Detection of large-scale X-ray bubbles in the Milky Way halo (2012.05840v1)

Abstract: The halo of the Milky Way provides a laboratory to study the properties of the shocked hot gas that is predicted by models of galaxy formation. There is observational evidence of energy injection into the halo from past activity in the nucleus of the Milky Way; however, the origin of this energy (star formation or supermassive-black-hole activity) is uncertain, and the causal connection between nuclear structures and large-scale features has not been established unequivocally. Here we report soft-X-ray-emitting bubbles that extend approximately 14 kiloparsecs above and below the Galactic centre and include a structure in the southern sky analogous to the North Polar Spur. The sharp boundaries of these bubbles trace collisionless and non-radiative shocks, and corroborate the idea that the bubbles are not a remnant of a local supernova but part of a vast Galaxy-scale structure closely related to features seen in gamma-rays. Large energy injections from the Galactic centre are the most likely cause of both the {\gamma}-ray and X-ray bubbles. The latter have an estimated energy of around 10$^{56}$ erg, which is sufficient to perturb the structure, energy content and chemical enrichment of the circumgalactic medium of the Milky Way.

• The paper identifies large-scale X-ray bubbles in the Milky Way halo with mirror symmetry around the Galactic plane, akin to the gamma-ray Fermi bubbles.
• It employs the sensitive eROSITA all-sky survey to map over a million X-ray point sources and approximately 20,000 extended sources, enabling detailed structural analysis.
• The study estimates an energetic contribution of roughly 10^56 erg, implicating past activity from Sgr A* in shaping the circumgalactic medium.

Observations and Implications of Large-Scale X-ray Bubbles in the Milky Way Halo

The investigation conducted by Predehl et al. details the detection of expansive X-ray bubbles located above and below the Galactic center of the Milky Way. Utilizing the eROSITA telescope aboard the Spektr-RG mission, this paper presents these structures as significant extensions of the known Fermi bubbles, suggesting a common origin linked to past energetic activity within the Galactic center.

Key Findings and Numerical Results

The paper identifies large X-ray bubbles that are analogous in morphology to the gamma-ray Fermi bubbles, initially observed in 2010. These X-ray structures, revealed through the eROSITA all-sky survey, are approximately several kiloparsecs in size and display a notable mirror symmetry around the Galactic plane. The eROSITA telescope's ability to detect these structures stems primarily from its sensitivity to softer X-ray energies, which allowed for the observation of over a million X-ray point sources and approximately 20,000 extended sources across the sky.

A significant outcome of this research is the reported total energetic contribution of the bubbles, estimated at around $10^{56}$ erg. This energy level, sourced from the Galactic nucleus, is purportedly sufficient to impact the structure and chemical makeup of the Milky Way's circumgalactic medium. The detected X-ray emission surface brightness from these bubbles supports a scenario of substantial thermal energy propagation outward from the Galaxy's core, suggestive of large-scale shock phenomena.

Interpretative Models and Implications

Preliminary analysis of the morphological and spectral data suggests that the X-ray bubbles are likely produced by non-radiative, collisionless shocks. This finding refutes earlier hypotheses of the structures being mere remnants of local supernovae. The analysis aligns with the scenario wherein the X-ray and gamma-ray bubbles result from significant outbursts or sustained activity from the supermassive black hole at the Galactic center, Sgr A*, potentially during a phase of Seyfert-like activity or vigorous star formation.

The implications of this work extend beyond the characterization of the bubbles themselves. It demonstrates the importance of AGN feedback in galaxy halo dynamics and the possibility of substantial re-heating events that can perturb the halo even in seemingly quiescent galaxies. This challenges the current understanding of energy mechanisms affecting galactic evolution and emphasizes the role of episodic central activity in shaping the circumgalactic medium.

Future Prospects

The ongoing analysis and future surveys by eROSITA and complementary observational platforms hold the promise of refining our understanding of these phenomena. The encapsulation and modeling of the eROSITA bubbles offer a framework through which similar features in other galaxies can be studied, aiding in the broader comprehension of galactic feedback processes.

In conclusion, the work by Predehl et al. provides substantial evidence of energetic phenomena capable of affecting large scale structures within galaxy halos. The robustness of the conclusions drawn from the eROSITA data will likely fuel further investigations into the dynamic processes occurring in the Milky Way and beyond, fostering advancements in the field of astrophysical feedback mechanisms.