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# An X-ray chimney extending hundreds of parsecs above and below the Galactic Centre

## Abstract

Evidence has mounted in recent decades that outflows of matter and energy from the central few parsecs of our Galaxy have shaped the observed structure of the Milky Way on a variety of larger scales1. On scales of 15 parsecs, the Galactic Centre has bipolar lobes that can be seen in both the X-ray and radio parts of the spectrum2,3, indicating broadly collimated outflows from the centre, directed perpendicular to the Galactic plane. On larger scales, approaching the size of the Galaxy itself, γ-ray observations have revealed the so-called ‘Fermi bubble’ features4, implying that our Galactic Centre has had a period of active energy release leading to the production of relativistic particles that now populate huge cavities on both sides of the Galactic plane. The X-ray maps from the ROSAT all-sky survey show that the edges of these cavities close to the Galactic plane are bright in X-rays4,5,6. At intermediate scales (about 150 parsecs), radio astronomers have observed the Galactic Centre lobe, an apparent bubble of emission seen only at positive Galactic latitudes7,8, but again indicative of energy injection from near the Galactic Centre. Here we report prominent X-ray structures on these intermediate scales (hundreds of parsecs) above and below the plane, which appear to connect the Galactic Centre region to the Fermi bubbles. We propose that these structures, which we term the Galactic Centre ‘chimneys’, constitute exhaust channels through which energy and mass, injected by a quasi-continuous train of episodic events at the Galactic Centre, are transported from the central few parsecs to the base of the Fermi bubbles4.

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## Data and code availability

The datasets analysed during the current study and the software to perform the analysis are available in the XMM-Newton, Chandra and ROSAT repository: http://nxsa.esac.esa.int, https://www.cosmos.esa.int, http://cxc.harvard.edu and http://www.xray.mpe.mpg.de/rosat/archive.

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## Acknowledgements

This work is dedicated to the memory of A. M. Ponti, who contributed to its realization by focusing the energy and motivation of G.P. G.P. thanks A. Merloni for discussions and the Max-Planck-Institut für Astronomie Heidelberg for hospitality. This research has made use of data obtained both with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA, and with the Chandra Data Archive and software provided by the Chandra X-ray Center. G.P. and F. Hofmann acknowledge financial support from the Bundesministerium für Wirtschaft und Technologie/Deutsches Zentrum für Luft- und Raumfahrt (BMWI/DLR; grants FKZ 50 OR 1604, FKZ 50 OR 1715 and FKZ 50 OR 1812) and the Max Planck Society. A.G. and R.T. acknowledge support from CNES and ANR (grant ANR-17-CE31-0014) and M.R.M. acknowledges support from NASA.

### Reviewer information

Nature thanks Masha Chernyakova and the other anonymous reviewer(s) for their contribution to the peer review of this work.

## Author information

Authors

### Contributions

G.P. and M.R.M. conceived and successfully proposed the two XMM-Newton large projects that led to this work. F. Haberl ran and improved the advanced mosaicking pipeline for XMM-Newton data developed by the high-energy group at MPE. F. Hofmann performed the mosaicking of the Chandra data, and implemented the contour-binned spectral extraction technique and advanced smoothing techniques. G.P. and F. Hofmann performed the spectral analysis. E.C. verified all analysis tasks employing an alternative pipeline. G.P., F. Hofmann, E.C. and M.R.M. led the interpretation, modelling and discussion of this work and together they wrote the manuscript. F. Haberl, K.N., R.T., M.C. and A.G. read and commented on the manuscript.

### Corresponding author

Correspondence to G. Ponti.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Diffuse Galactic Centre X-ray emission at different scales.

a, ROSAT (0.9–2-keV energy band) large-scale map of the Galactic Centre. The X-ray counterparts of the Fermi bubbles are strong X-ray emitters. The edges (white ellipses) are clearly detected on scales of several degrees, whereas they become confused (because of the short exposure of 200–300 s and soft X-ray energy band) close to the plane. The red dashed line indicates the XMM-Newton area covered by our survey. b, XMM-Newton map zooming into the central degrees of the Milky Way. The magenta dashed line intersects both chimneys, passing through Sgr A*. The map shows the X-ray emissivity within the 1.5–2.6-keV energy band (see Extended Data Fig. 2). c, Schematic view of the main diffuse X-ray-emitting features within the central 500 pc or so from Sgr A*. The red star and the yellow ellipses indicate the position of Sgr A* and of Sgr A’s ±15-pc bipolar lobes. The large violet ellipses indicate the location and extension of the X-ray counterpart of the Galactic Centre lobe with shell-like morphology (the northern chimney), of its eastern protrusion and of its roughly symmetric southern counterpart (the southern chimney). The orange filled ellipse, the two red circles and the red ellipse indicate the location of the Arc super-bubble (SB), the Quintuplet and Arches clusters and the super-bubble candidate G359.77–0.09 (ref. 8). The pink regions indicate the location of the edges to the Fermi bubbles. The dotted circles and solid ellipses indicate the position of bright X-ray sources with intense dust-scattering halos (DSH) and known supernova remnants (SNR). d, Chandra RGB map zooming into the central tens of parsecs of the Galaxy. The ±15-pc lobes are clearly visible (orange). The dashed circles have radii of 1 pc, 5 pc and 15 pc.

### Extended Data Fig. 2 X-ray emission from the central degrees of the Milky Way.

Bright X-ray emission traces the coherent edge brightened shell-like feature, dubbed the northern chimney, located north of Sgr A* and characterized by a diameter of about 160 pc. On the opposite side, the southern chimney appears as a bright linear feature. Bright X-ray emission is observed at high latitude ($$\left|b\right|$$ 1°), corresponding to the X-ray counterparts of the Fermi bubble. The magenta dashed line intersects both chimneys, passing through Sgr A*. The map shows the X-ray emissivity within the 1.5–2.6-keV energy band. The contours indicate the location of the radio Galactic Centre lobe as it appears in the background-filtered surface brightness map at 10.55 GHz (see ref. 56). Point sources have been removed. Larger circles have been excised to remove the dust-scattering haloes around bright sources.

### Extended Data Fig. 3 Continuum-subtracted S xv line emission.

The map highlights sources characterized by bright soft X-ray emission lines, such as diffuse thermally emitting plasma. On the other hand, point sources and dust-scattering halos are efficiently removed. The white dashed tilted line indicates a linear ridge, about 450 pc long, that appears to run west of the chimneys.

### Extended Data Fig. 4 Chandra 1–2 keV map of the Galactic Centre.

The same structures seen in the XMM-Newton 1.5–2.6-keV image (Extended Data Fig. 2, footprint indicated by red dashed box) are observed also in the shallower Chandra image. Although the 1–2-keV band map is more affected by neutral absorption towards the plane compared with the 1.5–2.6-keV band (Extended Data Fig. 2), we display it here in order to emphasize the residual high-latitude emission (for example, inside the Fermi bubbles). GX 3+1 is an accreting X-ray binary.

### Extended Data Fig. 5 Chandra spectra (with relative extraction regions) used to derive the physical properties of the chimneys.

a, b, Chandra spectra used to derive the latitudinal profiles at l = 0° and shown in Fig. 2 for the southern (a) and northern (b) sides, respectively. c, XMM-Newton 1.5–2.6 keV emission map in greyscale, showing the extraction areas of the Chandra spectra used to derive the latitudinal profiles at l = 0°, with colours corresponding to those of the spectra. The areas at l ≈ −0.7° indicate the extraction regions of the spectra used to derive the grey constraints in Fig. 2. d, e, XMM-Newton spectra used to derive the latitudinal profiles of the diffuse emission in Fig. 2. The error bars represent 1σ.

### Extended Data Fig. 6 XMM-Newton spectra (with relative extraction regions) used to derive the physical properties of the chimneys.

b, c, XMM-Newton spectra used to measure the longitudinal profiles shown in Extended Data Fig. 7. a, XMM-Newton 1.5–2.6-keV map displaying the regions used to extract the spectra corresponding to a longitudinal cut through the northern shell (at b ≈ 0.7°) and through the jet-like feature (at b ≈ −1.2°). The error bars represent 1σ.

### Extended Data Fig. 7 Longitudinal profiles of the physical properties of the chimneys.

From top to bottom, temperature, density and pressure profiles are shown as a function of longitude cutting through the northern and southern chimneys at latitudes b ≈ 0.7° and b ≈ −1.2° (see Extended Data Fig. 6). Positive values indicate Galactic west (‘west’ is ‘right’ as in an image in Galactic coordinates b and l). Black squares and red circles indicate the longitudinal cut through the northern and southern chimneys, respectively. The densities have been estimated assuming a volume V = (PA)3/2, where PA is the projected area. Vertical error bars represent 1σ uncertainties; horizontal error bars show the radial range of the extraction regions.

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Ponti, G., Hofmann, F., Churazov, E. et al. An X-ray chimney extending hundreds of parsecs above and below the Galactic Centre. Nature 567, 347–350 (2019). https://doi.org/10.1038/s41586-019-1009-6

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• DOI: https://doi.org/10.1038/s41586-019-1009-6

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