > −0.7) found34 over 23 to 41 GHz for the haze3 . Moreover, polarized2.3 GHz emission is observed considerably outside the γ-ray-defined edges of the Bubbles at high Galacticlatitudes (and towards Galactic west). These considerations indicate that a second, high-energy and veryhard electron population is either locally accelerated (perhaps powered by magnetic field reconnection) orinjected as secondaries (from collisions between cosmic ray protons and the Bubbles’ low-density thermalplasma5 in situ. This is consistent with the fact that the cooling time of the high-energy electrons required togenerate the γ-rays is too short for these particles to be transported from the plane out to the full extensionof the Bubbles/Lobes given the speed of the outflow (Figure 4, main text).3.1 Spectral index between 2.3 and 23 GHz polarized emissionThe spectral index between polarized emission at 2.3 GHz measured by S-PASS and at 23 GHz measuredby WMAP is shown in Figure S4. S-PASS and WMAP polarization maps have been binned to 2◦ × 2◦pixels to improve the signal–to–noise ratio of the 23 GHz data. Noise debiasing has been applied beforemeasurement of the spectral index. As stated in the main text, there is a clear tendency for the spectrum ofsynchrotron radiation to steepen with distance from the plane (the very flat spectrum in the the plane itselfis a spurious result of Faraday depolarization at 2.3 GHz near the Galactic plane). This is a clear indicationfor the ageing of the synchrotron-emitting cosmic electrons and consistent with their being transported outfrom the plane.3.2 Broandband limits on magnetic fieldBroadband considerations allow also us to derive a rough lower bound on the magnetic field intensitythroughout the volume of the Lobes/Bubbles: the magnetic field must be strong enough that the in situ 2 We note that the γ-ray spectrum for the ‘jet’ feature identified by Su and Finkbeiner3 and claimed by us to be more-or-lesscoincident with the GC Spur, is distinct from that of the general Fermi Bubble emission surrounding it. In particular, the jetspectrum is both harder and does not exhibit the same low energy cut-off seen in the general Bubble spectral energy distributionbelow ∼1 GeV. This phenomenology is consistent with the jet γ-rays being largely supplied by inverse Compton emission dueto primary electrons advected from the plane while, in contrast, the general Bubble γ-ray emission might largely be supplied byproton-proton collisions5 . 3 Though note that the spectrum that we determine, on the basis of the WMAP data, for the GC Spur between 23 and 33 GHz is,at α33 ≃ −1.25, considerably steeper than that determined for the haze. 23 6 W W W. N A T U R E . C O M / N A T U R E | 5
  10. 10. RESEARCH SUPPLEMENTARY INFORMATION electron population (required to generate, via synchrotron emission, the observed non-thermal microwave intensity) must not be so numerous that the inverse Compton emission4 from the same electrons surpasses the observed γ-ray intensity in the Fermi band (Eγ dNγ /dEγ ≃ 4 × 10−7 GeV/cm2 /s/sr at a few GeV fol- 2 lowing ref. [2 ]). Such reasoning implies B > 9 µG field for a hard-spectrum electron population with a 1 ∼ eV cm−3 photon background to up-scatter and γe = 2.4 (for dNe /dEe ∝ Ee e , the particle spectral index −γ corresponding to the steepest allowed spectral index from analysis of the haze emission, αhaze = −0.7[31 ], which generates the most conservative lower limit on the field amplitude), and B > 16 µG for γe = 2.0 ∼ (corresponding to αhaze = −0.5, the central value of the haze spectral index31 ). Given the evidence that the coincident γ-ray and microwave emission originates from the same cosmic ray electron population (indeed, from electrons in the same energy range), we may apply similar reasoning to the above to determine rough but robust limits to both the lower and upper allowed field strength in the GC Spur. Adopting the intensity reported3 for the jet-like γ-ray feature recently claimed in the Fermi data (also Eγ dNγ /dEγ ≃ 4×10−7 GeV/cm2 /s/sr at a few GeV) which is coincident with the GC Spur (identified 2 at radio continuum and microwave frequencies) at b ∼ 15 − 25◦ and using the spectral index measured by us between the polarized emission at 23 and 33 GHz, α33 ≃ 1.25, the polarized surface brightness at 23 23 GHz (1520 Jy/sr) and assuming the polarization fraction of 0.25 measured at 2.3 GHz also applies at 23 GHz, we derive a Stokes I surface brightness of 6100 Jy/sr and determine a lower limit to the total magnetic field amplitude in the GC Spur of 11 µG. We derive a conservative upper limit on the magnetic field from demanding that the cosmic ray electron population that supplies the inverse Compton γ-ray flux from the GC Spur saturate, via synchrotron emission, the whole total intensity at 23 GHz detected over the GC Spur solid angle, 16100 Jy/sr. This saturation point is attained for an ∼18 µG field, implying a rough upper limit to the field at this amplitude. In the case of the GC Spur, these lower and upper limits to the field imply that – if equipartition holds – the proton to electron number ratio, K0 is in the range 30 - 200. 4 Visibility of emission from rear windings at various wavelengths Our explanation of the geometry of the Ridges – in particular their curvature to Galactic west – requires that, while they wrap around the entirety of the cones defined by the global outflow, rear-side emission from the Ridges is attenuated with respect to the front side emission (the emission from the putative rear part of each Ridge would curve to Galactic east contrary to observations). This relative attenuation must function at 2.3, 23, and 33 GHz; we find that it cannot, then, be a result of simple Faraday depolarization which, for reasonable parameters of magnetic field intensity, plasma density, and path length through the volume of the Lobes, could not appreciably Faraday rotate the polarization angle at microwave frequencies. In fact, the relative attenuation of the rear, polarized emission can naturally be explained as a conse- quence of three simple effects which work equally well at radio continuum and microwave frequencies: In order to calculate the spectrum and luminosity of inverse Compton radiation we employ the RadiationField class39 from 4 the GalProp code available at http://galprop.stanford.edu/code.php. RMC thanks Troy Porter for assistance with using GalProp’s Galactic interstellar radiation field data. 76 | W W W. N A T U R E . C O M / N A T U R E
  11. 11. SUPPLEMENTARY INFORMATION RESEARCH 1. The synchrotron intensity scales approximately as Bperp , where Bperp is the component of the mag- 2 netic field perpendicular to the line-of-sight. From simple geometrical considerations (see caption to figure S5 ) the vertical component of Bperp is appreciably less in the rear part of a conical outflow than in the front part. The magnetic field direction, moreover, is largely vertical closer to the plane (figure 3 of the main text). 2. Furthermore, along any particular sightline the rear surface is intersected at a greater physical height from the plane than the front surface. This has the consequence that the electron population on the rear is ‘older’ (more cooled) than the front population. 3. A further (likely) consequence of the greater physical height of the intersection of any given line of sight with the rear surface (relative to the front surface) is that the local magnetic field amplitude at the rear on this line of sight is relatively attenuated (given that the magnetic field is also injected as ‘frozen-in’ field lines in the plasma outflowing from the plane and will have had more time to reconnect/relax while ascending to a greater height above the plane).Figure S5 shows the approximate ratio of front-side to rear-side synchrotron intensity taking these effectsinto account. It is also important to note that horizontal component of Bperp completely disappears at thetangent points of the projected outflow edges.4.1 Upper limit on volume magnetic field from tentative detection of 2.3 GHz polarized emission from rear surfaceNote, however, that a blanket statement that polarized emission from the Ridges on the rear surface of theoutflow is invisible from our vantage point does not seem to be correct, though such emission is certainlyobscured as discussed in the previous section. A careful examination of the Southern lobe in the 2.3 GHzpolarization map (Figure S6 and Figure 1 in the main text) reveals features curving in the opposite senseto the Ridges. A clear ridge–like structure – with possible counterparts at other wavelengths – is a lineardepolarization feature running from (l, b) ∼ (350◦ , −17◦ ) to (9, −32) (see Figure S6). A likely explanationof this feature is that it runs almost perpendicular to the Southern Ridge so that the polarization angles of thetwo structures are perpendicular (the magnetic angle is aligned with the Ridges). In turn, both Stokes Q andU have opposite signs for the two ridges and tend to cancel. An important implication of this explanation forthe phenomenology is that – even at the comparatively low frequency of 2.3 GHz – intrinsically polarizedemission from the rear surface is not Faraday depolarized by its passage through the magnetised plasmainhabiting the volume of the Southern lobe (rather the three geometrical factors outlined in the previoussection are responsible for the attenuation of the rear-side synchrotron emission relative to the front-side).We can use this inference to then place an upper limit on the magnetic field in this region of ∼ 43 µG: seeTable S2. 8 W W W. N A T U R E . C O M / N A T U R E | 7
  12. 12. RESEARCH SUPPLEMENTARY INFORMATION 5 More robust estimate on plasma outflow speed in ridges The Galactic Centre Ridge diverges from the projected co-rotating point in a fashion that is consistent with angular momentum being conserved in the outflow and the Galactic Centre Spur being ‘wound-up’ in the outflow21 . This same analysis points to an initial ratio between the circulation and vertical velocities of the outflow of vcirc /vvert ∼ 0.07. This generates a more accurate estimation of its ascension speed of ∼ 1100 km/s vcirc /(80 km/s) where we normalise to an 80 km/s circulation speed in the inner ∼100 pc as suggested by recent Herschel observations13 . 6 Global analysis 6.1 Considerations around the far infrared-radio continuum correlation The 60 micron (infrared) flux density19 of the inner ∼200 pc × 80 pc region around the Galactic centre – essentially the Nuclear Bulge – is 2 MJy [20 ]. On the basis of the far infrared-radio continuum correlation19 , this level of far infrared emission should be accompanied by a radio continuum flux density of 20.2 kJy at 1.4 GHz. In dramatic contrast, the detected radio continuum flux from the same region is ∼ 1.6 kJy at 1.4 GHz, less than 10% of expectation or around 4σ shy of the correlation40 . Even integrating the radio continuum flux density out to scales of 800 pc in diameter (thereby encompassing the distinct ‘diffuse non- thermal source’ identified by LaRosa et al.41 surrounding the Galactic centre), the detected radio continuum flux reaches only 25% of expectation. As has been argued at length elsewhere40 , the explanation for this phenomenology is that the vast bulk of cosmic ray electrons – accelerated in concert with star formation (and consequent supernova activity) in the Galactic centre region – is advected out of the region before the electrons can lose their energy, radiatively, in situ. Similarly, the γ-ray luminosity of this same inner region is in significant deficit with respect to the expectation42 were the hadronic cosmic rays accelerated in the region to lose their energy in situ (via col- lisions on ambient gas); i.e., the system is very far from a ‘calorimeter’. Again, the inference that can be made is that the vast bulk of the hadronic cosmic rays also escape the region on an outflow40 . Where does this power represented by the escaping cosmic ray ions and electrons go? The γ-ray luminosity of the Fermi Bubbles matches the expectation if supplied by hadronic collisions of the cosmic ray protons and ions leaving the Galactic centre5,6 . Equally, the S-PASS data allow us to determine that the total radio continuum flux density from the Lobes is 21 kJy at 2.3 GHz or νFν = 4.9 × 10−10 erg/cm2 /s; the 20.2 kJy at 1.4 GHz predicted by the correlation corresponds42 to νFν = 2.3 × 10−10 erg/cm2 /s at 2.3 GHz or, the expected 2.3 GHz flux density is 11.2 kJy assuming a spectral index of -1.2 between 1.4 and 2.3 GHz. The observed and predicted total flux densities at 2.3 GHz are, therefore, within a factor 1.9 of each other, corresponding to a quite acceptable difference of ∼1.1 σ (adopting the 0.26 dex scatter in the empirical correlation from ref. [19 ]). Given a number of uncertainties – particularly the effect that the spreading of the outflow cones will mean that the r 2 -biased emission from the front of the outflow takes place significantly 98 | W W W. N A T U R E . C O M / N A T U R E
  13. 13. SUPPLEMENTARY INFORMATION RESEARCHcloser than the ∼ 8 kpc distance to the GC (at which distance the Galactic centre star-formation-related farinfrared light is emitted) – we view this level of agreement, as stated in the main text, as a strong argumentthat the Lobes’ non-thermal radio emission is supplied by synchrotron emission from cosmic ray electronsaccelerated by star-formation activity in the Galactic Centre.6.2 Gross EnergeticsThe current star-formation rate in the inner ∼ 100 pc (in radius) region around the GC is slightly below0.1 M⊙ /year (see ref. [6 ] and references therein). Given the ∼ 109 M⊙ mass of the stellar populationinhabiting this region (the Nuclear Bulge20,43 ), the current value is close to the time-averaged value over thelast ∼ 10 Gyr. Using, conservatively, standard assumptions (i.e., the initial mass function, IMF, for the zero-age, main sequence stellar masses is given by a Kroupa44 IMF with a minimum stellar mass of 0.07 M⊙ ,the total mechanical energy release per core-collapse supernova is 1051 erg irrespective of the progenitor’szero-age, main sequence mass) this star formation rate translates to a mechanical power injection rate fromthe region’s core-collapse supernovae of ∼ 1040 erg/s[6,15 ]. By way of comparison, the power requisite toinflate the magnetic fields of the expanding Ridges and supply their cosmic ray content is ∼ 2 × 1039 erg/s(see Table S3) assuming that the equipartition approximation holds. These energetics can be satisfied by themechanical power available from the region’s supernovae under the most conservative assumptions. structure volume uBeq UBeq [†] age[⋆] texp [⋆] ˙ Emag [‡] name [cm3 ] [eV cm−3 ] [erg] [Myr] [Myr] [erg/s] S-PASS Lobes vol.-filling 2.0 × 1067 cm3 0.8 3 × 1055 300§ 300§ shell only 1.2 × 1066 cm3 3 8 × 1054 90§ 90§ northern ridge 5.0 × 1063 cm3 5 4 × 1052 4.7 1.4 8 × 1038 GC Spur 5.2 × 1063 cm3 4 7 × 1052 2.8 2.8 8 × 1038 southern ridge 1.4 × 1064 cm3 4 9 × 1052 4.7 3.4 9 × 1038Table S3: Derived quantities II: ⋆ Assumes expansion velocity vexp ≡ 1400 km/s. In principle, both thequoted ages and expansion times are lower limits because each Ridge structure disappears around the edgeof the general outflow. † Total magnetic energy assuming equipartition. ‡ Emag ≡ UBeq /texp . § Assumes ˙magnetic power injected at a rate 3 × 1039 erg/s.7 Discussion of thermal X-ray fluxesFrom ROSAT X-ray data covering the Lobes29,38 we find a background-subtracted count rate of 300 × 10−6cnt/s/arcmin2 over the R6 band (0.91 - 1.31 keV) for the bright X-ray counterpart to the southern Ridge(see Figure S2). This corresponds to an intensity of ∼ 8 × 10−8 erg/cm2 /s/sr. Obviously, this region isatypically bright in comparison to the whole solid angle of the Lobes but we use this intensity in the contextof generating various limits. (Also note, given the scale of other uncertainties, we are not correcting for 10 W W W. N A T U R E . C O M / N A T U R E | 9
  14. 14. RESEARCH SUPPLEMENTARY INFORMATION photoelectric absorption. This we expect to be a reasonably small correction given that we infer from the data presented in ref. 38 an optical depth at ∼ keV of ∼ 0.4) We first use this intensity to derive a (somewhat temperature-dependent) upper limit on the plasma density. Assuming a volume-filling plasma of ∼ 3 × 107 K, we derive from ref.45 , an upper limit on the density of this plasma of ∼ 3 × 10−3 cm−3 . Assuming, more naturally, that the observed X-rays are generated by plasma entrained in the Southern Ridge outflow, assuming a 300 pc pathlength through this structure, we obtain a plasma density of ∼ 2 × 10−2 cm−3 for the same assumed temperature (we obtain ∼ 9 × 10−3 cm−3 for T = 107 K) which defines an upper limit on the plasma density for the region. For a 1100 km/s outflow, this density then implies a ∼ 2 − 4 × 10−2 M⊙ /year mass flux along the Southern Ridge. Very roughly, this suggests that the mass flux along all of the ridges is ∼ 0.1 M⊙ /year, a comfortable fraction of the mass accretion rate on to the GC of ∼ 0.3 M⊙ /year (see ref. [6 ] and references therein). 7.1 Inferred Alfven velocities Given the upper limit on the plasma density and assuming the equipartition magnetic field amplitude, we may obtain the Alfven velocity, vA ≡ B 2 /(4πmp np ). The component of this resolved into the vertical 2 vert ≃ 300 km/s (for T ∼ 107 K and B = 15 µG). direction is vA 8 Relating Ridges to GC super-stellar clusters Assuming a reasonable fraction (∼20%) of the typical mechanical energy of a supernova (1051 erg) ends up in cosmic ray and magnetic energy, the Ridges each require 400 − 1000 supernovae or the formation of a total stellar mass of (3 − 9) × 104 M⊙ . This requires the accumulation of > (0.4 − 1) × 106 years’ star formation given the star-formation rate in the region. Such a timescale and the total stellar mass quoted are comparable to those associated with the formation of the observed massive stellar clusters in the Galactic Centre (e.g., ref. 46 ). As we have already noted, the gross energetics of the Ridges can be supplied by core- collapse supernovae occurring with the frequency implied by the Galactic Centre’s current star-formation rate. Note, however, a complicating factor: for any discrete star-formation event there is a delay of ∼ 3 Myr (e.g., ref. 47 ) between the onset of star-formation and the first core-collapse supernovae (originating in the most massive stars). In the strong tidal fields of the Galactic centre, moreover, stellar clusters are completely disrupted over a timescale ∼ 10 Myr[48 ] or, at least, suffer sufficient dissolution that they become invisible against the high stellar density background within a similar timeframe49 . In general terms, this means that, whereas the most massive stars of the super-stellar clusters contribute to the outflows forming the Ridges (both in terms of their winds and their supernovae which occur soon enough after cluster formation that the cluster is still coherent), core-collapse supernovae arising in less massive stars are more broadly distributed through the region and would seem to be prime candidates for energising the general bi-conical outflow 111 0 | W W W. N A T U R E . C O M / N A T U R E
  15. 15. SUPPLEMENTARY INFORMATION RESEARCHfeeding into the extended Lobes/Bubbles. Consistent with this picture, the mechanical power injected by thecombined stellar winds of a ∼ 5 × 104 M⊙ super-stellar cluster (a reasonable estimate for, e.g. the initialmass of either the Arches or Quintuplet clusters) is at least a few ×1039 erg/s (see, e.g., Figure 1 of ref.50 ),enough to initially supply the Ridges’ magnetic fields and cosmic ray content. Likewise, the mass flux alongthe Southern Ridge represented by the outflowing plasma, ∼ 2− 4× 10−2 M⊙ /year as derived above, is wellaccounted-for by mass loss due to massive stellar winds from a similarly-sized cluster (see, e.g., Figure 11of ref.50 ), allowing for an expected mass-loading of 3-10[51 ] and further mass injection by supernovae. Thismass flux also represents a comfortable fraction (∼ 10%) of the model-derived6 total plasma mass flux intothe entire outflow.9 Collimation of the RidgesOne significant aspect of the winding’s phenomenology is that they remain coherent over many kpc withrather constant widths. Aside from the implication that the windings present a channel to deliver cosmic raysfrom the Galactic nucleus out into the halo with little adiabatic loss, their collimation likely implies a par-ticular magnetic field topology: a ‘force-free’ configuration where the toroidal and longitudinal componentsof the field satisfy Bφ > B|| and the magnetic structure is self-confined52 . Confirmation of this speculation ∼and exactly how such a field configuration is produced is a subject of ongoing investigation.References[30] Kerr, F. J., & Sinclair, M. W. A Highly Symmetrical Pattern in the Continuum Emission from the Galactic Centre Region, Nature, 212, 166 (1966)[31] Dobler, G., & Finkbeiner, D. P., Extended Anomalous Foreground Emission in the WMAP Three-Year Data Astrophys. J., 680, 1222 (2008)[32] McQuinn, M., & Zaldarriaga, M. Testing the Dark Matter Annihilation Model for the WMAP Haze, Mon. Not. Roy. Astron. Soc., 414, 3577 (2011)[33] Gold, B., Odegard, N., Weiland, J. L., et al. Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Galactic Foreground Emission, Astrophys. J. Supp., 192, 15 (2011)[34] Dobler, G. A Last Look at the Microwave Haze/Bubbles with WMAP, Astrophys. J., 750, 17 (2012)[35] Bell, A. R. The acceleration of cosmic ray shock fronts – 1, Mon. Not. Roy. Astron. Soc., 182, 147 (1978)[36] Ginzburg, V. L., & Ptuskin, V. S. On the origin of cosmic rays: Some problems in high-energy astro- physics, Reviews of Modern Physics, 48, 161 (1976)[37] Finkbeiner, D. P. Microwave Interstellar Medium Emission Observed by the Wilkinson Microwave Anisotropy Probe, Astrophys. J., 614, 186-193 (2004) 12 W W W. N A T U R E . C O M / N A T U R E | 1 1
  16. 16. RESEARCH SUPPLEMENTARY INFORMATION [38] Snowden, S. L., et al. ROSAT Survey Diffuse X-Ray Background Maps. II., Astrophys. J., 485, 125 (1997) [39] Porter, T. A., Moskalenko, I. V., Strong, A. W., Orlando, E., & Bouchet, L. Inverse Compton Origin of the Hard X-Ray and Soft Gamma-Ray Emission from the Galactic Ridge, Astrophys. J., 682, 400 (2008) [40] Crocker, R. M., Jones, D. I., Aharonian, F., et al. γ-rays and the far-infrared-radio continuum correla- tion reveal a powerful Galactic Centre wind, Mon. Not. Roy. Astron. Soc., 411, L11-L15 (2011a) [41] LaRosa, T. N., Brogan, C. L., Shore, S. N., Lazio, T. J., Kassim, N. E., & Nord, M. E. Evidence of a Weak Galactic Center Magnetic Field from Diffuse Low-Frequency Nonthermal Radio Emission, Astrophys. J., 626, L23 (2005) [42] Thompson, T. A., Quataert, E., & Waxman, E. Starbursts and Extragalactic γ-ray Background, Astro- phys. J., 654, 219 (2007) [43] Serabyn, E., & Morris, M., Nature, 382, 602 (1996) [44] Kroupa, P. On the variation of the initial mass function, Mon. Not. Roy. Astron. Soc., 322, 231 (2001) [45] Raymond, J. C., Cox, D. P., & Smith, B. W., Radiative cooling of a low-density plasma, Astrophys. J., 204, 290 (1976) [46] Harfst, S., Portegies Zwart, S., & Stolte, A. Reconstructing the Arches cluster - I. Constraining the initial conditions, Mon. Not. Roy. Astron. Soc., 409, 628-638 (2010) [47] Mo, H., van den Bosch, F. C., & White, S. , Galaxy Formation and Evolution. Cambridge University Press, 2010. ISBN: 9780521857932 (2010) [48] Kim, S. S., Figer, D. F., Lee, H. M., & Morris, M. N-Body Simulations of Compact Young Clusters near the Galactic Center, Astrophys. J., 545, 301 (2000) [49] Portegies Zwart, S. F., Makino, J., McMillan, S. L. W., & Hut, P. The Lives and Deaths of Star Clusters near the Galactic Center, Astrophys. J., 565, 265 (2002) [50] Cˆ t´ , B., Martel, H., Drissen, L., & Robert, C. Galactic outflows and evolution of the interstellar oe medium, Mon. Not. Roy. Astron. Soc., 421, 847 (2012) [51] Strickland, D. K., & Heckman, T. M. Supernova Feedback Efficiency and Mass Loading in the Star- burst and Galactic Superwind Exemplar M82, Astrophys. J., 697, 2030 (2009) [52] Bicknell, G. V., & Li, J. The Snake: A Reconnecting Coil in a Twisted Magnetic Flux Tube, Astro- phys. J., 548, L69-L72 (2001) 131 2 | W W W. N A T U R E . C O M / N A T U R E
  17. 17. SUPPLEMENTARY INFORMATION RESEARCH -0.16 -0.13 -0.096 -0.064 -0.032 0.00016 0.032 0.064 0.096 0.13 0.16 -10 21 52 83 114 145 176 207 238 269 300Figure S1: Top: Stokes Q image of the area around the Galactic Centre. The Galactic plane is horizontalacross the picture and the emission unit is Jy/beam with a beam of FWHM=10.75’. The green dashed lineindicates the two areas of depolarization on either side of the Galactic Centre and the belt encompassingthem of emission modulated to small angular scales by Faraday Rotation effects. Bottom: H-α emissionimage of the same area from the SHASSA survey. The emission unit is decirayleighs (dR); The resolutionis FHWM=6’. The area affected by Faraday Rotation effects is reported as well and corresponds to H-αemission regions from the Sagittarius and Scutum-Centaurus arms – see text. 14 W W W. N A T U R E . C O M / N A T U R E | 1 3
  18. 18. RESEARCH SUPPLEMENTARY INFORMATION 0 0.017 0.034 0.051 0.068 0.085 0.1 0.12 0.14 0.15 0.17 Figure S2: S-PASS and X-ray emission. Data are: i) polarized emission from S-PASS shown in colour – unit is Jy/beam); ii) X-ray emission as detected by ROSAT (white contour levels, ranging from 250 to 550 × 10−6 cts/s/arcmin2 with steps of 75). ROSAT data are the average of the bands 5 and 6 and the band 7 subtracted to remove the large scale emission and emphasise substructures. The thick dashed lines show the edges of the S-PASS Lobes and the thinner dashed lines the edges of the γ–ray Fermi Bubbles as defined by Su et al.2 . 151 4 | W W W. N A T U R E . C O M / N A T U R E
  19. 19. SUPPLEMENTARY INFORMATION RESEARCHFigure S3: Linear polarization emission component Stokes U at 23 GHz from WMAP8 . An X-shapestructure centred at the Galactic Centre matches the biconical Lobe base as traced by X-ray emission (cf.Figure 6c of ref. [12 ] and ref. [38 ]) and could be limb brightening of the Lobes (the 2.3 GHz Lobe edgesshown by the black solid line). Stokes U is less contaminated by spiral arm emission contamination thanStokes Q because the magnetic angle of the arm emission is largely parallel to the Galactic plane8 . Themap is in Galactic coordinates, centred at the Galactic Centre. Grid lines are spaced by 15◦ . The emissionintensity is in Brightness Temperature, the unit is K. Data have been binned in 1◦ × 1◦ pixels. 16 W W W. N A T U R E . C O M / N A T U R E | 1 5

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