Definition
A galactic corona is a region of hot, diffuse plasma that surrounds a galaxy, extending beyond the galaxy’s visible stellar disk and often beyond the traditional stellar halo. The term is used primarily to describe the tenuous, X‑ray‑emitting gas that is gravitationally bound to the galaxy and typically has temperatures of 10⁶–10⁷ K.
Physical Characteristics
| Property | Typical Range / Description |
|---|---|
| Temperature | 1–10 million kelvin (≈10⁶–10⁷ K), producing thermal X‑ray emission. |
| Density | Extremely low, n ≈ 10⁻⁴–10⁻³ cm⁻³ near the galaxy, decreasing with radius. |
| Extent | Can reach out to several hundred kiloparsecs (kpc), comparable to the virial radius of the host galaxy. |
| Composition | Primarily ionized hydrogen and helium, with trace amounts of heavier elements (metals) that provide line emission in the X‑ray band. |
| Pressure | Roughly in equilibrium with the galaxy’s gravitational potential and any surrounding intergalactic medium (IGM). |
Observational Evidence
- X‑ray Emission – Space‑based telescopes such as ROSAT, Chandra, and XMM‑Newton detect soft X‑ray halos around massive spiral galaxies (e.g., the Milky Way, Andromeda) and elliptical galaxies.
- Absorption Lines – Ultraviolet and X‑ray spectroscopy of background quasars reveals absorption by highly ionized species (e.g., O VII, O VIII) consistent with hot halo gas.
- Sunyaev‑Zel’dovich Effect – Small distortions of the cosmic microwave background caused by scattering off hot electrons in the corona have been measured for some nearby galaxies.
Formation and Maintenance
- Gravitational Heating – Gas falling into a galaxy’s dark‑matter halo is shock‑heated to the virial temperature, establishing the hot corona.
- Feedback Processes – Energy input from supernovae, stellar winds, and active galactic nuclei (AGN) can heat and sustain the corona.
- Accretion and Recycling – The corona can act as a reservoir, gradually cooling and delivering gas to the galaxy’s disk, or it can be replenished by ongoing accretion from the intergalactic medium.
Role in Galaxy Evolution
- Gas Reservoir – Provides a source of material that may later cool, condense, and fuel star formation in the galactic disk.
- Regulation of Star Formation – The pressure and temperature of the corona influence the ability of cool gas clouds to survive and collapse, thereby affecting the galaxy’s star‑formation rate.
- Metal Enrichment – Outflows from the galaxy enrich the corona with heavy elements, which can later mix back into the interstellar medium.
Milky Way Example
- The Milky Way’s corona has been inferred from X‑ray observations of O VII and O VIII emission and from absorption toward distant quasars.
- Estimates suggest a mass of roughly 1–5 × 10¹⁰ M☉ within a radius of ≈200 kpc, comparable to the galaxy’s stellar mass.
Open Questions and Uncertainties
- The precise density profile and total mass of galactic coronas remain uncertain, especially for low‑mass galaxies where detection is challenging.
- The relative contributions of gravitational heating versus feedback mechanisms in establishing and maintaining the corona are active research topics.
Related Concepts
- Galactic Halo – A broader term that includes both the hot corona and the dark‑matter, stellar, and globular‑cluster components of a galaxy’s outer region.
- Circumgalactic Medium (CGM) – Often used interchangeably with “galactic corona,” though CGM can encompass multiple gas phases (cold, warm, hot).
References (selected)
- Anderson, M. E., & Bregman, J. N. (2011). Hot Gas in the Galactic Halo. ApJ, 734, 48.
- Tumlinson, J., et al. (2017). The Circumgalactic Medium. Annual Review of Astronomy and Astrophysics, 55, 389‑432.
- Miller, M. J., & Bregman, J. N. (2015). The Milky Way’s Hot Gaseous Halo. ApJ, 800, 14.
