The North Polar Spur: Evidence for Gaseous Plumes Originating from Star-Forming Regions 3-5 kpc from the Galactic Center

Core Concepts

The North Polar Spur (NPS) and similar structures observed in radio and X-ray wavelengths are likely formed by metal-enriched gaseous plumes ejected from active star-forming regions in the Milky Way's disk, particularly those located at the tangent to the 3-5 kpc rings.

Abstract

Bibliographic Information:

Churazov, E., Khabibullin, I. I., Bykov, A. M., et al. (2024). North Polar Spur: gaseous plume(s) from star-forming regions at ∼3-5 kpc from Galactic Center? Astronomy & Astrophysics manuscript no. current.

Research Objective:

This research letter proposes an alternative explanation for the formation of the North Polar Spur (NPS), challenging the prevailing view of a shock front origin. The authors investigate the possibility that the NPS arises from gaseous plumes ejected from star-forming regions in the Milky Way.

Methodology:

The researchers analyze X-ray data from the eROSITA all-sky survey, focusing on the morphology and spectral properties of the NPS. They develop a morphological model to simulate the trajectories of gaseous plumes originating from star-forming regions, considering factors like initial velocity, scale heights, and Galactic rotation.

Key Findings:

• The X-ray morphology of the NPS, particularly the sharp outer edge and flat inner brightness profile, suggests a flattened sheet-like structure rather than a spherical shell expected from a shock front.
• The model demonstrates that plumes ejected from star-forming regions, particularly those at the tangent to the 3-5 kpc rings, can reproduce the observed morphology of the NPS.
• The model predicts a high metal abundance in the NPS gas, consistent with observational hints.

Main Conclusions:

The authors argue that the NPS and similar structures are likely formed by the accumulation of metal-enriched gaseous plumes ejected from active star-forming regions. These plumes, influenced by Galactic rotation and interaction with the halo gas, create the observed spiral-like structures.

Significance:

This research offers a new perspective on the origin of the NPS and similar structures, highlighting the role of stellar feedback in shaping the Milky Way's halo. The proposed model provides testable predictions for future observations, particularly regarding the gas metallicity and ionization state.

Limitations and Future Research:

The study acknowledges the need for more detailed spectral analysis to confirm the presence of high metallicity and rule out non-equilibrium ionization signatures expected in shock-driven scenarios. Future observations with high-resolution X-ray spectrometers like the Line Emission Mapper (LEM) are crucial for testing the model's predictions and further understanding the NPS's origin.

Stats

The distance of the NPS base from the Sun is approximately 6.6 kpc.
The physical size (depth) of the NPS is estimated to be on the order of its distance from the Sun, around 6 kpc.
The estimated proton number density of the hot gas in the NPS is ~7 × 10^-4 cm^-3, assuming a metallicity equal to the Solar value.
The cooling time of the gas in the NPS is estimated to be ~2 × 10^9 years.
The total energy (enthalpy) of the NPS is estimated to be ~2 × 10^55 erg.
An estimated ~2 × 10^6 solar masses of gas need to be converted into stars to provide enough energy to power the NPS.
Assuming a star formation rate of 0.1 solar masses per year, it would take ~20 million years to generate enough energy for the NPS.

Quotes

"In this Letter, we consider another scenario for NPS formation motivated by the morphological and spectral properties of the soft X-ray emission measured in the course of the eROSITA all-sky survey."
"We propose that NPS is produced by a break-out of the massive star formation regions associated with the end of the Galactic bar, which locates its base at ∼5 kpc from us."
"In this model, it is the advection of the enriched gas and relativistic particles, rather than a shock, that is responsible for the appearance of NPS, predicting the high metal abundance of X-ray emitting gas and lack of evolutionary signatures in the direction perpendicular to the NPS edge."

Key Insights Distilled From

North Polar Spur: gaseous plume(s) from star-forming regions at $\sim$3-5 kpc from Galactic Center?

by E.Churazov, ... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2408.00752.pdf

Deeper Inquiries

How do the authors' proposed gaseous plume model and the observed properties of the NPS relate to similar structures observed in other galaxies?

The authors suggest that the North Polar Spur (NPS) might be a Galactic analog of extraplanar magnetic structures observed in other galaxies. These structures, often observed reaching significant distances above the galactic disk, are thought to be shaped by intense stellar feedback episodes. This feedback, in the form of powerful outflows and galactic winds driven by supernovae and massive star formation, transports gas, metals, and magnetic fields from the disk into the halo.
Here's how the proposed NPS model relates to observations in other galaxies:

Extraplanar Magnetic Structures: Galaxies like NGC 4217 exhibit prominent magnetic structures extending far above their disks. These structures, often observed in radio wavelengths, trace synchrotron emission from relativistic electrons spiraling in magnetic fields. The authors propose a similar mechanism for the NPS, where the observed radio emission and polarization arise from relativistic electrons within the rising, magnetized plumes.
Disk-Halo Interaction: The presence of these extraplanar structures highlights the dynamic interplay between a galaxy's disk and its surrounding halo. The NPS, if formed by gaseous plumes, would serve as a compelling example of this interaction within the Milky Way. The plumes, driven by stellar feedback, would act as conduits, transporting material and energy from the disk into the halo, influencing the halo's composition, structure, and evolution.
Galactic Winds and Fountains: Many galaxies exhibit large-scale outflows known as galactic winds, driven by the collective energy output from supernovae and massive stars. These winds can transport gas and metals far into the circumgalactic medium, enriching and heating the surrounding environment. The proposed NPS plumes could be a manifestation of a similar process, albeit on a smaller scale, where the plumes represent channels of outflow from the Milky Way's disk.

Could the interaction of the Milky Way with a dwarf galaxy, as opposed to stellar feedback, explain the formation of the North Polar Spur?

While the authors focus on stellar feedback as the driving mechanism for the NPS, the interaction of the Milky Way with a dwarf galaxy is an alternative scenario worth considering. Here's an exploration of this possibility:
Arguments in Favor:

Tidal Interactions: The gravitational interaction between the Milky Way and a passing or merging dwarf galaxy could generate tidal forces capable of disrupting the dwarf's gas and stars. This disrupted material could be drawn into the Milky Way's halo, potentially forming structures like the NPS.
Asymmetric Structures: The interaction with a dwarf galaxy could naturally explain the observed asymmetry of the NPS. The infalling dwarf galaxy's trajectory and the resulting tidal forces would not be symmetric with respect to the Milky Way's disk, leading to the formation of off-center or lopsided structures.
Challenges and Considerations:

Metallicity: The authors' model predicts a relatively high metallicity for the NPS gas, consistent with an origin from the Milky Way's disk, where star formation has enriched the gas with heavy elements. Gas stripped from a dwarf galaxy might have a lower metallicity, depending on the dwarf's star formation history. Observational constraints on the NPS metallicity are crucial for distinguishing between these scenarios.
Lack of Observable Remnants: If a dwarf galaxy interaction occurred relatively recently, there might be observable remnants of the dwarf, such as tidal streams of stars or gas. The absence of such clear remnants would pose a challenge to this scenario.
Alternative Explanations for Asymmetry: Asymmetries in the Milky Way's halo gas distribution and rotation could also contribute to the observed asymmetry of the NPS, even in the absence of a dwarf galaxy interaction.
In summary, while a dwarf galaxy interaction could potentially contribute to the formation of structures in the Milky Way's halo, the specific characteristics of the NPS, such as its metallicity and the lack of clear dwarf remnants, present challenges to this scenario. Further observations and detailed modeling are needed to definitively assess the viability of this alternative explanation.

If the NPS is indeed formed by gaseous plumes, what does this tell us about the long-term evolution of the Milky Way and the distribution of heavy elements in its halo?

The confirmation of the gaseous plume model for the NPS would provide valuable insights into several key aspects of the Milky Way's evolution:

Galactic Fountains and Recycling: The plumes, acting as galactic fountains, would demonstrate the cyclical flow of gas between the Milky Way's disk and halo. Supernovae and stellar winds eject metal-enriched gas into the halo, which then cools, condenses, and eventually falls back onto the disk, fueling future generations of stars. This recycling process plays a crucial role in the chemical evolution of galaxies.
Metal Distribution and Mixing: The plumes would transport heavy elements synthesized in stars out into the halo, influencing the metallicity gradient of the Milky Way. The spatial distribution and mixing of these metals within the halo provide clues about the efficiency of feedback processes and the dynamics of gas flows over the Milky Way's lifetime.
Halo Gas Properties: Studying the NPS plumes would offer insights into the properties of the Milky Way's halo gas, including its temperature, density, and ionization state. These properties are essential for understanding the halo's overall structure, its interaction with the disk, and its role in shaping the evolution of the Milky Way.
Star Formation History: The rate and distribution of star formation within the Milky Way's disk directly influence the strength and frequency of these plume-forming events. The observed properties of the NPS, such as its size, morphology, and metallicity, could provide constraints on the Milky Way's past star formation activity.
Overall, the gaseous plume model, if confirmed, would paint a picture of a dynamic and evolving Milky Way, where stellar feedback continuously shapes the galaxy's structure and composition. The NPS would serve as a tangible record of these processes, offering a glimpse into the complex interplay between the Milky Way's disk, halo, and the cycle of stellar birth, death, and chemical enrichment.