A new look at microlensing

Through a Hubble Space Telescope analysis of stars that have undergone gravitational microlensing, astronomers have collected strong evidence that microlensing events are caused by compact dark matter in the halo of the Milky Way. The findings were presented Monday by Lab researchers Cailin Nelson and Kem Cook on behalf of the Massive Compact Halo Objects (MACHO) collaboration during the annual meeting of the American Astronomical Society in San Diego.

Astronomers believe the dark matter in the Milky Way is distributed in a spherical halo of matter, which extends more than 10 times farther than the disk of visible stars. Some of this matter may be in one of many primarily baryonic forms including planets, brown dwarfs, very old low-mass stars, neutron stars and low-mass black holes. They are collectively known as MACHOs.

Though MACHOs emit some light, their level of emission is below present-day detection thresholds. They can be found indirectly by noting the gravitational signature they produce as they interact with other visible objects. One such signature is microlensing. In a microlensing event, a MACHO passes through an observer’s line of sight to an ordinary, luminous star. The gravitational presence of the MACHO bends the light from the star, and, acting like a lens, causes a temporary apparent increase in the brightness of the star. The brightened star in a microlensing event is called the source star.

The MACHO project has been monitoring the sky — through the use of the 1.27-meter telescope at Mount Stromlo Observatory in Australia — for microlensing events in a line of sight toward a nearby galaxy, the Large Magellanic Cloud (LMC), for eight years. The LMC provides a convenient backdrop of source stars. Earlier MACHO project results show that MACHOs can account for about 20 percent mass in the Milky Way halo and that MACHO mass is most likely between 0.15 and 0.9 times the mass of the Sun.

Some astronomers, however, have remained skeptical that the microlensing events are actually produced by MACHOs in the halo of the Milky Way, instead speculating that it is faint stars in the LMC lensing other stars in the LMC, which cause microlensing events. To make these non-dark matter theories for microlensing viable, these models also must include adjustments to the generally accepted structure of the LMC.

These adjustments require different arrangements of the source stars. If dark matter causes the microlensing events, the source stars will be randomly distributed in the LMC. Conversely, if normal stars in the LMC cause the microlensing events, the source stars will be found toward the far side of the LMC. This subtle effect can be detected by taking Hubble Space Telescope images of the microlensing source stars. The MACHO project recently has completed this analysis.

"Our analysis has determined that it is very unlikely that microlensing is due to some strange population of source stars behind the LMC," said Nelson, a University of California at Berkeley graduate student, who works at Livermore’s Institute for Geophysics and Planetary Physics. "We can also say that it is somewhat unlikely that microlensing is due to any sort of spherical distribution of stars in a halo around the LMC. The most likely explanation remains that microlensing events are caused by dark matter MACHOs in the halo of the Milky Way."

It is generally impossible to measure the characteristics needed from ground-based data to determine the arrangement of source stars. The ground-based MACHO images taken on the Mount Stromlo Observatory are very crowded, causing several nearby stars in the LMC to blend together and appear as one blended "object" in the ground-based image. Only one of the stars in the microlensed object is actually lensed and thus the ground-based data provides little detail about the properties of the actual source star. To eliminate this confusion, Hubble Space Telescope data of the area surrounding each microlensing event was obtained. Using a technique known as difference-image analysis, it was then possible to identify the source star of each microlensing event.

Using the brightness and color of the source stars, the LLNL team determined the distribution of source stars in the LMC. They found no evidence that the source stars are not randomly distributed in the LMC.

Additionally, this analysis ruled out with high confidence (99 percent) any model in which all of the source stars are located behind the LMC disk. It also ruled out with some confidence (about 80 percent) models in which two thirds of the source stars were located behind the LMC disk.

Astronomers have long known that most of the matter in the universe is invisible. This dark matter gives off no light, yet can be detected through its gravitational interaction with other luminous forms of matter such as stars and galaxies. The dark matter pervades all space, both within galaxies and in the vast "empty" space between them.

A significant component of the dark matter must be made up of some sort of exotic elementary particle that has yet to be detected. However, a small fraction of the dark matter consists of baryons. Studies show that our universe holds several times more baryons than we can count as visible light in stars and galaxies.

The MACHO collaboration consists of scientists from the Lab (Kem Cook, Andrew Drake, Stuart Marshall, Cailin Nelson and Piotr Popowski); the University of Pennsylvania; the Australian National Supercomputing Facility; STScI; the Mount Stromlo Observatory; Bell Labs; University of Notre Dame; UC Santa Cruz; UC San Diego; Universidad Catolica; University of Washington; the European Southern Observatory; the University of Oxford; and McMaster University.