Short answers, pulled from the story.
A galactic halo is an extended, roughly spherical component of a galaxy that reaches far beyond the galaxy's visible main body. It has three distinct parts: a stellar halo of old stars and globular clusters, a galactic corona of hot gas, and a dark matter halo whose mass exceeds all visible components of the galaxy.
Most stars in the Milky Way's stellar halo are greater than 12 billion years old. They are also metal-poor, meaning they formed early in the universe before later generations of stars enriched interstellar space with heavier elements.
The Navarro-Frenk-White profile is a widely accepted density profile describing how dark matter density varies with distance from a galaxy's centre. Established through numerical simulations, it expresses mass density as a function of distance using a characteristic radius, the critical density tied to the Hubble constant, and a dimensionless constant.
Astronomers study the galactic halo by observing its effect on light from distant quasars that lie in the line of sight beyond the galaxy. The galactic corona can also be detected through its emission spectrum, which reveals atomic neutral hydrogen and features measurable by X-ray spectroscopy.
The Gaia Sausage is a named merger event thought to have contributed to the formation of the Milky Way's stellar halo. It left a detectable signature in the kinematics of halo stars.
NGC 4236 has an active stellar halo where star formation continues. Most galaxies, including the Milky Way, ceased stellar halo star formation long ago, making NGC 4236 a notable exception.