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A disk-dominated and clumpy circumgalactic medium of the Milky Way seen in X-ray emission


The Milky Way galaxy is surrounded by a circumgalactic medium1 that may play a key role in galaxy evolution as the source of gas for star formation and a repository of metals and energy produced by star formation and nuclear activity2. The circumgalactic medium may also be a repository for baryons seen in the early universe, but undetected locally3. The circumgalactic medium has an ionized component at temperatures near 2 × 106 K studied primarily in the soft-X-ray band4,5. Here we report a survey of the southern Galactic sky with a soft-X-ray spectrometer optimized to study diffuse soft-X-ray emission6. The X-ray emission is best fitted with a disk-like model based on the radial profile of the surface density of molecular hydrogen, a tracer of star formation, suggesting that the X-ray emission is predominantly from hot plasma produced via stellar feedback. Strong variations in the X-ray emission on angular scales of ~10° indicate that the circumgalactic medium is clumpy. Addition of an extended, and possibly massive, halo component is needed to match the halo density inferred from other observations7,8,9.

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Fig. 1: Polar plots in Galactic coordinates of the EM and kT of the HaloSat fields with b < −30°.
Fig. 2: Model fits to the EM data.
Fig. 3: Autocorrelation of the deviations from the best-fit empirical disk-like model.
Fig. 4: Combined disk and halo model fit.

Data availability

The OMNI data are available at and the ACE data at The first year of HaloSat data are available at NASA’s HEASARC. The additional HaloSat data analysed in this study are available on request to P.K. The spectral fitting results will be made available for download at the CDS and on request to P.K.

Code availability

The computer codes used to analyse the data in this study are available from the corresponding author on request.


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We acknowledge support from NASA grant NNX15AU57G and the Iowa Space Grant consortium. D.K.’s modelling work was supported by CNES and performed with the High Performance Computer and Visualisation (HPCaVe) platform hosted by UPMC-Sorbonne Université. We thank J. Raines and the SWICS team for providing the solar-wind data.

Author information

Authors and Affiliations



P.K. carried out the X-ray data analysis and wrote the text; D.K. carried out the heliospheric SWCX modelling; D.M.L. wrote the code to calculate the absorption column densities and the LHB emission measures; D.K., K.D.K., E.H.-K., K.J., D.M.L., A.Z., R.R., J.B. and H.G. read and commented on the manuscript.

Corresponding author

Correspondence to P. Kaaret.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Randall Smith and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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

Extended data

Extended Data Fig. 1 HaloSat X-ray spectra.

X-ray spectra of the HaloSat field at (l = 122.616, b = − 55.418). Data from the three detectors are shown, detector 14 in black, 54 in orange, and 38 in blue. Errors indicate 1-σ confidence intervals. The average exposure per detector is 28 ks. The best fitted model and the powerlaw used to model the instrumental background is shown in the same colour for each detector. The model components are shown as black lines. At 0.6 keV, the highest is the sum of the astrophysical components (all components except the instrumental background), the thermal plasma for the halo, the instrumental background, the cosmic X-ray background, the Ovii oxygen line, the thermal plasma for local hot bubble, and the Oviii oxygen line.

Extended Data Fig. 2 HaloSat instrumental background model.

The photon index of the instrumental background for each detector is calculated from the count rate in the 3-7 keV band using the slope and intercept values in the table in the equation: Photon index = slope × (hard rate - hr0) + intercept, where hr0 = 0.05 c/s.

Extended Data Fig. 3 Heliospheric Solar wind charge exchange.

Illustration of the sum used to calculate the line flux due to heliospheric solar wind charge exchange (SWCX) along a line of sight. The Sun is at the origin and the figure is limited to the first 2 AU. Ecliptic longitude is zero along the horizontal axis to the right and increases anti-clockwise. Each triangle represents a term in the sum at the radial distance form the Sun (Dj) with the solar wind intensity evaluated for the time of flight Tj from a radius of 1 AU. The inset shows the H and He neutral densities scaled by the step size (ds) and by \({(1{\rm{AU}}/{D}_{j})}^{2}\) as a function of radial distance from the Sun for the full radial scale of the simulations. The density profiles were calculated for the Earth at 126.56 ecliptic longitude and a look direction of (λ = 39.88, β = − 7.64) that crosses the He-focusing cone and the H-ionization cavity resulting in a high oxygen line flux and a high He/H neutral density ratio.

Extended Data Fig. 4 Density Model Fit Results.

Results of fitting the EM data to various density profiles. The first column specifies the model, model parameters, and goodness of fit statistics. Results from fitting using a patchiness parameter are given in the second column and using a Huber loss function in the third column. The Fit statistic is either χ2 with the patchiness parameter included or the value of the Huber loss function.

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Kaaret, P., Koutroumpa, D., Kuntz, K.D. et al. A disk-dominated and clumpy circumgalactic medium of the Milky Way seen in X-ray emission. Nat Astron 4, 1072–1077 (2020).

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