Galactic halo

• 1. Defining The Galactic Halo

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  A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. This structure surrounds most disk galaxies and some elliptical galaxies of type cD. In spiral galaxies, the spherical shape of the halo contrasts sharply with the flat disc below it. An elliptical galaxy shows no sharp transition between its other components and the halo. Astronomers study this region by observing how it affects light from distant bright objects like quasars that lie in line of sight beyond the galaxy in question. Several distinct components comprise the total halo: the stellar halo, the galactic corona, and the dark matter halo.

• 2. Stellar Population Analysis

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  The Milky Way's stellar halo contains globular clusters and RR Lyrae stars with low metallicity. Most stars within this population are older than 12 billion years old. These ancient stars are also metal-poor compared to those found elsewhere. A low amount, about one percent, of a galaxy's stellar mass resides in the stellar halo. Consequently, its luminosity remains much lower than other components of the galaxy. Star formation in the stellar halo of the Milky Way ceased long ago. The halo stars have an observed radial velocity dispersion of about 200 kilometres per second. They maintain a low average velocity of rotation near zero. Some halo star clusters show metal content similar to disk stars despite their age.

• 3. Corona And Gas Dynamics

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  A galactic corona is a distribution of gas extending far away from the center of the galaxy. It can be detected by the distinct emission spectrum it gives off. This spectrum shows the presence of atomic neutral hydrogen known as the H I region. Other features become detectable through X-ray spectroscopy observations. The hot gas exists as a plasma that stretches outward into space. Scientists analyze these signals to map the invisible gaseous envelope surrounding the visible galaxy. The detection relies on specific spectral lines rather than direct visual observation. This method allows researchers to study regions too faint for standard telescopes.

• 4. Dark Matter Distribution Models

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  The dark matter halo is a theorized distribution of dark matter which extends throughout the galaxy. Its mass is far greater than the mass of the other components combined. The existence of this component is hypothesized to account for the gravitational potential determining dynamics within galaxies. The Navarro, Frenk, White profile represents a widely accepted density model determined through numerical simulations. This formula describes mass density as a function of distance from the galactic center. A characteristic radius defines the scale of the model while critical density relates to the Hubble constant. Most measurements remain relatively insensitive to the outer halo's mass distribution due to Newton's laws. Only the escape velocity provides a lower bound on the mass profile of the outer edges.

• 5. Bottom-Up Formation History

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  The formation of stellar halos occurs naturally in a cold dark matter model of the universe. Evolution of systems such as halos proceeds from the bottom-up, meaning large scale structures form starting with small objects. Halos composed of both baryonic and dark matter form by merging with each other. Evidence suggests that increased gravity and the presence of primordial black holes also influence formation. Gas from halo mergers goes toward forming central galactic components while stars and dark matter remain in the halo. The halo of the Milky Way Galaxy is thought to derive from the Gaia Sausage event. This specific merger shaped the current structure we observe today.