What Is The MACHO Project:

More than 20 years ago, astronomers came face to face with an unsettling finding: The tug exerted by all the visible material in our galaxy is not nearly enough to keep it intact. To explain why the rapidly rotating stars and gas at the edge of the galaxy don’t simply fly away, scientists have been forced to assume that a vast halo of dark matter, extending thousands of light-years beyond the Milky Way’s visible outline, envelops the galaxy.

The MACHO Project is a collaboration between scientists at the Mt. Stromlo & Siding Spring Observatories, the Center for Particle Astrophysics at the Santa Barbara, San Diego, & Berkeley campuses of the University of California, and the Lawrence Livermore National Laboratory. Their primary aim is to test the hypothesis that a significant fraction of the dark matter in the halo of the Milky Way is made up of objects like brown dwarfs or planets: these objects have come to be known as MACHOs, for MAssive Compact Halo Objects. The signature of these objects is the occasional amplification of the light from extragalactic stars by the gravitational lens effect. The amplification can be large, but events are extremely rare: it is necessary to monitor photometrically several million stars for a period of years in order to obtain a useful detection rate. For the purpose of detection they have built a two channel system thatemploys eight 2048*2048 CCDs, mounted on the 50 inch telescope at Mt. Stromlo.

Mt. Stromlo Observatory, Canberra, Australia Macho Camera System

The Detection of MACHOs:

Based on 50 years of accumulated observations of the motions of galaxies and the expansion of the universe, most astronomers believe that as much as 90 percent of the stuff constituting the universe may be objects or particles that cannot be seen. In other words, most of the universe's matter does not radiate--it provides no glow that we can detect in the electromagnetic spectrum.

Understanding something you cannot see is difficult--but not impossible. Not surprisingly, astronomers currently study dark matter by its effects on the bright matter that we do observe. For instance, when we watch a nearby star wobbling predictably, we infer from calculations that a "dark planet" orbits around it. Applying similar principles to spiral galaxies, we infer dark matter's presence because it accounts for the otherwise inexplicable motions of stars within those galaxies.

MACHOS can possibly be detected if they pass close enough (line-of-sight) to a luminous background object. The resulting distortion of space has a lensing effect (microlensing) that increases the background object intensity. Surveys of the sky are now being carried out (EROS, OGLE, MACHO and MOA, a New Zealand program) to detect such microlensing events.

The amplification of light from stars, sometimes called ``microlensing'', was proposed by Paczynski (1986) as a means to detect MAssive Compact Halo Objects (MACHOs). This prediction of photometric lensing events in the light from stars in the large Magellanic Cloud (LMC) was confirmed by Alcock et al. (1993) and Aubourg et al. (1993) through their detection of three events, probably caused by MACHOs of about solar masses as they passed within less than one milliarcsecond (mas) of the line-of-sight to a star. Nine lensing events in the light from stars in the Galactic bulge have been reported by Udalski et al. (1993, 1994) and pacz94 with characteristic times between 8.6 and 62 days.

The Large Magellanic Cloud Macho images? Located in the Hubble Deep Field North

The MACHO collaboration announced the observation of several events in which the light from a star in the Large Magellanic Cloud brightened as a result of gravitational microlensing by a nonluminous object lying in the galactic halo along our line of sight. The MACHO group is able to discover this by taking pictures of the 9 million stars every night and looking for telltale light enhancements. The event sample is not very large, but the researchers are so confident of their events that they use the number to estimate a value of at least 50% as the likely contribution of MACHOs to the dark matter believed to be lurking in the halo.

Still To Come With MACHOs:

Due to the fact that this is a fairly new subject there are still many questions. Dark matter could consist of exotic, unfamiliar particles that we have not figured out how to observe. Physicists theorize about the existence of these particles, although experiments have not yet confirmed their presence. A third possibility is that our understanding of gravity needs a major revision--but most physicists do not consider that option seriously.

Because our nearest neighboring spiral galaxy, Andromeda, lies a mere two million light-years away, we now realize that our galaxy's halo may indeed span a significant fraction of the distance to Andromeda and its halo. We have also determined that clusters of galaxies lie embedded in even larger systems of dark matter. At the farthest distances for which we can deduce the masses of galaxies, dark matter appears to dwarf luminous matter by a factor of at least 10, possibly as much as 100.

Overall, we believe dark matter associates loosely with bright matter, because the two often appear together. Yet, admittedly, this conclusion may stem from biased observations, because bright matter typically enables us to find dark matter.

In some sense, our ignorance about dark matter's properties has become inextricably tangled up with other outstanding issues in cosmology--such as how much mass the universe contains, how galaxies formed and whether or not the universe will expand forever. So important is this dark matter to our understanding of the size, shape and ultimate fate of the universe that the search for it will very likely dominate astronomy for the next few decades.

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