The findings were made public today in a paper delivered by David Bennett, a MACHO team scientist associated with the Lawrence Livermore National Laboratory and the National Science Foundation's Center for Particle Astrophysics, at the American Astronomical Society's national meeting here. The paper was based on analysis of data collected over a two-year period.
The analysis of the first-year data focused on searching for MACHOs ranging from the mass of Jupiter to one-tenth the mass of the Sun. This mass range (0.001-0.1 solar mass) includes objects known as brown dwarfs which were considered the type of MACHO most likely to comprize the dark matter halo.
In fact, only three "microlensing" events were observed the first year, too few to support the hypothesis that the bulk of the dark matter is made up on brown dwarfs. Microlensing is the term given to the apparent brightening of a star caused by the gravitational field of a massive object passing almost directly in front of it. The brightening effect is caused by the bending of light rays by gravity.
An additional year of data and improved analyses allowed the MACHO team to extend the sensitivity of their dark matter search down to objects less massive than the earth and up to objects as massive as the Sun (0.000001-1.0 solar mass). The improved analysis yielded a total 7 microlensing events from both years that appear to be caused by more massive objects, most likely between 0.1 and 1.0 solar mass.
Calculating a hypothetical percentage of dark matter in the halo of the Milky Way is a function of both the number of objects detected and the mass of those objects. Thus, even though the number of microlensing events observed in the first year and second year remained roughly constant, the estimate of dark matter in the halo was increased to approximately 50 percent because the seven events seen in the second year analysis represented - on average - more massive objects than the events seen in the first year analysis.
"The new events have an average timescale of more than 2.5 months, which puts the estimated mass of the lensing objects in the range of white dwarfs - stars which long ago exhausted their nuclear fuel," said Bennett. "The events reported last year had an average timescale of less than a month, which was consistent with lensing by brown dwarfs - objects which are not massive enough to shine like stars."
Following discovery of the first microlensing events in 1993, several authors proposed that the microlensing events seen towards the Large Magellanic Cloud might not be due to dark matter. They suggested that the lenses might be faint stars in the Milky Way disk or the Large Magellanic Cloud itself. Based on its own calculations, the MACHO team expected to see approximately one lensing event due to a faint star, when in fact seven events have been detected, a significant excess.
"The simplest hypothesis is that we have detected a portion of the halo dark matter, but there may be other models that can explain these results," said Bennett.
One of the newly discovered lensing events is an exotic binary microlensing event in which the lens consists of two objects in orbit about each other. This event should offer researchers an opportunity to learn much more about the location and masses of the lensing objects, if detailed light curves can be obtained.
Earth is located near the edge of the Milky Way, a disk of stars that is more than 100,000 light years across. The Milky Way (like other spiral galaxies) is rotating about its center. Astronomers have found that stars - visible matter - account for only a small portion of the matter necessary to keep galaxies from flying apart.
Indeed, 90 percent or more of gravitational mass in the Milky Way is unaccounted for. In the last two decades, the existence of this galactic dark matter has become widely accepted, but this has given rise to a new question: what form does this unseen matter take? Theories range from hypothetical subatomic particles to MACHOs. The latter term is a generic name for any massive ordinary object that is dark. The possibilities include black holes, planet-like objects, white dwarfs and neutron stars.
The MACHO team's current findings do not preclude the possibility that halo dark matter is composed entirely of MACHOs.
"The uncertainties in our results are large enough to be consistent with a dark halo composed entirely of MACHOs, if the halo has somewhat less mass than is usually supposed." explains Bennett. "It is also possible that more MACHOs will be discovered when we extend our mass sensitivity to masses larger than that of the Sun."
To probe the halo, the MACHO team aims its telescope toward a satellite galaxy of the Milky Way known as the Large Magellanic Cloud. Only visible in the southern sky, the Large Magellanic Cloud is beyond much of the dark halo surrounding the Milky Way. Thus, from the platform of earth, astronomers can use the stars in the Large Magellanic Cloud as a backdrop in front of which objects in the dark halo of the Milky Way can be viewed as they pass. Figure 1 shows the geometry of a microlensing event as well as photographs of the Large Magellanic Cloud and the Great Melbourne Telescope at the Mount Stromlo Observatory near Canberra, Australia.
The collaboration monitors 9 million stars in the Large Magellanic Cloud with two sophisticated camera systems, each of which contains 16 million picture elements. Images of these 9 million stars are taken nightly and processed by computer to obtain brightness measurements of each of these stars. These brightness measurements are then assembled into lightcurves that show how the brightness of each star varies with time. Figure 2 is a portion of the lightcurve for a microlensing event that was observed to brighten by more than a factor of 40.
The team has also searched for MACHOs with masses in the range of the planets in our solar system, and these results have been reported in a paper by team member Matt Lehner of the University of California, San Diego and the Center for Particle Astrophysics. No short timescale events consistent with lensing by planetary-mass MACHOs have been detected, and this has allowed the MACHO team to set tight constraints on the abundance of low mass MACHOs. The new MACHO limits are somewhat lower than the limits set by the EROS group, a French microlensing search team.
MACHO team member Mark Pratt of the University of Washington and Center for Particle Astrophysics presented results of the MACHO Global Microlensing Alert Network, which has detected and monitored more than 40 events toward both the Galactic center and the Large Magellanic Cloud. (The MACHO project looks for microlensing by faint stars toward the Galactic center, in addition to the halo, to test the microlensing hypothesis and to better determine the structure of the inner parts of the Galaxy.)
Two of these events have shown behavior which provides additional clues about the MACHOs involved. One appears to be microlensing by a binary MACHO and the other event is the microlensing of a giant star by a lensing object which transits the face of the star, the first detection of this effect. This rare alignment provides an accurate measurement of the angular speed of the MACHO and thus removes some of the confusion about its mass and location. The binary microlens is the third such event found in the MACHO data and the first witnessed in progress.
Individual microlensing events normally do not provide unambiguous information about the Macho involved. Typically only a single number can be measured for each event and this is a combination of the MACHO's mass, speed and position.
"It is possible, with high quality data and a little luck, to obtain additional information about individual events. MACHO-GMAN enables us to improve substantially our chances of detecting and characterizing these exotic microlensing events, and this will allow us to learn more about the lensing objects," said Pratt. "A more ambitious version of the GMAN will even enable us to detect Earth-mass planets around faint stars in the disk of the Galaxy." The GMAN includes astronomers and telescopes in Australia, Chile, New Zealand and Israel in addition to the MACHO team.
Intrinsic variable stars in the Large Magellanic Cloud are a background that must be characterized in order to detect true microlensing events. MACHO has catalogued thousands of variable stars including Cepheids, RR Lyraes, eclipsing binaries, and what appear to be an entirely new class of blue variable stars.
David Alves of University of California, Davis and the Lawrence Livermore National Laboratory reports on the Cepheids discovered by MACHO, "The non-dark matter science coming out of MACHO is a goldmine. We've more than doubled the number of Cepheids known in the Large Magellanic Cloud. These MACHO data are critical for the complete understanding of Cepheid variables that serve as important yardsticks for measuring the size of the universe."
The MACHO collaboration, which consists of 18 researchers from eight different institutions, is funded by the Department of Energy's Lawrence Livermore National Laboratory; the National Science Foundation through the Center for Particle Astrophysics (headquartered at UC Berkeley); and the Australian National University at the Mount Stromlo and Siding Spring Observatories. The search for dark matter will continue for at least four more years.
For further information, contact:
Dr. David Bennett Lawrence Livermore Nat'l Laboratory (510) 423-0656 bennett@igpp.llnl.gov Professor Kim Griest University of Calif., San Diego (619) 534-0924 griest@astrophys.ucsd.edu Professor Christopher Stubbs Center for Particle Astrophysics University of California, Santa Barbara and University of Washington (206) 543-9375 Professor Alex Rodgers Mt. Stromlo Observatory Australian National University 011+61+6+249-0262 alex@merlin.anu.edu.au Dr. Kem Cook Lawrence Livermore Nat'l Laboratory (510) 423-4634 kcook@igpp.llnl.gov Dr. Will Sutherland Oxford University 011+44+1865 273340 w.sutherland@physics.ox.ac.ukThe MACHO Project home page may be found at the URLs:
The MACHO collaboration consists of Charles Alcock, Robyn Allsman, David Alves, Timothy Axelrod, Andrew Becker, David Bennett, Kem Cook, Ken Freeman, Kim Griest, Jerry Guern, Mathew Lehner, Stuart Marshall, Bruce Peterson, Mark Pratt, Peter Quinn, Alexander Rodgers, Christopher Stubbs, William Sutherland and Doug Welch.
Participating institutions include Lawrence Livermore National Laboratory, Livermore, Calif.; Center for Particle Astrophysics, University of California, Berkeley, Calif.; Supercomputing Facility, Australian National University, Canberra, Australia; Mt. Stromlo and Siding Spring Observatories, Australian National University, Weston, Australia; Department of Physics, University of California, San Diego, Calif; Department of Physics, University of California, Santa Barbara, Calif; Departments of Astronomy and Physics, University of Washington, Seattle, Washington; Department of Physics, University of Oxford, U.K.
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Figure 1: This diagram shows the geometry of the microlensing events observed by the MACHO Collaboration. The target stars reside in the Large Magellanic Cloud, and a microlensing event can be observed when a MACHO in the Milky Way's dark matter halo happens to pass very close to the line of sight to a target star. The light rays are bent slightly by the MACHO's gravity which causes the star to appear brighter from the Earth. Also shown is the Great Melbourne Telescope at the Mount Stromlo Observatory which is used for the collaboration's observations. For further information, please contact David Bennett at (510) 423-0656 (bennett@igpp.llnl.gov) or Kem Cook at (510) 423-4634 (kcook@igpp.llnl.gov).
Figure 2: The lightcurve of a microlensing event observed by the MACHO collaboration is plotted as a function of time. The MACHO data is indicated by the dots with error bars while the solid curve is the best fit theoretical light curve. This particular event was observed to brighten by more than a factor of 40 which is currently the largest brightening ever observed in a microlensing event.