insight - Astrophysics - # Origin and structure of super-virial and virial temperature gas in the circumgalactic medium of the Milky Way

Disk-like Extraplanar Regions of Hot Emitting Gas in the Circumgalactic Medium of the Milky Way

Q: How do the properties of the super-virial and virial temperature gas in the extraplanar regions, such as their dynamics and chemical composition, evolve over time?

The properties of the super-virial (SV) and virial temperature gas in the extraplanar regions of the Milky Way's circumgalactic medium (CGM) evolve significantly over time due to stellar feedback processes, particularly from supernovae (SNe) and star formation activities. Initially, the gas in these regions is characterized by distinct temperature profiles, with the SV gas reaching temperatures around (10^7) K and the virial gas around (10^6) K. As time progresses, particularly within the first 15 million years post-star formation, the dynamics of the gas are influenced by the outflows generated from OB associations in the Galactic disk. The SV gas is pushed upward, creating a "puffed-up" disk-like structure that extends vertically, with the height of this region increasing over time. The simulation results indicate that at around 10 Myr, the SV gas reaches heights of approximately 1 kpc, and by 15 Myr, this extends to about 2 kpc. In terms of chemical composition, the SV gas is expected to be metal-enriched due to the contributions from SNe, which inject high metallicity gas into the CGM. The mixing of this enriched gas with the lower metallicity gas from the CGM leads to a complex metallicity gradient, where the SV gas retains a higher metallicity signature compared to the surrounding halo gas. This evolution highlights the dynamic nature of the CGM, where the interplay between stellar feedback and gas mixing plays a crucial role in shaping the properties of the hot gas over time.

Q: Can the model explain the observed column densities of the "super-virial" gas detected in absorption, or is the origin of the absorption and emission components fundamentally different?

The model presented in the study primarily focuses on the emission characteristics of the super-virial gas, suggesting that it originates from outflows driven by star formation in the Galactic disk. However, when it comes to explaining the observed column densities of the super-virial gas detected in absorption, the model faces challenges. The estimated column densities of oxygen along various lines of sight, such as IES 1553+113 and Mrk 421, indicate values significantly higher than what the model predicts. For instance, the model predicts total column densities of (1.1 \times 10^{19}) cm(^{-2}) for IES 1553+113, which is lower than the observed range of (0.8 - 5.7 \times 10^{17}) cm(^{-2}). To reconcile these discrepancies, the model suggests that a higher oxygen mass fraction is required, which is not consistent with the typical supernova ejecta composition. This indicates that the origins of the absorption and emission components may indeed be fundamentally different. While the emission phase is linked to the outflows from the disk, the absorption phase could involve different physical processes or regions of the CGM that are not adequately captured by the current model. Therefore, it is plausible that the mechanisms driving the super-virial gas in emission and absorption are distinct, necessitating further investigation to fully understand their relationship.

Q: What are the implications of the disk-like structure of the hot gas for our understanding of the overall energy and mass budget of the circumgalactic medium of the Milky Way?

The disk-like structure of the hot gas in the circumgalactic medium (CGM) has significant implications for our understanding of the energy and mass budget of the Milky Way. The presence of both super-virial and virial temperature gas in a disk configuration suggests that the CGM is not merely a static halo of gas but rather a dynamic and evolving system influenced by ongoing star formation and stellar feedback. Firstly, the disk-like profile indicates that a substantial amount of mass is concentrated in the extraplanar regions, with the model estimating masses of (1.7 \times 10^8) M⊙ for the virial phase and (3.5 \times 10^7) M⊙ for the super-virial phase. This mass is significant, although it remains small compared to the total baryonic mass of the Milky Way. The existence of these hot gas phases suggests that they play a crucial role in the overall energy balance of the CGM, as they are likely to be involved in the processes of energy injection and thermalization due to stellar activities. Moreover, the disk-like structure implies that the hot gas is continuously interacting with the cooler gas in the interstellar medium (ISM) and the surrounding halo. This interaction can lead to mixing and redistribution of metals, affecting the chemical evolution of the galaxy. The high metallicity of the super-virial gas, enriched by supernovae, contributes to the overall metallicity of the CGM, influencing the formation of future stars and the evolution of the galaxy. In summary, the disk-like structure of the hot gas in the CGM enhances our understanding of the mass and energy dynamics within the Milky Way, highlighting the importance of stellar feedback in shaping the circumgalactic environment and its role in the broader context of galaxy evolution.

Conceitos essenciais

The super-virial (∼10^7 K) and virial (∼10^6 K) temperature gas in the circumgalactic medium of the Milky Way occupy disk-like extraplanar regions, likely produced by stellar feedback in and around the Galactic disk.

Resumo

The paper investigates the origin and structure of the hot gas (∼10^7 K) detected in emission in the circumgalactic medium (CGM) of the Milky Way, in addition to the previously known virial temperature (∼10^6 K) gas.

Key highlights:

Observational data suggests the ∼10^7 K "super-virial" and ∼10^6 K "virial" temperature gas occupy disk-like extraplanar regions around the Galactic disk, rather than being distributed in a spherical halo.
The authors perform MCMC analysis to determine the parameters (scale height, scale radius, central density) of these disk-like extraplanar regions for both the super-virial and virial temperature gas.
Hydrodynamical simulations show the ∼10^7 K super-virial gas is likely produced by stellar feedback and outflows from the Galactic disk, forming a disk-like extraplanar structure.
The super-virial and virial temperature gas in the extraplanar regions is metal-enriched and not in hydrostatic equilibrium with the halo, but rather in a dynamical state.
The origin of the "super-virial" gas observed in emission is distinct from the "super-virial" gas detected in absorption, which the model cannot fully explain.

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Estatísticas

The Milky Way halo has a virial temperature of ~3 × 10^6 K.
The central density of the virial temperature halo gas is 8.8 × 10^-4 cm^-3.
The total mass in the extra-planar virial and super-virial phases is 1.7 × 10^8 M⊙ and 3.5 × 10^7 M⊙ respectively.

Citações

"The presence of the ≈10^6 K gas in the circumgalactic medium of the Milky Way has been well established."
"Recent observations have, however, indicated the presence of ∼10^7 K (0.8 keV) gas in addition to the gas at lower temperatures."
"We show that both the 'virial' and the 'super-virial' temperature gas as observed in emission occupy disk-like extraplanar regions, in addition to the diffuse virial temperature gas filling the halo of the Milky Way."

Principais Insights Extraídos De

On the origin of the $10^7$ K hot emitting gas in the Circumgalactic medium of the Milky Way

by Mukesh Singh... às arxiv.org 10-01-2024

https://arxiv.org/pdf/2408.14344.pdf

On the origin of the $10^7$ K hot emitting gas in the Circumgalactic medium of the Milky Way

Perguntas Mais Profundas

How do the properties of the super-virial and virial temperature gas in the extraplanar regions, such as their dynamics and chemical composition, evolve over time?

The properties of the super-virial (SV) and virial temperature gas in the extraplanar regions of the Milky Way's circumgalactic medium (CGM) evolve significantly over time due to stellar feedback processes, particularly from supernovae (SNe) and star formation activities. Initially, the gas in these regions is characterized by distinct temperature profiles, with the SV gas reaching temperatures around (10^7) K and the virial gas around (10^6) K.
As time progresses, particularly within the first 15 million years post-star formation, the dynamics of the gas are influenced by the outflows generated from OB associations in the Galactic disk. The SV gas is pushed upward, creating a "puffed-up" disk-like structure that extends vertically, with the height of this region increasing over time. The simulation results indicate that at around 10 Myr, the SV gas reaches heights of approximately 1 kpc, and by 15 Myr, this extends to about 2 kpc.
In terms of chemical composition, the SV gas is expected to be metal-enriched due to the contributions from SNe, which inject high metallicity gas into the CGM. The mixing of this enriched gas with the lower metallicity gas from the CGM leads to a complex metallicity gradient, where the SV gas retains a higher metallicity signature compared to the surrounding halo gas. This evolution highlights the dynamic nature of the CGM, where the interplay between stellar feedback and gas mixing plays a crucial role in shaping the properties of the hot gas over time.

Can the model explain the observed column densities of the "super-virial" gas detected in absorption, or is the origin of the absorption and emission components fundamentally different?

The model presented in the study primarily focuses on the emission characteristics of the super-virial gas, suggesting that it originates from outflows driven by star formation in the Galactic disk. However, when it comes to explaining the observed column densities of the super-virial gas detected in absorption, the model faces challenges.
The estimated column densities of oxygen along various lines of sight, such as IES 1553+113 and Mrk 421, indicate values significantly higher than what the model predicts. For instance, the model predicts total column densities of (1.1 \times 10^{19}) cm(^{-2}) for IES 1553+113, which is lower than the observed range of (0.8 - 5.7 \times 10^{17}) cm(^{-2}). To reconcile these discrepancies, the model suggests that a higher oxygen mass fraction is required, which is not consistent with the typical supernova ejecta composition.
This indicates that the origins of the absorption and emission components may indeed be fundamentally different. While the emission phase is linked to the outflows from the disk, the absorption phase could involve different physical processes or regions of the CGM that are not adequately captured by the current model. Therefore, it is plausible that the mechanisms driving the super-virial gas in emission and absorption are distinct, necessitating further investigation to fully understand their relationship.

What are the implications of the disk-like structure of the hot gas for our understanding of the overall energy and mass budget of the circumgalactic medium of the Milky Way?

The disk-like structure of the hot gas in the circumgalactic medium (CGM) has significant implications for our understanding of the energy and mass budget of the Milky Way. The presence of both super-virial and virial temperature gas in a disk configuration suggests that the CGM is not merely a static halo of gas but rather a dynamic and evolving system influenced by ongoing star formation and stellar feedback.
Firstly, the disk-like profile indicates that a substantial amount of mass is concentrated in the extraplanar regions, with the model estimating masses of (1.7 \times 10^8) M⊙ for the virial phase and (3.5 \times 10^7) M⊙ for the super-virial phase. This mass is significant, although it remains small compared to the total baryonic mass of the Milky Way. The existence of these hot gas phases suggests that they play a crucial role in the overall energy balance of the CGM, as they are likely to be involved in the processes of energy injection and thermalization due to stellar activities.
Moreover, the disk-like structure implies that the hot gas is continuously interacting with the cooler gas in the interstellar medium (ISM) and the surrounding halo. This interaction can lead to mixing and redistribution of metals, affecting the chemical evolution of the galaxy. The high metallicity of the super-virial gas, enriched by supernovae, contributes to the overall metallicity of the CGM, influencing the formation of future stars and the evolution of the galaxy.
In summary, the disk-like structure of the hot gas in the CGM enhances our understanding of the mass and energy dynamics within the Milky Way, highlighting the importance of stellar feedback in shaping the circumgalactic environment and its role in the broader context of galaxy evolution.