eROSITA Bubbles: Galactic X-Ray Outflows

• eROSITA Bubbles are giant, bipolar soft X-ray structures centered on the Galactic Center, highlighting large-scale nuclear feedback mechanisms.
• They feature shell-like morphologies with multiple thermal components and shock diagnostics from X-ray, radio, and γ-ray data.
• Current models debate their origin—from continuous winds to episodic AGN jets—underscoring their role as laboratories for Galactic outflow studies.

The eROSITA Bubbles are giant bipolar soft X-ray structures centered on the Galactic Center, extending to very high Galactic latitudes and generally interpreted as a large-scale manifestation of Milky Way nuclear feedback. In the discovery picture, they are shell-like, thermal X-ray structures that surround the smaller Fermi Bubbles and include the North Polar Spur/Loop I as the bright northern rim, while the southern counterpart completes the bipolar system (Predehl et al., 2020). Subsequent work has treated them as a key laboratory for Galactic outflows, shock physics, halo thermodynamics, and the relation between thermal X-ray shells and nonthermal radio, $\gamma$-ray, and possibly neutrino or cosmic-ray signatures (Sarkar, 2024, Keshet et al., 20 Jan 2026).

1. Discovery, nomenclature, and large-scale morphology

The first clear all-sky identification of the eROSITA Bubbles came from the first six months of the Spektr-RG/eROSITA all-sky survey. In that work, the bubbles were described as soft-X-ray-emitting structures extending approximately $14$ kiloparsecs above and below the Galactic Center, with shell-like morphology rather than the more volume-filling appearance of the Fermi Bubbles (Predehl et al., 2020). The most useful discovery band was $0.6$–$1.0$ keV, where the eROSITA effective area is high and the emission from a $\sim 0.3$ keV plasma peaks (Predehl et al., 2020).

The northern bubble incorporates the North Polar Spur/Loop I complex as its brightest part, while the southern bubble appears as a much fainter analogous annulus. The overall morphology is close to bilateral symmetry about the Galactic Center and the Galactic plane, which was one of the main arguments against interpreting the northern arc as merely an isolated nearby supernova remnant (Predehl et al., 2020). In later review language, the eROSITA Bubbles were summarized as $\sim 12\!-\!14$ kpc soft X-ray shells surrounding the $\sim 8\!-\!10$ kpc Fermi Bubbles, with the nested geometry regarded as one of the strongest empirical clues to a common outflow system (Sarkar, 2024).

The terminology is not fully uniform across the literature. Some authors abbreviate the eROSITA or ROSAT/eROSITA Bubbles as the “RBs” and emphasize that they are the larger, older analogs of the Fermi Bubbles, extending to $|b|\sim 80^\circ$ while the Fermi Bubbles reach only $|b|\sim 50^\circ$ (Keshet et al., 20 Jan 2026). The term “eRObub” also appears in work focused on the western Galactic hemisphere (Yeung et al., 4 May 2026). Despite these naming differences, the referent is the same: a giant bipolar X-ray structure, larger than the Fermi Bubbles and plausibly rooted in past Galactic-center activity.

2. Distance scale, geometry, and relation to Loop I and the North Polar Spur

One of the longest-running controversies concerns distance. Competing views have treated the eROSITA Bubbles either as a $\sim 10$ kpc-scale Galactic-center structure or as a $14$0 pc-scale local feature fortuitously projected toward the Galactic Center direction (Liu et al., 2024). Morphological work based on 3D dust reconstructions identified three isolated clouds, G19+18, G33+25, and G308+21, whose projected morphologies match X-ray shadows on the North Polar Spur and the Lotus Petal Cloud. In that analysis, the Lotus Petal Cloud was assigned a robust lower limit of $14$1 pc and the North Polar Spur root a robust lower limit of $14$2 pc, leading to the conclusion that the northern eROSITA Bubble is most plausibly a giant, distant structure rooted at the Galactic Center (Liu et al., 2024).

The same study argued that the North Polar Spur and the Lotus Petal Cloud are not two independent distant features but parts of a single giant bubble, and that the border can be fit by the line-of-sight tangent of a 3D skewed cup model rooted in the Galactic Center (Liu et al., 2024). This does not eliminate local foreground complexity, but it moves the dominant northern X-ray structure well beyond the immediate Solar neighborhood.

A complementary western-hemisphere analysis fit the eROSITA Bubble morphology with a parameterized blast-wave geometry and found that the horizontal size of both bubbles is comparatively well constrained, with semi-minor axis $14$3 kpc, whereas the vertical extent is poorly constrained because the observed X-ray emission is nearly insensitive to whether a bubble cap exists and where it lies (Yeung et al., 4 May 2026). In that same work, reproducing the projected northern bubble required a tilt of $14$4 toward $14$5, while the southern bubble required little tilt (Yeung et al., 4 May 2026). This suggests that the projected north-south asymmetry is not solely a matter of brightness but also of 3D orientation or environmental asymmetry.

The relation to Loop I and the North Polar Spur remains nontrivial. The discovery paper argued that Loop I is not an isolated local arc but part of a much larger bipolar Galactic structure (Predehl et al., 2020). However, newer spectroscopy found that the North Polar Spur exhibits higher abundances, $14$6, than the western eROSITA Bubble interior, which has $14$7; taken at face value, that abundance contrast disfavors a simple common origin for the North Polar Spur and the western eROSITA Bubble gas (Yeung et al., 4 May 2026). This does not remove the morphological connection, but it sharpens the possibility of superposed components.

3. Thermal structure, surface brightness, and shell physics

The initial eROSITA interpretation emphasized a shell of shock-heated gas with characteristic $14$8 keV, contrasted with an ambient halo near $14$9 keV. Using that temperature jump, the discovery paper inferred a shock Mach number $0.6$0, a shock speed $0.6$1, an expansion time $0.6$2, and total energy in the high-$0.6$3 to low-$0.6$4 erg range (Predehl et al., 2020). Under a shell model with inner radius $0.6$5 kpc and outer radius $0.6$6 kpc, it estimated $0.6$7 and thermal energy $0.6$8 per bubble (Predehl et al., 2020).

Later Suzaku spectroscopy argued that this single-temperature picture is incomplete. In that work, the shell and ambient medium were both better described by a two-temperature thermal model. In the bubble/shell region, the warm-hot component had mean $0.6$9 keV and the hot component mean $1.0$0 keV; in the extended halo, the corresponding means were $1.0$1 keV and $1.0$2 keV (Gupta et al., 2022). The temperatures are therefore similar inside and outside the bubbles, whereas the emission measures are significantly higher in the shell, by factors of about $1.0$3 for the warm-hot component and $1.0$4 for the hot component (Gupta et al., 2022). The explicit conclusion was that the shells are X-ray bright because they trace denser gas, not because they are hotter, and that the combination of high compression with similar pre- and post-shock temperatures rules out an adiabatic-shock interpretation (Gupta et al., 2022).

The same Suzaku analysis also reported non-solar abundance ratios, especially enhanced Ne/O along 10 bubble sightlines and enhanced Mg/O along one sightline, which it interpreted as favoring stellar feedback models over a simple AGN-wind origin (Gupta et al., 2022). This chemical argument is distinct from the geometry or temperature arguments and remains part of the origin debate.

A later eROSITA spectral study of the western Galactic hemisphere again found multiple thermal components. The western eROSITA Bubble interior was best characterized by a hotter component at $1.0$5 keV and a colder one at $1.0$6 keV, with the colder component’s emission measure about five times higher on average (Yeung et al., 4 May 2026). That work found sub-solar abundances, $1.0$7, no conclusive evidence for $1.0$8-element enhancement, and spectrally confirmed an apparent cool shell at $1.0$9–$\sim 0.3$0 keV surrounding the northern bubble (Yeung et al., 4 May 2026). Taken together, these studies indicate that the eROSITA Bubbles are not well described by a single uniform shell temperature.

4. Shock diagnostics and nonthermal edges

A major recent development was the use of eROSITA-defined X-ray edges as geometric priors for stacking radio and $\sim 0.3$1-ray data. In the high-latitude tip analysis, the $\sim 0.3$2–$\sim 0.3$3 keV eROSITA map was processed with a first-order directional Gaussian-derivative edge detector, and radio and Fermi-LAT data were then stacked parallel to the detected edges to recover weak nonthermal signals (Keshet et al., 20 Jan 2026). This analysis reported $\sim 0.3$4 detections of both bubble tips in radio and $\sim 0.3$5-rays overall, including the first secure nonthermal detection of the southern bubble tip in radio (Keshet et al., 20 Jan 2026).

The radio spectrum was used as a shock diagnostic through the standard synchrotron/DSA relation

$\sim 0.3$6

where $\sim 0.3$7 and $\sim 0.3$8 is the shock Mach number (Keshet et al., 20 Jan 2026). For the northern bubble tip, the measured slope was $\sim 0.3$9, implying $\sim 12\!-\!14$0. For the southern tip, the nominal result was $\sim 12\!-\!14$1, implying $\sim 12\!-\!14$2. Under alternative foreground treatments, the inferred Mach numbers can be even higher, so the paper’s conservative summary was that the edges are forward shocks with $\sim 12\!-\!14$3 (Keshet et al., 20 Jan 2026).

These radio-derived Mach numbers are substantially higher than the $\sim 12\!-\!14$4 estimate inferred earlier from simple X-ray temperature jumps (Predehl et al., 2020, Keshet et al., 20 Jan 2026). The 2026 study interpreted the hard radio slopes as incompatible with weak shocks and argued that cooling is negligible at the observed frequencies if the bubbles are not much older than a few $\sim 12\!-\!14$5 Myr (Keshet et al., 20 Jan 2026). It also emphasized a factor $\sim 12\!-\!14$6 jump in stacked X-ray brightness across the edge, supporting a strong shock qualitatively even without a formal Rankine–Hugoniot conversion (Keshet et al., 20 Jan 2026).

A pronounced north–south asymmetry accompanies these detections. The southern bubble is nearly an order of magnitude fainter in radio, and the northern shell is brighter in X-rays by a factor of $\sim 12\!-\!14$7–$\sim 12\!-\!14$8. Interpreting the thermal X-ray emissivity as approximately density-squared, the 2026 analysis inferred that the northern bubble propagates into a medium denser by about $\sim 12\!-\!14$9, so the southern shock expands into a halo of roughly half the density (Keshet et al., 20 Jan 2026). The paper further argued that the eROSITA Bubbles are older, evolved counterparts of the Fermi Bubbles, arising from an earlier collimated Galactic-center outburst (Keshet et al., 20 Jan 2026).

A region-specific southeastern study pushed the shock interpretation further. Treating the southeastern arc as a forward shock, it inferred $\sim 8\!-\!10$0–$\sim 8\!-\!10$1 kpc, $\sim 8\!-\!10$2–$\sim 8\!-\!10$3 Myr, current velocity $\sim 8\!-\!10$4, upstream density $\sim 8\!-\!10$5, and metallicity $\sim 8\!-\!10$6, emphasizing that these are effectively in situ constraints on the circumgalactic medium at $\sim 8\!-\!10$7–$\sim 8\!-\!10$8 kpc (Churazov et al., 21 Mar 2026). That interpretation is local to the southeastern sector rather than a full-bubble solution, but it shows how a cleaner shell segment can yield stronger dynamical constraints.

5. Formation scenarios

No single formation scenario has displaced all others, and the literature contains several distinct models.

In continuous-wind interpretations, the eROSITA Bubbles trace the forward-shocked circumgalactic medium around a long-lived Galactic nuclear outflow. A recent review emphasized that a weak-shock, long-lived wind with $\sim 8\!-\!10$9, $|b|\sim 80^\circ$0, and $|b|\sim 80^\circ$1 can match the eROSITA shell and naturally place the Fermi Bubbles at the contact discontinuity inside it (Sarkar, 2024). In that picture, elevated Central Molecular Zone star formation over the last $|b|\sim 80^\circ$2 Myr would be energetically relevant (Sarkar, 2024).

Single-event AGN jet models remain prominent. One 3D hydrodynamic-plus-cosmic-ray model proposed a brief bipolar jet episode from Sgr A* $|b|\sim 80^\circ$3 Myr ago, lasting $|b|\sim 80^\circ$4 Myr, with total injected energy $|b|\sim 80^\circ$5 erg, in which the eROSITA Bubbles are the thermal forward shock and the Fermi Bubbles are the contact discontinuity / CR-filled cocoon (Yang et al., 2022). A related relativistic-hydrodynamic study showed that even jets tilted by $|b|\sim 80^\circ$6 can still yield symmetric eROSITA and Fermi Bubbles if a dense Galactic disk disrupts jet collimation into “failed jets,” after which buoyancy in the stratified halo renders the bubbles vertical; in that model the eROSITA edge again corresponds to a forward shock (Tseng et al., 2024).

Multiple-episode models take the nested geometry literally. In a double-episode jet scenario, the first jet pair launched $|b|\sim 80^\circ$7 Myr ago produces the eROSITA Bubbles, now extending to $|b|\sim 80^\circ$8 kpc with gas temperatures of $|b|\sim 80^\circ$9 keV, while a second jet pair launched $|b|\sim 50^\circ$0 Myr ago produces the Fermi Bubbles, now reaching $|b|\sim 50^\circ$1 kpc with temperatures $|b|\sim 50^\circ$2–$|b|\sim 50^\circ$3 keV; in that interpretation, the sharp edges of both systems are forward shocks (Zhang et al., 18 Jul 2025).

Repeated tidal disruption events have also been proposed as the engine. In that framework, TDEs recurring every $|b|\sim 50^\circ$4 kyr over $|b|\sim 50^\circ$5 Myr, with mean power $|b|\sim 50^\circ$6, can inflate superbubbles whose forward shock matches the eROSITA morphology in a $|b|\sim 50^\circ$7-model halo, while the Fermi $|b|\sim 50^\circ$8-rays would arise near the contact discontinuity where cosmic rays are accelerated in situ (Scheffler et al., 30 Jan 2025).

The observed asymmetry has motivated an environmental explanation. A hydrodynamic study argued that a circumgalactic medium wind of order $|b|\sim 50^\circ$9, blowing from roughly east by north, can explain the westward distortion and eastern brightening of the northern bubble together with the much fainter southern bubble. In that picture the wind preconditions the halo before the Galactic-center outflow begins (Mou et al., 2022). This does not directly solve the age or driver question, but it addresses the pronounced north–south and east–west asymmetries.

6. Timescales, high-energy extensions, and unresolved issues

Age estimates remain model-dependent. The original eROSITA interpretation yielded $\sim 10$0 Myr from weak-shock kinematics (Predehl et al., 2020). A Rayleigh–Taylor analysis argued that the smooth outer rims imply the shells have not yet undergone disruptive RT fragmentation and favored a current age of about $\sim 10$1 Myr in a $\sim 10$2-model Galactic halo, while ruling out a simple exponential halo because that would fragment the shell too early (Schulreich et al., 2021). By contrast, the southeastern forward-shock solution gave $\sim 10$3–$\sim 10$4 Myr for that cleaner sector (Churazov et al., 21 Mar 2026), and the high-latitude radio slopes were explicitly interpreted as requiring that the bubbles are not much older than a few $\sim 10$5 Myr if synchrotron cooling is to remain negligible (Keshet et al., 20 Jan 2026). A plausible implication is that “the age of the eROSITA Bubbles” is not yet a single settled number but a quantity entangled with geometry, sector selection, and emission model.

Possible high-energy counterparts remain suggestive rather than definitive. A study of AMS-02 positron data argued that a $\sim 10$6 GeV feature, if interpreted as an inner-Galaxy burst signature, corresponds to an age $\sim 10$7–$\sim 10$8 Myr and could be associated with the Fermi Bubbles, substructures within them, or the eROSITA Bubbles; the authors were explicit that the eROSITA identification is speculative rather than established (Cholis et al., 2022). An IceCube analysis reported significant neutrino detections for the Fermi Bubbles and only preliminary evidence for the eROSITA Bubble shells, with the northeastern shell sector showing a hadronic-model preference at $\sim 10$9 and the southern shell remaining insufficient for modeling (Keshet et al., 21 Jun 2026). A Galactic cosmic-ray transport model further proposed that the eROSITA outflow acts as an advective escape boundary, with an effective local inner halo boundary of $14$00 kpc and outflow speed $14$01, offering a transport-based explanation for the $14$02 GV hardening (Schroer, 2 Apr 2026).

Several issues remain unresolved. The physical relation between the eROSITA Bubbles and the North Polar Spur/Loop I is still complicated by possible local or chemically distinct superposed components (Yeung et al., 4 May 2026). The thermodynamic interpretation of the shell ranges from weak-shock, $14$03 keV forward shocks (Predehl et al., 2020) to denser-but-not-hotter shells that do not trace adiabatic shocks (Gupta et al., 2022) to strong $14$04–5 forward shocks inferred from radio spectra at the high-latitude tips (Keshet et al., 20 Jan 2026). The origin of the outflow remains divided among nuclear star formation, short AGN jet episodes, repeated TDEs, and multi-episode jet histories (Sarkar, 2024, Yang et al., 2022, Scheffler et al., 30 Jan 2025, Zhang et al., 18 Jul 2025). The present state of the field therefore supports a Galactic-scale, bipolar, shock-related structure centered on the Milky Way nucleus, while leaving the exact driving history, 3D geometry, and degree of physical unity with neighboring large-scale structures as active research questions.