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A magnetized Galactic halo from inner Galaxy outflows

Nature Astronomy volume 8, pages 1416–1428 (2024)Cite this article

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Abstract

Magnetic halos of galaxies are crucial for understanding galaxy evolution, galactic-scale outflows and feedback from star formation activity. Identifying the magnetized halo of the Milky Way is challenging because of the potential contamination from foreground emission arising in local spiral arms. In addition, it is unclear how our magnetic halo is influenced by recently revealed large-scale structures such as the X-ray-emitting eROSITA Bubbles detected by the extended Roentgen Survey with an Imaging Telescope Array (eROSITA). Here we report the identification of several kiloparsec-scale magnetized structures on the basis of their polarized radio emission and their gamma-ray counterparts, which can be interpreted as the radiation of relativistic electrons in the Galactic magnetic halo. These non-thermal structures extend far above and below the Galactic plane and are spatially coincident with the thermal X-ray emission from the eROSITA Bubbles. The morphological consistency of these structures suggests a common origin, which can be sustained by Galactic outflows driven by active star-forming regions located in the Galactic Disk at 3–5 kpc from the Galactic Centre. These results reveal how X-ray-emitting and magnetized halos of spiral galaxies can be related to intense star formation activities and suggest that the X-shaped coherent magnetic structures observed in their halos can stem from galactic outflows.

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Fig. 1: Polarized radio counterpart of the eROSITA bubbles.
Fig. 2: Distance measurements to the eROSITA bubbles.
Fig. 3: Distance measurements for magnetized ridges by Faraday rotation depolarization.
Fig. 4: Radio/γ-ray analysis and SED.
Fig. 5: Energy sources of the Galactic outflows.

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Data availability

We use the following surveys in our paper to analyse the magnetic halo: synchrotron and dust data (https://lambda.gsfc.nasa.gov/), Fermi gamma-ray data (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/14yr_catalog/), 3D dust extinction map (https://astro.acri-st.fr/gaia_dev/about) and the ROSAT all-sky survey (https://cade.irap.omp.eu/dokuwiki/doku.php?id=rass). The other data are taken from the maps of published papers, for which we provide references in the Supplementary Information.

Code availability

The following software and code packages have been used in our analysis: Python (https://www.python.org/) with the packages Numpy (https://numpy.org/), Healpy (https://healpy.readthedocs.io/), Astropy (https://www.astropy.org/) and Fermipy (https://fermipy.readthedocs.io/); Jupyter Notebook (https://jupyter-notebook.readthedocs.io/), Matplotlib (https://matplotlib.org/); and DS9 (https://sites.google.com/cfa.harvard.edu/saoimageds9). The package naima (https://naima.readthedocs.io/) is used to model the multi-wavelength results. The electron distribution is from the package ymw16 (https://www.atnf.csiro.au/research/pulsar/ymw16/). The JF12 magnetic field model comes from the package CRPropa (https://crpropa.github.io/CRPropa3/api/classcrpropa_1_1PlanckJF12bField.html).

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Acknowledgements

H.S.Z. acknowledges support by the X-riStMAs project (Seal of Excellence no. 0000153) under the National Recovery and Resilience Plan (PNRR), Mission 4, Component 2, Investment 1.2 – Italian Ministry of University and Research, funded by the European Union – NextGenerationEU. H.S.Z. also acknowledges computing support from the PLEIADI supercomputer from INAF. H.S.Z., G.P., N.L., X.Z., Y.Z. and G.S. acknowledge financial support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program HotMilk (grant agreement no. 865637). G.P. also acknowledges support from Bando per il Finanziamento della Ricerca Fondamentale 2022 dell’Istituto Nazionale di Astrofisica (INAF): GO Large program and from the Framework per l’Attrazione e il Rafforzamento delle Eccellenze (FARE) per la ricerca in Italia (R20L5S39T9). M.R.M. acknowledges support from NASA under ADAP grant 80NSSC24K0639. The authors acknowledge X.Y. Li, X. Wu, M. Sasaki, G. Pareschi and G. Ghisellini.

Author information

Authors and Affiliations

  1. INAF Osservatorio Astronomico di Brera, Merate, Lecco, Italy

    He-Shou Zhang, Gabriele Ponti, Nicola Locatelli & Giovanni Stel

  2. Max-Planck-Institut für Extraterrestrische Physik, Garching bei München, Germany

    Gabriele Ponti, Nicola Locatelli, Xueying Zheng, Yi Zhang, Andrew Strong, Michael C. H. Yeung & Andrea Merloni

  3. INAF Istituto di Radioastronomia, Bologna, Italy

    Ettore Carretti

  4. School of Astronomy and Space Science, Nanjing University, Nanjing, China

    Ruo-Yu Liu & Hai-Ming Zhang

  5. Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Nanjing, China

    Ruo-Yu Liu

  6. Department of Physics and Astronomy, University of California, Los Angeles, CA, USA

    Mark R. Morris

  7. Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands

    Marijke Haverkorn

  8. Dublin Institute for Advanced Studies, Dublin, Ireland

    Felix Aharonian

  9. Max-Planck-Institut für Kernphysik, Heidelberg, Germany

    Felix Aharonian

  10. Yerevan State University, Yerevan, Armenia

    Felix Aharonian

  11. Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning, China

    Hai-Ming Zhang

  12. DiSAT, Università degli Studi dell’Insubria, Como, Italy

    Giovanni Stel

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  1. He-Shou Zhang
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Contributions

H.S.Z. and G.P. led the project. H.S.Z. performed the analysis. E.C., H.S.Z. and M.H. led the radio data analysis and magnetic field measurement. G.P., H.S.Z., N.L. and X.Z. led the X-ray study. R.Y.L., H.S.Z., F.A., H.M.Z., M.R.M., Y.Z. and G.S. led the gamma-ray study. H.S.Z., G.P., E.C., R.Y.L. and M.R.M. led the multi-wavelength comparison and wrote the manuscript. All authors contributed to improving the analysis and the manuscript.

Corresponding authors

Correspondence to He-Shou Zhang, Gabriele Ponti, Ettore Carretti, Ruo-Yu Liu or Mark R. Morris.

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Nature Astronomy thanks Jun Kataoka and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Comparison between the X-ray surface brightness of the eROSITA all-sky map (0.6-1.0 keV) and the magnetic ridges.

Panel (a) presents the magnetic ridges from Fig. 1(b), with four constant latitude cuts at the roots of the X-ray outer halos. The comparisons between the polarized intensity WMAP-K Band (ref. 6) (PIsyn, red) and the 0.6-1.0 keV eROSITA (ref. 5) X-ray emission (green) for the four cuts are presented in panel (b). The detected magnetic ridges are clear peaks of the red curves in (b). At the edges of the X-ray outer halo (highlighted in dotted boxes), the enhancements of the polarized synchrotron intensity (PIsyn,max) are observed with an offset of only a few degrees. While the large-scale magnetic structures surrounding the northern cap of the X-ray outer halo appear to largely enclose the Bubble, the same is not obviously true for the southern halo; there, only the southeastern magnetic ridge approaches the cap of the southern X-ray outer halo.

Extended Data Fig. 2 Comparison between dust and synchrotron polarization.

(a) Magnetic field ridges detected by synchrotron polarization (Bsyn, WMAP at 22.8 GHz). The coherent ridges are enhanced in their polarized synchrotron intensity and connected by the magnetic field lines. (b) Comparison between the polarized emission of thermal dust (PDust, background black bars for direction and filled color for polarized intensity by Planck at 353 GHz from ref. 28) and the magnetic ridges deduced from synchrotron (white lines adapted from panel a). The polarized E-vectors of the dust emission shows a general perpendicular direction to the magnetic field from synchrotron in the Galactic plane (∣b∣ < 5°), while some of the polarized E-vectors of dust emission is parallel to the magnetic field in the known local structure within the Serpens-Aquila Rift. However, most of the magnetic ridges presented in (a) have no dust counterparts, hence they are Galactic structures.

Extended Data Fig. 3 Faraday depolarization by the turbulent Galactic magnetic field out to different distances from the Sun.

The first column is the observed polarized emission at 1.4 GHz (a) and 2.3 GHz (b). Columns 2–5 show the Faraday depolarization fdepol and angle dispersion ΔΦ due to the turbulent Galactic magnetic field, estimated as described in the Methods section. These maps represent the depolarization effects for polarized synchrotron radiations at distances of 1, 3, 5, and 7 kpc from us, shown at frequencies of 1.4 GHz (top two rows) and 2.3 GHz (bottom two rows), respectively. The white color in ΔΦ maps indicates where the dispersion of polarization angles is expected to be more than 45°. At 1.4 GHz frequency, the depolarization screen shows notable growth in latitude between L = 1–5 kpc. The L = 5-kpc case matches well the observed depolarization and the emission at mid and high Galactic latitudes, and therefore the radiation from the magnetic ridges must arise at a distance beyond 5 kpc.

Extended Data Fig. 4 Depolarization of the polarized radio emission ridges and local structures, such as the Fan region and Loop III.

(a) Depolarization analyses (fdepol, ΔΦdepol) at 1.4 GHz; (b) observations for polarized intensity at 1.4 GHz (refs. 25,63) and 22.8 GHz (refs. 6,55). No Faraday depolarization is expected for local emission down to the Galactic disc. At 1.4 GHz, the radio counterpart of the eROSITA Bubbles is depolarized at Galactic latitudes ∣b∣ ≲ 20°, whilst no depolarization is observed for the Fan region or Loop III.

Extended Data Fig. 5 γ − ray diffuse emission intensity maps.

Following Fig. 4a and the calculations in Methods, we calculate the relative excess of the γ − ray flux density compared to patch R0 for γ − ray photons with (a) Eγ ≳ 1 GeV; (b) Eγ ≳ 10 GeV. The background area is selected in the yellow triangle in the northeast (the same as Patch R0 in Fig. 4a). The two lines are the edges of the eROSITA (solid) and Fermi (dashed) Bubbles.

Extended Data Fig. 6 Comparison between the X-ray surface brightness (0.6-1.0 keV) and gamma-ray intensity (Eγ ≳ 100 GeV) at high Galactic latitudes.

Two cuts are considered in the Galactic north for (a): l = + 70°, (b): l = + 65°, with X-ray (red dashed lines) and gamma-ray (blue lines). The all-sky significance map is shown in (c), where the span of cuts are marked. The cut in the Galactic south (d): l = − 60° is considered. Lower latitudes are not considered to avoid the influence of foreground structures or the Fermi Bubbles. The two energy bands have shown enhancements beyond the background within the edges of the X-ray outer halo, and the edges of the enhancements are in agreement with a separation of only a few degrees. The consistencies are observed in the southern cut, but the enhanced plateau is less evident for the southern Bubble.

Extended Data Fig. 7 Power-law fits for observed fluxes in radio and γ − ray bands.

Upper plots fit the radio fluxes with respect to the energy \({F}_{\nu }\propto {E}_{\gamma }^{\,{\alpha }_{r}}\). Lower plots fit the gamma-ray fluxes with respect to the energy for Platz+ (ref. 17) –\(EdN/dE\propto {E}_{\gamma }^{{\alpha }_{Platz}}\), Fermi14yr – \(EdN/dE\propto {E}_{\gamma }^{\,{\alpha }_{F14}}\). The error-bars reported here for the gamma-ray data are based on the statistical uncertainties (see ‘Data and errors for SED’ in the Methods section for details). The reference frequencies are listed in the Supplementary Table 2. The error-bars reported for the radio fluxes are calculated based on the flux density calibration accuracy and beam sensitivity of the corresponding surveys, as defined in the Supplementary. The definitions of the fitting parameters are described ‘Fitting results’ in the Methods section. (a) northeastern outer outflows (R1-R2). αr = − 1.07 ± 0.04, αPl = − 1.499 ± 0.004, and αF14 = − 1.422 ± 0.008. (b) southeastern outer outflows (R3-R4). αr = − 1.14 ± 0.11, αPl = − 1.415 ± 0.006, and αF14 = − 1.344 ± 0.010. (c) southeastern Fermi Bubble cap (R5-R3). αr = − 1.13 ± 0.22, αPl = − 1.239 ± 0.002, and αF14 = − 1.144 ± 0.001.

Extended Data Fig. 8 Active star forming clumps.

(a) an artist’s view of the Galaxy (NASA/JPL-Caltech/R. Hurt) with the active star-forming clumps and their Galactic longitude overlaid. The specific star formation rates are measured from ref. 32 binned by a resolution of 0.5 × 0.5 kpc2, and the clumps with ΣSFR ≳ 0.02 M⊙yr−1kpc−2/bin are considered. (b) The footpoints of the magnetic ridges correspond to the marked clumps with a high star-formation rate on the Galactic plane (from measurements in the Fig. 4 of ref. 32).

Extended Data Fig. 9 Projection effect of the outer halo.

The geometric check for the projection of an open ‘bouquet’ outer halo is presented with four different heights: (a) H = 4 kpc (having an inner vacant cylinder with bottom radius of 3 kpc); (b) H = 7 kpc (with a 2-kpc thickness); (c) H = 10 kpc (with a 2-kpc thickness); (d) H = 100 kpc (with a 2-kpc thickness). We assume that the density in the outer halo is uniform, the temperature and the metallicity in the outer outflow are T=0.3 keV, metallicity is 0.2 solar (refs. 5,30). Their corresponding projections are presented in the middle column. In the right column, cuts at western sky at l = 330° for the three projections (green line) are compared with the observation (red, ref. 5), the ‘P20’ model (blue) and ‘P20 thin’ model (magenta). This figure shows that a ‘bubble-shape’ with cap can be reproduced by an open halo because of the projection effect. The intrinsic 3D structure of the X-ray emitting outer halo cannot be inferred from only a 2D projection.

Extended Data Table 1 Calculations for the outer outflows
Full size table

Supplementary information

Supplementary Information

Supplementary Figs. 1−4, Tables 1 and 2 and Discussion.

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Zhang, HS., Ponti, G., Carretti, E. et al. A magnetized Galactic halo from inner Galaxy outflows. Nat Astron 8, 1416–1428 (2024). https://doi.org/10.1038/s41550-024-02362-0

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