A massive astrophysical compact halo object (MACHO) is any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body that emits little or no radiation and drifts through interstellar space unassociated with any planetary system (and may or may not be composed of normal baryonic matter).
Since MACHOs are not luminous, they are hard to detect. MACHO candidates include black holes or neutron stars as well as brown dwarfs and unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs. The term was coined by astrophysicist Kim Griest.
A MACHO may be detected when it passes in front of or nearly in front of a star and the MACHO’s gravity bends the light, causing the star to appear brighter in an example of gravitational lensing known as gravitational microlensing.
Several groups have searched for MACHOs by searching for the microlensing amplification of light. These groups have ruled out dark matter being explained by MACHOs with mass in the range 1×10−8 solar masses (0.3 lunar masses) to 100 solar masses.
One group, the MACHO collaboration, claimed in 2000 to have found enough microlensing to predict the existence of many MACHOs with mean mass of about 0.5 solar masses, enough to make up perhaps 20% of the dark matter in the galaxy. This suggests that MACHOs could be white dwarfs or red dwarfs which have similar masses. However, red and white dwarfs are not completely dark; they do emit some light, and so can be searched for with the Hubble Telescope and with proper motion surveys. These searches have ruled out the possibility that these objects make up a significant fraction of dark matter in our galaxy.
MACHOs may sometimes be considered to include black holes. Isolated black holes without any matter around them are truly black in that they emit no light and any light shone upon them is absorbed and not reflected. A black hole can sometimes be detected by the halo of bright gas and dust that forms around it as an accretion disk being pulled in by the black hole’s gravity. Such a disk can generate jets of gas that are shot out away from the black hole because it cannot be absorbed quickly enough. An isolated black hole, however, would not have an accretion disk and would only be detectable by gravitational lensing.
Neutron stars, unlike black holes, are not heavy enough to collapse completely, and instead form a material rather like that of an atomic nucleus (sometimes informally called neutronium). After sufficient time these stars could radiate away enough energy to become cold enough that they would be too faint to see. Likewise, old white dwarfs may also become cold and dead, eventually becoming black dwarfs, although the universe is not thought to be old enough for any stars to have reached this stage.
Brown dwarfs have also been proposed as MACHO candidates. Brown dwarfs are sometimes called “failed stars” as they do not have enough mass for nuclear fusion to begin once their gravity causes them to collapse. Brown dwarfs are about thirteen to seventy-five times the mass of Jupiter. The contraction of material forming the brown dwarf heats them up so they only glow feebly at infrared wavelengths, making them difficult to detect. A survey of gravitational lensing effects in the direction of the Small Magellanic Cloud and Large Magellanic Cloud did not detect the number and type of lensing events expected if brown dwarfs made up a significant fraction of dark matter.