Stellar halo The stellar halo is a nearly spherical population of
field stars and
globular clusters. It surrounds most disk galaxies as well as some elliptical galaxies of
type cD. A low amount (about one percent) of a galaxy's stellar mass resides in the stellar halo, meaning its luminosity is much lower than other components of the galaxy. The
Milky Way's stellar halo contains
globular clusters,
RR Lyrae stars with low
metallicity, and
subdwarfs. In our stellar halo, stars tend to be old (most are greater than 12 billion years old) and metal-poor, but there are also halo star clusters with observed metal content similar to
disk stars. The halo stars of the Milky Way have an observed radial velocity dispersion of about 200 kilometres per second and a low average velocity of rotation of about . Star formation in the stellar halo of the Milky Way ceased long ago.
Galactic corona 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, showing the presence of atomic neutral hydrogen (the
H I region, pronounced "H-one") and other features detectable by
X-ray spectroscopy.
Dark matter halo The
dark matter halo is a theorized distribution of dark matter which extends throughout the galaxy extending far beyond its visible components. The mass of the dark matter halo is far greater than the mass of the other components of the galaxy. Its existence is hypothesized in order to account for the gravitational potential that determines the dynamics of bodies within galaxies. The nature of dark matter halos is an important area in current research in
cosmology, in particular its relation to
galactic formation and evolution. The
Navarro–Frenk–White profile is a widely accepted density profile of the dark matter halo determined through numerical simulations. It represents the mass density of the dark matter halo as a function of r, the distance from the galactic center: \rho (r) = \frac{\rho_\text{crit} \delta_{c}}{(r/r_{s})(1+r/r_{s})^{2}} where r_{s} is a characteristic radius for the model, \rho_\text{crit} = 3H^2/8 \pi G is the critical density (with H being the
Hubble constant), and \delta_c is a dimensionless constant. The invisible halo component cannot extend with this density profile indefinitely, however; this would lead to a diverging integral when calculating mass. It does, however, provide a finite gravitational potential for all r. Most measurements that can be made are relatively insensitive to the outer halo's mass distribution. This is a consequence of
Newton's laws, which state that if the shape of the halo is spheroidal or elliptical there will be no net gravitational effect from halo mass a distance r from the galactic center on an object that is closer to the galactic center than r. The only dynamical variable related to the extent of the halo that can be constrained is the
escape velocity: the fastest-moving stellar objects still gravitationally bound to the Galaxy can give a lower bound on the mass profile of the outer edges of the dark halo. == Formation of galactic halos ==