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From the April 2023 problem
Lone black holes are enormous but dark — a tempting explanation for dark matter. But astronomers never think these objects are the culprit.
Scientists think that the Milky Way, like most galaxies, is surrounded by a dark matter halo (blue), as depicted in this artist’s idea. Credit: ESO/L. Calçada
Can isolated black holes account for dark matter?
Extended Beach, California
Astronomers estimate about one hundred million black holes wander the Milky Way. Considering that black holes emit no light, we rely on indirect strategies to infer their presence. The initial approach is to search for a black hole’s gravitational influence on nearby objects, such as stars. The second is to observe an accretion disk about a feeding black hole. Each strategies, even so, need a single or far more stars to reside close to the black hole. For that reason, it is no surprise that scientists know of only about 20 black holes in the Milky Way — all of which are in binary systems, therefore why we could obtain them. That does leave a lot of potentially unaccounted-for black holes in our galaxy.
Surrounding the disk of a galaxy is the stellar halo, a spherical population of stars and clusters. When the Milky Way’s halo extends far more than 300,000 light-years, it also sports a halo of dark matter (as do almost all identified galaxies) that may perhaps extend even additional. Simply because stars positioned in the halo are generally older, researchers have certainly wondered if unseen black holes — along with other faint objects like neutron stars, white dwarfs, and planets — in the halo could account for a galaxy’s dark matter. Collectively, these faint, compact objects are identified as Enormous Compact Halo Objects, or MACHOs for quick.
But primarily based on observations of MACHOs, astronomers believe these objects probably only make up 20 % of the anticipated quantity of dark matter in the Milky Way’s halo. That leaves one more 80 % of dark matter present in our galaxy that can’t be produced up of black holes.