The stellar halo of a galaxy refers to the component of its

galactic halo A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of galaxies comprise the halo: * the stellar halo * the galactic corona (hot gas, i.e. a plas ...

containing stars. The halo extends far outside a galaxy's brightest regions and typically contains its oldest and most metal poor stars.

Observation history

Early studies, investigating the shape of the stellar halo of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

, found some evidence that it may vary with increasing distance from the galaxy. These studies found halos with spherically shaped outer regions and flatter inner regions. Large surveys in the 21st century such as the

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 a ...

have allowed the shape and distribution of the stellar halo to be investigated in much more detail; this data has been used to postulate a triaxial or oblate halo. More recent studies have found the halo to be flattened with a

broken power law In statistics, a power law is a functional relationship between two quantities, where a relative change in one quantity results in a proportional relative change in the other quantity, independent of the initial size of those quantities: one q ...

radius dependence; evidence for triaxiality is unclear. As a result of their faint brightness, observations of stellar halos in distant galaxies have required very long

exposure time In photography, shutter speed or exposure time is the length of time that the film or digital sensor inside the camera is exposed to light (that is, when the camera's shutter is open) when taking a photograph. The amount of light that re ...

s, the stacking of data from numerous galaxies to obtain averaged properties, or observing only the resolved stellar populations. Individual resolved stars in stellar halos can only be measured in the Milky Way and Andromeda. The furthest stellar halos detected are at a redshift distance of 1.

Structure/properties

In the

Lambda-CDM model The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parameterization of the Big Bang cosmological model in which the universe contains three major components: first, a cosmological constant denoted by Lambda (Greek Λ) associated with ...

of the universe, galaxies grow by mergers. Such mergers are the cause of substructure observed in the stellar halo of galaxies; streams of stars from disrupted

satellite galaxies A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy (also known as the primary galaxy). Satellite galaxies and their constituents are bound ...

are detectable through their coherence in space or velocity; a number of these streams are observable around the Milky Way. As a result of the buildup from an assortment of satellite galaxies, variations in properties such as metallicity are present across stellar populations in halos.

Astrophysical simulations Computational astrophysics refers to the methods and computing tools developed and used in astrophysics research. Like computational chemistry or computational physics, it is both a specific branch of theoretical astrophysics and an interdiscip ...

of galaxies have predicted that stellar halos should have two components; one inner region dominated by stars which formed within the galaxy, and an outer region primarily composed of stars accreted through merger events. Predictions for these components include different structure and rotation directions. Observational evidence for this dual halo in the Milky Way has been claimed but contested.

Milky Way

Studies of the

galaxy have found that approximately 0.1–1% of its total stellar mass is contained within the stellar halo, and that it extends to over 100 kilo

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...

s from the galactic centre.

References

{{reflist, 25em Galaxies

Observation history

Structure/properties

Milky Way

See also

References