Massive compact halo object

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For other uses of the term macho, see Macho.

Massive compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a small chunk of normal baryonic matter, which emits little or no radiation and drifts through interstellar space unassociated with any solar system. Since MACHOs would not emit any light of their own, they would be very hard to detect. MACHOs may sometimes be black holes or neutron stars as well as brown dwarfs or unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs.

Other forms of MACHO are seen when the MACHO passes in front of or nearly in front of a star and the MACHO's gravity bends the light, causing the star to appear smaller and 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 0.00000001 solar masses to 100 solar masses. One group, the Macho collaboration, claims to have found enough microlensing to predict the existence of many MACHOs with 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. Another group, the EROS2 collaboration does not confirm the signal claims by the MACHO group. They did not find enough microlensing effect with a sensitivity higher by a factor 2.

MACHOs are thought to be in some abundance in the halo around our galaxy but the Hubble Space telescope shows only about six percent of the halo is composed of brown dwarfs. Therefore, the missing mass problem is not entirely solved by MACHOs.

MACHOs may sometimes be considered to include black holes. Black holes are truly black in that they emit no light and any light shined upon them is sucked in before any illumination could occur. It is thought possible that there is a halo of black holes surrounding the galaxy, and these could therefore be considered MACHOs. A black hole is detected by the halo of bright gas and dust that forms around it as an accretion disc being pulled in by the black hole's gravity. Also at times hot gas is shot out of a black hole because it cannot be absorbed quickly enough. Cosmologists doubt they make up a majority of dark matter because the black holes are at isolated points of the galaxy. The largest contributor to the missing mass must be spread throughout the galaxy to balance the gravity.

Neutron stars are somewhat like black holes, but are not heavy enough to collapse completely, instead forming into 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 yet old enough for any stars to have reached this stage.

The next candidate for MACHOs are the brown dwarfs mentioned above. These are sometimes called jupiters because the planet Jupiter is a small form of this heavenly body. Brown dwarfs are semi-stars. They do not have enough mass for nuclear fusion to begin and simply glow a dull brown, radiating the energy released through their own gravitational contraction, and may therefore be faintly visible in some circumstances. Most brown dwarfs are about ten to seventy-five times the mass of Jupiter. However, their composition and structure are virtually the same.

Recent work has also suggested that ancient MACHOs are not likely to account for the large amounts of dark matter now thought to be present in the universe; the Big Bang as it is currently understood simply couldn't produce enough baryons without causing major problems in the observed elemental abundances, particularly the abundance of deuterium. However, some MACHOs may still exist (towards the upper end of their mass scale they would be brown dwarfs or objects formed from long dead stars).

[edit] See also

• WIMPs (An alternate theory of Dark Matter)