Lone black holes are large however darkish — a tempting rationalization for dark matter. However astronomers do not consider these objects are the wrongdoer.
Scientists consider that the Milky Way, like most galaxies, is surrounded by a dark matter halo (blue), as depicted on this artist’s idea. Credit score: ESO/L. Calçada
Can remoted black holes account for dark matter?
Rick Pinson
Lengthy Seashore, California
Astronomers estimate about 100 million black holes wander the Milky Way. Since black holes emit no gentle, we depend on oblique strategies to deduce their presence. The primary methodology is to seek for a black hole’s gravitational affect on close by objects, resembling stars. The second is to watch an accretion disk round a feeding black hole. Each strategies, nonetheless, require a number of stars to reside close to the black hole. Subsequently, it’s no shock that scientists know of solely about 20 black holes within the Milky Way — all of that are in binary programs, therefore why we might discover them. That does go away a number of doubtlessly unaccounted-for black holes in our galaxy.
Surrounding the disk of a galaxy is the stellar halo, a spherical inhabitants of stars and clusters. Whereas the Milky Way’s halo extends greater than 300,000 light-years, it additionally sports activities a halo of dark matter (as do practically all identified galaxies) which will lengthen even additional. As a result of stars situated within the halo are usually older, researchers have certainly puzzled if unseen black holes — together with different faint objects like neutron stars, white dwarfs, and planets — within the halo might account for a galaxy’s dark matter. Collectively, these faint, compact objects are often known as MAssive Compact Halo Objects, or MACHOs for brief.
However based mostly on observations of MACHOs, astronomers suppose these objects possible solely make up 20 p.c of the anticipated quantity of dark matter within the Milky Way’s halo. That leaves one other 80 p.c of dark matter current in our galaxy that can not be made up of black holes.
Caitlyn Buongiorno
Associate Editor
