Fig.1: Left: X-ray (top) and radio (bottom) images of the sky in galactic coordinates, showing prominent features corresponding to the horizontal galactic disk and vertical extensions above and below the disk. Right: zoomed images, showing the central part of the galaxy. The northern side, dominated by the North Polar Spur, appears much brighter and has a more complicated morphology than the shell-like southern bubble. Credit: X-ray: Predehl et al. 2020, Sunyaev et al. 2020; Radio: Haslam et al. 1982
The largest features visible in the sky are from our home galaxy, the Milky Way. In the optical spectrum, this is the disc of our galaxy, which is prominent across the entire electromagnetic spectrum, from radio waves to ultra-high-energy gamma rays. In the X-ray sky, the second-largest feature is the so-called 'eROSITA bubbles', giant structures above and below the galactic disk that span almost half the sky. These were mapped by the eROSITA X-ray telescope on board the SRG Observatory. Recent studies suggest that these 'bubbles' consist of overlapping heterogeneous structures (different in the north and south), which are partly driven by the release of energy at the centre of our galaxy and partly powered by outflows from star-forming regions in the disk. Although separating these structures is difficult, it is well worth the effort: we can learn about transient outbursts in our galactic centre and measure the properties of low-density gas far from the disk.
The eROSITA bubbles are arc-like structures above the disc of the galaxy that are visible in eROSITA images in the general direction of the galactic centre (see Fig. 1 and these press releases from 2020 and 2021). Direct measurements of the distance to these structures are problematic, but the hypothesis that they are associated with the centre of the Milky Way is attractive, placing the bubbles some 30 thousand light-years away from us. According to this hypothesis, the energy required to create the bubbles originates from a powerful energy release in the galactic centre, possibly from the supermassive black hole Sgr A* or an outburst of star formation in close proximity to the Galactic Centre. A similar hypothesis was first proposed decades ago to explain a bright feature in the radio band (the North Polar Spur, or NPS), and was later extended to include hourglass-shaped structures visible in gamma rays (the Fermi Bubbles).
The eROSITA bubbles fit into this picture as the outer edge of a shock wave propagating through the gas of our galaxy, compressing and heating it (see Press Release 2021). However, the Northern and Southern bubbles differ in morphology and brightness, suggesting the presence of additional effects affecting their appearance.
North is North
Two recent studies argue that, in addition to the shock wave, plumes of hot, metal-enriched gas rise buoyantly from the most active star-forming regions in the galactic disk and contribute to X-ray emission in the northern hemisphere. This makes the northern eROSITA bubble much more prominent than the southern one.
The physics of plume formation is well understood in principle. Supernova explosions from the most massive stars in a star-forming region release energy and produce hot, metal-enriched gas. Initially confined by dense, cold ambient matter, the hot gas breaks through and forms a plume that rises into the galactic atmosphere, away from the disk. As the break is more likely to occur on one side of the region (above or below the galactic disk), a one-sided plume is expected. The differential rotation of the gas above the disk gives the plume a shape reminiscent of smoke rising from a steam locomotive. This plume is hypothesised to account for the remarkable brightness and complex structures of the Northern Bubble. The W43 complex, one of the most active star-forming regions in the Milky Way, lies directly beneath the NPS. In this model, it is the most plausible source of the plume.
The physics of plume formation is well understood in principle. Supernova explosions from the most massive stars in a star-forming region release energy and produce hot, metal-enriched gas. Initially confined by dense, cold ambient matter, the hot gas breaks through and forms a plume that rises into the galactic atmosphere, away from the disk. As the break is more likely to occur on one side of the region (above or below the galactic disk), a one-sided plume is expected. The differential rotation of the gas above the disk gives the plume a shape reminiscent of smoke rising from a steam locomotive. This plume is hypothesised to account for the remarkable brightness and complex structures of the Northern Bubble. The W43 complex, one of the most active star-forming regions in the Milky Way, lies directly beneath the NPS. In this model, it is the most plausible source of the plume.
Fig. 2: Detailed image of the northern and southern eROSITA bubbles in the 0.7–1.05 keV band (in stereographic projection). At these energies, nearby hot gas contributes little, providing a clearer view of the eROSITA bubbles, which consist of denser, hotter gas. The bottom-right plot shows the X-ray brightness profile across the southern eROSITA bubble radial profile compared with the shell model. The top-right compares the observed X-ray spectrum of the southern bubble shell with predictions from an expanding spherical shock (red line). These data provide us with information on the density, temperature, and heavy element abundance in the gas some 30 000 light years away from the Sun. Adopted from Churazov et al. (2026) .
Fig. 3: On the left, the simplest geometrical model of the shells associated with propagating shocks. From the Sun's position, only a small portion of the putative spherical shell can be seen; the rest is obscured and contaminated by emission from the galactic disk. The green box marks the Sun's position. Right: the X-ray map of the entire sky from the MAXI telescope, with regions affected by the propagating shock in the current scenario delineated. In the simplest model, similar shell-like structures should appear below and above the disk. Recent models postulate that a 'metal-rich plume' rises buoyantly from a star-forming region in the disc, sitting atop these structures. This plume corresponds to NPS in the radio maps and a bright X-ray arc. The plume dominates the northern sky, with the 'proper' northern eROSITA bubble (similar to the southern bubble) barely visible as a faint extension to the left of the plume. Adopted from Churazov et al. (2026) - MAXI .
In contrast, the southern eROSITA bubble, with its much simpler morphology, appears to be consistent with a shockwave driven by a transient energy release at the centre of the Milky Way. Its shell-like appearance (Fig. 2) is reminiscent of a supernova remnant, albeit 100–1000 times larger, and consistent with expectations for a shock driven by a 5–10 Myr-old explosion that has already reached distances of the order of 20,000 light-years from the galactic centre. This makes its boundary appear almost at zenith in the galactic coordinate system. This hypothesis can be further tested by measuring the X-ray spectrum of the southern bubble shell, which reasonably supports the shock scenario.
Interpreting the southern eROSITA bubble as a shock wave propagating through tenuous gas high above the galactic plane is an attractive way to measure its density, temperature, and metal abundance. This is usually challenging because it is difficult to separate the emission from this gas from the contributions of much closer, denser gas. However, the shock compresses and heats this gas, making it suddenly 'visible' (see Fig. 3) as a brighter shell surrounded by a dimmer background. The density and metal abundance can be determined through X-ray spectral analysis.
The gas through which the shockwave is propagating is around 200 times hotter than the interstellar medium in the disk, has a density that is several thousand times lower, and has a 5–10 times lower abundance of heavy elements, such as oxygen and iron. Thus, the eROSITA bubbles provide an in situ probe of Galactic gas located far from us (see Fig. 3).
If the southern eROSITA bubble is indeed a 'proper' shock, there may be a similar structure in the northern sky. Unlike the plume discussed above, which can be highly asymmetric, the massive energy release in the galactic centre should produce comparable bubbles above and below the disk. Such structures can be identified in the northern bubble, just outside the much brighter NPS plume (Figures 1 and 3). It is comparable in size and brightness to the southern bubble, but the bright plume outshines it in the overlapping regions.
In future, high-resolution X-ray spectroscopy could provide the most direct probe of our current understanding of the properties and origin of the eROSITA bubbles by measuring line velocities and broadening at different positions across the bubbles with sensitive X-ray microcalorimeters. This could be achieved by combining a large grasp with high spectral resolution, such as the recently proposed Line Emission Mapper mission. In particular, a double-horn profile of emission lines with a separation of approximately 2 eV would strongly suggest the presence of an expanding shell in the southern bubble. This would place tight constraints on the age and energetics of the past energy release in the galactic centre.
In contrast, the southern eROSITA bubble, with its much simpler morphology, appears to be consistent with a shockwave driven by a transient energy release at the centre of the Milky Way. Its shell-like appearance (Fig. 2) is reminiscent of a supernova remnant, albeit 100–1000 times larger, and consistent with expectations for a shock driven by a 5–10 Myr-old explosion that has already reached distances of the order of 20,000 light-years from the galactic centre. This makes its boundary appear almost at zenith in the galactic coordinate system. This hypothesis can be further tested by measuring the X-ray spectrum of the southern bubble shell, which reasonably supports the shock scenario.
Interpreting the southern eROSITA bubble as a shock wave propagating through tenuous gas high above the galactic plane is an attractive way to measure its density, temperature, and metal abundance. This is usually challenging because it is difficult to separate the emission from this gas from the contributions of much closer, denser gas. However, the shock compresses and heats this gas, making it suddenly 'visible' (see Fig. 3) as a brighter shell surrounded by a dimmer background. The density and metal abundance can be determined through X-ray spectral analysis.
The gas through which the shockwave is propagating is around 200 times hotter than the interstellar medium in the disk, has a density that is several thousand times lower, and has a 5–10 times lower abundance of heavy elements, such as oxygen and iron. Thus, the eROSITA bubbles provide an in situ probe of Galactic gas located far from us (see Fig. 3).
If the southern eROSITA bubble is indeed a 'proper' shock, there may be a similar structure in the northern sky. Unlike the plume discussed above, which can be highly asymmetric, the massive energy release in the galactic centre should produce comparable bubbles above and below the disk. Such structures can be identified in the northern bubble, just outside the much brighter NPS plume (Figures 1 and 3). It is comparable in size and brightness to the southern bubble, but the bright plume outshines it in the overlapping regions.
In future, high-resolution X-ray spectroscopy could provide the most direct probe of our current understanding of the properties and origin of the eROSITA bubbles by measuring line velocities and broadening at different positions across the bubbles with sensitive X-ray microcalorimeters. This could be achieved by combining a large grasp with high spectral resolution, such as the recently proposed Line Emission Mapper mission. In particular, a double-horn profile of emission lines with a separation of approximately 2 eV would strongly suggest the presence of an expanding shell in the southern bubble. This would place tight constraints on the age and energetics of the past energy release in the galactic centre.
Authors:
Eugene Churazo
Scientific Staff
Tel.: 2219
Email: echurazov@mpa-garching.mpg.de
Ildar Khabibullin
Guest
Email: ildar@MPA-Garching.MPG.DE
Rashid Sunyaev
Emeritus Director
Tel.: 2244
Email: rsunyaev@mpa-garching.mpg.de
Original publications:
1. Churazov E., Khabibullin I. I., Bykov A. M., Chugai N. N., Sunyaev R. A., Utrobin V. P., Zinchenko I. I.
Southern eROSITA bubble as a forward shock and the low-metallicity CGM. South-east side story
A&A (2026)
DOI
2. Churazov E., Khabibullin I. I., Bykov A. M., Chugai N. N., Sunyaev R. A., Utrobin V. P., Zinchenko I. I.
North Polar Spur: Gaseous plume(s) from star-forming regions ∼3─5 kpc from the Galactic Center?
A&A (2024)
DOI
3. Kraft R., et al.
(2022)
Source
4. Khabibullin, I., et al.
LEM All-Sky Survey: Soft X-ray Sky at Microcalorimeter Resolution
(2023)
Source
