| Astrophysics (Index) | About |
A dark matter halo is the dark matter portion of a galaxy which extends throughout the visible galaxy and further out, thus as a halo. Dark matter explains apparent additional gravity of a galaxy, galaxy cluster, or other large structures beyond that explained by the visible and inferred stars, interstellar media and other normal matter. The velocity distribution of orbiting stars depends upon the amount of gravity (and thus mass), and can be determined from Doppler shifts of the starlight, revealing the mass of the galaxy, including any not evident from the number and mass of its stars.
The typical inferred halo is spherical, densest in the center and about 5 times more massive than the visible portion of the galaxy. The halo of a substantial galaxy such as the Milky Way includes satellite galaxies and globular clusters, which are presumed to form around volumes of extra dark-matter density, termed subhalos (sub-halos). Lambda-CDM model predicts more subhalos than known evidence confirms, so any new evidence of additional subhalos (e.g., tidal effects on stellar streams) is of interest.
Dark matter halos are presumed to exist before the galaxies, i.e., galaxies form where the gravity of dark matter halos has gathered gas. The term minihalo refers to a small halo, and it is presumed that in the early universe, they were usual before gravity caused halos to combine into larger halos. Models of Population III star formation often take into account the possibility of such minihalos.
Dark matter halo can also refer to a region of dark matter throughout a galaxy cluster, presumably somewhat less dense than regions around the individual galaxies. The term cluster dark matter halo is sometimes used to distinguish this usage.