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The SRG/eROSITA diffuse soft X-ray background II. spectra and morphology of the eROSITA bubbles in the western Galactic hemisphere

This study utilizes SRG/eROSITA spectral and morphological data to characterize the western eROSITA bubbles as two-component hot gas structures with sub-solar abundances and a tilted geometry, while distinguishing them from the North Polar Spur and confirming their lack of correlation with Fermi Bubbles.

Original authors: Michael C. H. Yeung, Martin G. F. Mayer, Andy Strong, Michael J. Freyberg, Gabriele Ponti, Konrad Dennerl, Junjie Mao, Manami Sasaki, Xueying Zheng, Jeremy S. Sanders, Yi Zhang, Jiejia Liu, Liyi Gu, W

Published 2026-05-06

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CC BY 4.0

Original authors: Michael C. H. Yeung, Martin G. F. Mayer, Andy Strong, Michael J. Freyberg, Gabriele Ponti, Konrad Dennerl, Junjie Mao, Manami Sasaki, Xueying Zheng, Jeremy S. Sanders, Yi Zhang, Jiejia Liu, Liyi Gu, Werner Becker, Frank Haberl, Teng Liu, Andrea Merloni, Peter Predehl

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine our Milky Way galaxy as a giant, swirling city of stars. For a long time, astronomers thought this city was relatively quiet. But in 2020, a new space telescope named eROSITA (part of a satellite called SRG) discovered two massive, ghostly bubbles of hot gas erupting from the very center of our galactic city. These are the eROSITA bubbles.

This paper is like a detailed detective report on the western half of these bubbles. The scientists used the telescope to take "X-ray photos" and "spectral fingerprints" of the gas inside and around these bubbles to figure out what they are made of, how hot they are, and where they came from.

Here is a breakdown of their findings using simple analogies:

1. The "Two-Layer Cake" of Hot Gas

When the scientists looked at the gas inside the bubbles, they expected it to be one uniform temperature, like a single layer of warm soup. Instead, they found it was more like a two-layer cake:

The Bottom Layer (The Cool Layer): This is the main bulk of the gas. It's "cool" in astronomical terms, about 2 million degrees (0.2 keV). It's very dense, meaning there's a lot of it.
The Top Layer (The Hot Layer): Floating on top is a thinner, much hotter layer, about 7 million degrees (0.6 keV).

The Analogy: Imagine a pot of water on a stove. The water at the bottom is simmering (the cool layer), but right above it, there's a layer of steam that is much hotter (the hot layer). The scientists found that the "simmering" layer is about five times more massive than the "steam" layer.

2. The "Recipe" of the Gas (Chemical Composition)

By analyzing the light coming from the gas, the scientists could tell what elements were in it.

The Result: The gas is "sub-solar," meaning it has fewer heavy elements (like iron, oxygen, and neon) than our Sun does. It's about 10% to 30% as "rich" in metals as the Sun.
The Mystery: They found a slight hint that there is more Neon relative to Oxygen than expected.
The Clue: This "recipe" suggests the gas likely comes from the general halo of the galaxy (the space between stars), rather than being freshly cooked up by a massive star explosion right next to us. However, the North Polar Spur (a bright arc on the edge of the northern bubble) has a different, richer recipe, suggesting it might have a different origin, perhaps from a nearby star-forming region.

3. The "Cool Shell" Mystery

Surrounding the northern bubble, the scientists found a distinct "shell" of gas that is even cooler than the main bubble gas.

The Question: Is this shell part of the big bubble, or is it a separate object sitting in front of it?
The Theory: One idea is that this shell is actually part of a much older, smaller bubble called Loop I, which is a local structure in our neighborhood (only about 100–150 light-years away). Imagine looking at a giant balloon in the distance, but there's a small, clear plastic sheet floating in front of it. The "cool shell" might be that plastic sheet, not part of the giant balloon.

4. The Shape: A Tilted, Asymmetric Balloon

The scientists tried to build a 3D model of what these bubbles actually look like in space, not just how they appear on a 2D map.

The Tilt: The northern bubble isn't standing straight up; it's tilted. It leans toward the west and slightly toward us (our Solar System).
The Asymmetry: The southern bubble is different. It doesn't tilt as much and seems to have a different shape.
The "Cap" Problem: The scientists found it very hard to tell how tall the bubbles are. Because we are living inside the galactic disk (like sitting on the floor of a room looking at the ceiling), we can't see the "top" of the bubbles clearly. The gas at the very top is so thin and faint that it's invisible. It's like trying to guess the height of a foggy mountain when you can only see the base; the top might be 1,000 feet high or 10,000 feet high, and the X-ray data can't tell the difference.

5. How Did They Get There? (The Engine)

The bubbles are huge and energetic. To inflate them, something powerful must have happened in the center of the galaxy millions of years ago.

The Candidates:
1. The Black Hole: The supermassive black hole at the center of our galaxy (Sagittarius A*) might have had a "burp" or a jet of energy a few million years ago, blowing these bubbles like a giant air horn.
2. Star Power: Alternatively, a massive burst of star formation (a "starburst") could have created enough stellar winds and supernovae to push the gas out.
The Verdict: The data doesn't definitively pick a winner yet. However, the fact that the gas is relatively cool and the bubbles are tilted makes the "Black Hole Jet" theory plausible, but a "Starburst" theory is also possible if the outflow was shaped by the surrounding gas.

Summary

In short, this paper tells us that the eROSITA bubbles are massive, two-layered structures of hot gas extending from the center of our galaxy. They are made of "star-poor" material, they are tilted, and they might be hiding a smaller, older bubble (Loop I) in their northern shell. While we know they are there and what they are made of, the exact "engine" that blew them up and their true 3D height remain a bit of a mystery, much like trying to guess the shape of a cloud from the ground.

Technical Summary: The SRG/eROSITA Diffuse Soft X-ray Background II. Spectra and Morphology of the eROSITA Bubbles in the Western Galactic Hemisphere

Problem and Context
The eROSITA bubbles (eRObub), discovered in the first SRG/eROSITA All-Sky Survey (eRASS1), are among the most extended structures in the X-ray sky, appearing as two giant shells symmetric about the Galactic Center (GC). They enclose the non-thermal Fermi bubbles (FB) and are bounded in the north by the North Polar Spur (NPS). While their morphology suggests a physical connection to the GC and an energy budget of $\sim 10^{56}$ erg, their three-dimensional geometry, physical origin (AGN outburst vs. starburst), and internal gas properties remain debated. Previous studies have relied on pencil-beam observations or single-temperature models that fail to capture the complexity of the diffuse emission, particularly regarding metal abundances, temperature structure, and the nature of the cool shell surrounding the northern bubble.

Methodology
This work presents a comprehensive analysis of the eRObub in the western Galactic hemisphere using eRASS1 data. The methodology is divided into spectral and morphological analyses:

Spectral Analysis:
- Large Regions: The authors defined large, manually selected extraction regions ( $25\text{--}670 \text{ deg}^2$ ) to maximize photon statistics for specific features, including the FB, the NPS, the cool shell, and the bubble interior. These regions were paired with background regions at similar latitudes to constrain foreground/background components (Local Hot Bubble, Galactic Halo, Cosmic X-ray Background, and unresolved stellar emission).
- Small Regions: To map temperature and emission measure (EM) continuously, smaller regions ( $5\text{--}10 \text{ deg}^2$ ) were defined based on a constant signal-to-noise ratio (S/N $\approx 80$ ).
- Modeling: Spectra were fitted using the SPEX code (v3.08.1). The analysis tested single-temperature Collisional Ionization Equilibrium (CIE), plane-parallel shock (NEI), and two-temperature CIE models. A critical step involved refining the response matrix files (RMF) and ancillary response files (ARF) to address systematic residuals around emission lines. The authors also incorporated a stellar contribution model based on the projected stellar mass profile of the Milky Way to replace the previously assumed hot CGM component at $\sim 0.7$ keV.
Morphological Analysis:
- A parametric geometrical model was constructed to reproduce the observed X-ray surface brightness. The model assumes a bipolar blast wave originating from the GC, expanding into an idealized Galactic halo.
- The geometry consists of constricted prolate ellipsoids (to account for impeded expansion in the Galactic plane) with free parameters for semi-major/minor axes, tilt angles (polar and azimuthal), and shock Mach numbers.
- The model includes a quadrupolar density modulation to account for inhomogeneities in the circumgalactic medium (CGM) and an empirical background component to handle unresolved stellar and CXB contributions.

Key Results

Temperature Structure: The interior of the eRObub is best characterized by two emission components with relatively uniform temperatures across the hemisphere: a hotter component at $kT = 0.60 \pm 0.02$ keV and a colder component at $kT = 0.21^{+0.03}_{-0.01}$ keV. The colder component dominates the emission measure (EM) by a factor of $\sim 5$ . A single-temperature model fails to reproduce the observed Fe XVII line ratios ( $15\text{\AA}/17\text{\AA}$ ), a discrepancy that persists even when using different plasma codes (SPEX vs. AtomDB/apec).
Abundances: The gas within the eRObub interior exhibits sub-solar abundances ( $Z \approx 0.2 \pm 0.1 Z_\odot$ ), consistent with shock-heated CGM. While there are tentative indications of $\alpha$ -element enhancement (specifically Ne/O), the evidence is not conclusive due to systematic uncertainties in abundance tables. In contrast, the NPS shows significantly higher abundances ( $Z > 0.5 Z_\odot$ ), suggesting a different origin or enrichment mechanism compared to the rest of the bubbles.
The Cool Shell: A spectral cool shell ( $kT \sim 0.18\text{--}0.2$ keV) is confirmed surrounding the northern bubble. However, the authors find no conclusive evidence for a similar shell in the southern hemisphere, where the signal is comparable to the unresolved stellar background. The cool shell's properties are consistent with a foreground structure, potentially the Loop I superbubble, rather than a radiative cooling shell of the eRObub itself.
Fermi Bubble Connection: Spectra extracted from regions overlapping with the FB show no significant difference from the surrounding eRObub interior. There is no evidence for an additional non-thermal component or a distinct temperature jump at the FB boundary.
Morphology and Geometry: The geometrical model successfully reproduces the observed X-ray morphology.
- Horizontal Extent: The horizontal semi-minor axes are well-constrained at $\sim 5$ kpc for both hemispheres.
- Vertical Extent: The vertical extent is highly uncertain due to projection effects; the observed X-ray emission is insensitive to the existence or location of a "bubble cap."
- Tilt: The northern bubble requires a significant tilt ( $\sim 30^\circ$ ) towards $l \sim 220^\circ$ (west) to match the observed protrusion, whereas the southern bubble requires little tilt.
- Shock Velocity: The model implies a higher shock Mach number in the south ( $M \sim 2.9$ ) compared to the north ( $M \sim 1.8$ ), consistent with the southern shell appearing more limb-brightened.

Significance and Claims
The paper claims to provide the first spatially resolved, high-S/N spectral characterization of the eRObub across the western hemisphere. Its primary contributions are:

Resolution of the Fe XVII Anomaly: The authors demonstrate that the anomalous Fe XVII line ratio cannot be explained by plasma code uncertainties or single-temperature models, necessitating a two-temperature plasma structure.
Refined Energetics: By combining the two-temperature spectral fit with the geometrical model, the authors estimate a total thermal energy excess of $\sim 1.5 \times 10^{55}$ to $1.66 \times 10^{56}$ erg, requiring an energy injection power of $P \gtrsim 1.5 \times 10^{41}$ erg s $^{-1}$ over an age of $\sim 15\text{--}27$ Myr.
Origin Constraints: The sub-solar abundances generally favor a CGM origin (AGN or starburst driven), while the specific enhancement in the NPS suggests a separate origin linked to active star-forming regions in the Galactic disk. The lack of a temperature jump at the FB boundary and the absence of multi-keV plasma disfavor a short, powerful AGN jet scenario that would produce a hot, shocked interior, though long-term AGN wind or starburst scenarios remain viable.
Geometrical Degeneracy: The study highlights that current X-ray data cannot uniquely constrain the vertical extent of the bubbles due to the observer's location within the Galactic plane, emphasizing that the observed "shell" is likely a projection of a conical or open outflow rather than a closed spherical shell.

The authors conclude that while the eRObub and FB are likely physically connected, the NPS may have a distinct origin, and the global structure is best described as a tilted, asymmetric outflow with complex internal temperature stratification.