Halo of the Milky Way

The halo of our Galaxy is the large roughly spherical distribution of stars and ??? of our Galaxy. The stars (and globular clusters) are thought to have formed in the early stages of the formation of the Milky Way (and are referred to as Population II stars) and typically will therefore be stellar remnants, low mass stars, or other low mass objects (e.g., planets or brown dwarfs). Halo stars are not the first generation of stars, however, because they contain elements heavier than hydrogen and helium, that is, material which has been processed through stars at least once. According to the measurements of the mass of the Milky Way, it is inferred that the bulk of the mass of the Milky Way is contained in the halo. It is not clear in what form the mass resides, however. Over the years, the halo material has been suggested to be in the form of normal stars, planet-sized objets, brown dwarfs, black holes, neutron stars, white dwarfs, exotic particles (e.g., Weakly Interacting Massive Particles (WIMPs), ... . It is on this aspect of the halo (and, is an interesting question for the Universe at large) that I will spend most of my time.

Massive Compact Halo Objects (MACHOs)

An interesting experiment to study the possible types of compact objects in the halo of the Milky Way galaxy has been in progress for the last few years. In this work, one takes advantage of a prediction of the General Theory of Relativity (GTR) to probe the structure of the halo. GTR predicts that the path of a light ray will bend as it passes by a massive object,

that is, the intervening galaxy focuses the light from more distant objects acting like a lens, hence the name, gravitational lensing). We see the formation of rings (arcs), multiple images or amplification depending on how aligned are the Earth, the lensing mass, and the observed object, and on the uniformity of the mass distribution of the lensing object,

What do we do?

In the MACHO project, the idea is as follows. Astronomers point their telescopes in the direction of our nearest neighbor galaxy, the Large Magellanic Cloud (LMC), and the Bulge of our own Galaxy. The project team regularly take pictures of the LMC and the Bulge. The LMC and Bulge contains many billions of stars of which roughly twenty million are monitored by the MACHO team.

What is expected to be seen?

Since the objects in our halo orbit about the center of our Galaxy there is a chance that a halo object will pass in front of an LMC or Bulge star. If a halo object passes in front of an LMC or Bulge star, the light from the star will be amplified.

What has been found?

Based on the MACHO experiment, we infer that there is roughly 8x1010 M(Sun) of MACHO material in the halo roughly the same as the contribution as the luminous disk material. MACHOs may account for only `roughly 20 % of the halo mass. Each MACHO has a mass of roughly 0.1 to 1 M(Sun), that is, the observed MACHOs are of stellar-mass. The MACHOs may be of low mass but they are not planets, brown dwarfs, or comet-like objects; they are still too massive. MACHOs may make up a significant chunk of the halo mass, but still leave much material unaccounted for. Much of our halo may still be made up of more exotic types of matter.

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