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Searching for MACHOs

John G. Cramer

Alternate View Column AV-65
Keywords: dark matter problem Jupiter brown dwarf gravitational lensing light enhancement
Published in the May-1994 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 11/06/93 and is copyrighted ©1993 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

 

    On a mountain top on a clear moonless night the brilliant stars strewn across the sky press down almost oppressively, the Milky Way so full of them it seems about to burst. And yet, we have been learning in the past decade that the visible matter of the universe, the stars that we see, represent only a tiny fraction, perhaps less than one part in 200, of the mass of the universe. The question of what the remainder of the universe is made of is called the Dark Matter Problem. It is one of the most vexing questions in modern physics.

    Since the dark matter problem came to prominence in scientific thinking there has been no shortage of speculations as to its origins. Theorists have suggested that the extra mass might be in the form of massive neutrinos [see my AV columns in the 12/91, and 9/92 issues of Analog], axions [see my AV column in the 2/85 Analog], weakly interacting massive particles or WIMPS [see my AV column in the 5/86 Analog], or a host of other could-be particles. One unresolved possibility is that some part of dark matter could be simply an excess of normal matter in non-luminous forms. Cometary halos, Jupiter-like planets, and small dim stars may be more numerous that we estimate. This possibility presents a problem: if there is more non-luminous normal matter in the universe than expected, how could it be detected?

    Fortunately, there is a feasible detection scheme. When light passes near a massive jupiter- or star-size object, it will be deflected slightly by the object's gravitational field. If the alignment of object, light source, and observer is a perfect straight line, the "gravitational lens" formed by this deflection produces a ring image called an "Einstein Ring" [see my AV column "The Rainbows of Gravity" in the 11/88 Analog], a halo of light centered around the imaged star. Such perfect alignment is very improbable, and only a few Einstein Rings have ever been observed.

    A much more probably scenario is that a massive non-luminous objects might pass close to the line-of-sight to a distant star and brighten its pointlike image by deflecting more light toward the observer. This phenomenon is called "gravitational micro-lensing", and it can brighten a stellar image by up to factor of up to 10 or so if the conditions are right. Micro-lensing can be used to detect the presence of a massive non-luminous object as it brightens for a time the image of a background star in passing across the line of sight.

    There are, of course, many other reasons why a star can become brighter: eclipsing binaries, Cephid variables, solar flares, novas, supernovas, etc. The signatures of a micro-lensing event, as opposed to these other phenomena, are that (a) it will be independent of the type of star brightened, (b) the light-increase curve must have the same shape on the rising side and the falling side of the curve, (c) the expected time duration of such stellar brightening should be on the order of a few days to a month, and (d) the increase must be the same in shape and magnitude at all light wavelengths.

    Gravitational micro-lensing events are expected to be relatively rare. It is estimated that at any given evening of observation, the image of only about one star in two million will be observably increased in intensity by gravitational micro-lensing. For this reason, it is necessary to scan the images of a very large number of stars very efficiently in order to have any reasonable chance of seeing this phenomena.

    Three separate groups of physicists and astronomers have been seeking dark matter objects using the gravitational micro-lensing technique. These groups are the MACHO collaboration in the USA and Australia , EROS in France, and OGLE in Poland. MACHO is an acronym for MAssive Compact Halo Objects, and EROS stands for Expérience de Recherche d'Objets Sombre, all of the groups perhaps distinguished by their politically incorrect acronyms.

    I don't have the space to discuss in detail the techniques used by each group to observe the brightening of background stars, so I'll focus on the method used by the MACHO collaboration. They have decided to use the stars of the Large Magellanic Cloud (LMC) for their background because it is a relatively dense population of stars viewed through the halo of our own galaxy. The LMC is only visible in the southern hemisphere, so the collaboration has rehabilitated a venerable 19th century telescope, the 1.27 meter refracting telescope at the Mt. Stromlo Observatory in Australia, for their MACHO search. On each night with good "seeing" they scan the LMC in pre-programmed blocks using two very large charge-coupled-device imaging systems with about 16 million pixels each, one observing blue wavelengths and one observing red. On a good night of observation they record about 4-5 gigabytes of star-image data, which are then processed with sophisticated imaging software to identify star images that change in intensity. As of September 15, 1993, they had recorded over 12,000 wide-field images with the electronic imaging system. So far they have been concentrating on images near the center of the Large Magellanic Cloud, scanning about 1.8 million stars and analyzing about 250 images of each star taken over a period of one year. The EROS and OGLE groups use more conventional photographic plate imaging systems, with the photographic images then scanned and analyzed with computer software.

    The October 14, 1993 issue of Nature presented the first results from the MACHO and EROS searches. The MACHO group reports what appears to me a definitive micro-lensing event. A magnitude 19 star in the LMC (believed to be a "clump giant" metal-rich helium core burning star) shows no variation for most of the year of observation, but during a 33.9 ± 0.26 day period it shows an excursion in which the light output is increased by a factor of 6.89 ± 0.11. The event has the characteristic shape predicted for a micro-lens event, and shows this shape in both the red and blue imaging systems with the same magnitude, time position, and time width. The group has also observed 10 other less striking events that show light increases by a factor of 1.3 or more.

    In the same issue of Nature the EROS group reports their results. They have been monitoring 8 million stars in the LMC over a 3 year period by taking 304 plates with red or blue filters in a 5o 5o field of view, using the Schmidt telescope at the European Southern Observatory at La Scilla, Chile. The EROS group is also conducting a second program involving fast electronic imaging for seeking microlensing events with a time scales of hours or days.

    The EROS group reports two candidate events observed for background stars in the LMC, one showing an increase in brightness by a factor of 2.5 ± 0.3 and a duration of 27 ± 2 days and the other with an increase factor of 3.0 ± 0.6 and a duration of 30 ± 3 days. The data for these events are well fitted in both the red and blue images with increase curves predicted by micro-lensing.

    The EROS group also reports partial observation of the event reported by the MACHO group. The image of the star of interest is too weak for analysis using their blue plates, but it shows the reported characteristics in their red plates. The MACHO group, unfortunately, was not able to confirm the EROS results because one of the EROS events occurred outside their observation period and the image of the other EROS event fell in a gap between their CCD detectors. In a recent issue of Acta Astronomica, the OGLE group in Poland has also reported observation of an apparent micro-lensing event with an increase factor of 2.4 and a duration of 24 days observed in a background star in the "bulge" of our own galaxy.

    Because the massive object that has produced a micro-lensing event is not observed directly, estimates of the size of the object are correlated with estimates of its distance from the observer. For this reason, only a rather broad range of possible object masses can be extracted from the observation of individual microlensing events, no matter how good the data is. The MACHO collaboration estimates that the mass of the object producing their event, as inferred from the size and duration of the event, is between 0.03 and 0.5 times the mass of our sun, with a most probable value of about 0.12 solar masses. For reference, Earth has a mass of 3 x 10-6 solar masses, and Jupiter has a mass of about .001 solar masses.

    A star with a mass of 0.12 solar masses is not, strictly speaking, a "dark matter" object. For example, one of the stars in our galactic neighborhood with a mass of about 0.12 solar masses is Kreuger 60 A, a class M4 star about 12.8 light years from Earth. It is rather dim because of its small mass, with a visual magnitude of 9.7 and a net light output only about 0.17% that of our sun. We can observe Kreuger 60 A only because it is close. If it were about half way to the LMC, as is the MACHO object observed, it would not be visible. The significance of the gravitational micro-lensing technique is that, given sufficient statistics, it allows astrophysicists to "take the census" of objects in the halo of our galaxy that are too dim to be observed with conventional astronomical techniques.

    The three groups mentioned above are so far observing micro-lensing events at a rate expected if the extra mass in the halo of our galaxy deduced from the velocities of bright stars there were accounted for completely by MACHO objects, i.e., dim clumps of normal matter. It is also consistent with estimates from Big Bang cosmology of the density of normal matter required to synthesize deuterium, helium-3, helium-4, and lithium-6 in the early Big Bang. However, so far only a few events have been observed. Much better statistics will be required before meaningful conclusions about cosmology and the dark matter problem can be drawn from these data.

    Even if these early indications are supported by better statistics, however, only a part of the dark matter problem would be solved. There would remain the problems associated with the excess of mass present on galactic clusters, the excess of mass required to congeal the universe after the Big Bang to form the galaxies and galactic clusters we observe, and the excess of mass needed by cosmologists to reconcile our universe with the inflationary scenario of the Big bang, which requires a density 8 times larger that the most optimistic estimate of MACHO-contributed densities would imply.

    We may have been able to account for one kind of dark matter, but there remains at least a second variety which remains elusive and mysterious. Perhaps part of the universe is MACHOs and another part is WIMPs.


References:

MACHO and Dark Matter Overview:
C. J. Hogan, "In search of the halo grail", Nature 365, 602-603 (1993).

Microlensing Predictions:
B. Paczynski, Astrophysics Journal 304, 1-5, (1986).

MACHO Observations:
C. Alcock, et al., "Possible gravitational microlensing of a star in the Large Magellanic Cloud", Nature 365, 621-623 (1993).
E. Auburn, et al., "Evidence for gravitational microlensing by dark objects in the Galactic halo", Nature 365, 623-625 (1993).


SF Novels by John Cramer:  my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .

AV Columns Online: Electronic reprints of about 177 "The Alternate View" columns by John G. Cramer, previously published in Analog , are available online at: http://www.npl.washington.edu/av.


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