Feature

MOA Seeks Macho Companions

Astronomers are using Mt John Observatory to search for cold, dark matter, variable stars and supernovae.

Richard Dodd, John Hearnshaw, Denis Sullivan and Phil Yock

A group of New Zealand and Japanese astronomers and physicists have joined forces to carry out measurements from the Mt John University Observatory in Canterbury on dark matter, supernovae and planets orbiting distant stars in the galaxy.

The existence of dark matter in the universe has been inferred since the 1930s from studies of the motions of stars in our galaxy and of galaxies within clusters of galaxies. Theoretical analyses of the production of light elements in the first few minutes following the Big Bang, and the subsequent formation of galaxies, also suggest the presence of dark matter.

It is currently thought that some 90% of the matter in the universe may be dark. What form does this dark matter take? There are two popular candidates: WIMPs and MACHOs. The former are Weakly Interacting Massive Particles, which could include neutrinos or as yet undiscovered particles such as axions' or other hypothetical objects. The latter are MAssive Compact Halo Objects such as small stars (brown dwarfs') in the outskirts of the galaxy that shine too feebly to be seen directly, or smaller planet sized objects, or even black holes.

The New Zealand-based Microlensing Observations in Astrophysics programme (MOA) is to concentrate its study on MACHOs. Our monitoring telescope is the 61cm aperture Boller and Chivens based at the Mt John Observatory of the University of Canterbury near Lake Tekapo. Last year this telescope underwent a major modernisation with a computer-controlled acquisition and tracking system being fitted. This year new optics, designed by Norman Rumsey and manufactured by Garry Nankivell, were made for the telescope to give it a much wider field of view, roughly twice the apparent size of the full moon.

A CCD (Charge Coupled Device) electronic camera constructed by the National Astronomical Observatory of Japan was delivered in late 1995 and is now undergoing tests on the telescope. This camera is one of the most powerful of its type in the world with nine individual one-megabyte chips totalling in all nine million picture elements or pixels (for comparison, a standard computer monitor might have around 500,000 pixels).

What are we looking for? Albert Einstein, in a paper published in Science in 1936, showed that light from a distant star will apparently increase in brightness, as seen by an observer on Earth, if it traverses the gravitational field of a nearer star. For this effect to be observable, the stars in question need to be very closely aligned in the observer's line of sight. The nearer of the two bodies need not be another star but could be a planet-sized object or even a black hole.

Gravitational Micro-Lensing

This phenomenon, known as gravitational micro-lensing is likely to be very rare due to the stringent requirement of two astronomical bodies being lined up along the observer's line of sight. The odds of making such an observation may be dramatically improved by making the observations against a very dense background of distant stars. For this purpose, a satellite galaxy of the Milky Way known as the Large Magellanic Cloud (LMC) will be used, as it is the nearest external galaxy and it forms an ideal background of distant but resolvable stars. Mt John is very well situated for observations of this galaxy -- hence the interest of Japanese astronomers.

The observing sequence will be to image three adjacent star fields covering, in total, 3.25 square degrees of sky several times through red and blue glass filters every night. Each frame (picture) containing around one million images will be reduced using sophisticated image processing software. Comparisons can then be made of the brightness of each image through a time sequence of frames to look for images that vary in brightness between frames.

Massive dark bodies in our galaxy are in orbit about the centre of our galaxy and so move relative to the distant background of, in this case, stars in the Large Magellanic Cloud. Passing very close to the line of sight of an LMC star, such bodies will cause a temporary increase in the brightness of the distant star. The way in which the light varies is peculiar to such a gravitational lensing effect and is quite distinct from objects such as pulsating stars, eclipsing binary stars or novae.

This leads to an obvious by-product of the dark matter search, namely the discovery of previously unknown variable stars. Other groups conducting similar programmes have begun to produce catalogues of such stars.

Planet Searchers

Using the same equipment and observing the central bulge of our own galaxy, opens up the possibility, via gravitational lensing, of detecting planets orbiting distant stars thousands of light years away in the direction of the galactic centre. The light variation signature of a star plus planet is significantly different from that of a star alone. This work complements other techniques that are being followed to search for planets orbiting stars within a few light years of Earth.

These latter searches have recently produced positive and interesting results. Claims have been made for detections of two extra-solar planetary systems that are similar to ours, and for a third that is quite different. Further studies will yield invaluable information on the physical processes that are involved in the formation of planetary systems. Comparison of the results from the above studies of neighbouring systems, and the studies of distant systems by MOA, may enable us to determine if we live in a region of the galaxy with conditions that are especially favourable for the formation of planets.

Supernova Watch

The final MOA monitoring programme is that of a selected group of clusters of galaxies. These will be searched for supernovae. A supernova is a spectacular outburst of energy caused by the explosive disruption of a massive star in the final stages of its evolution. This outburst can equal in energy the total output of the galaxy to which the supernova belongs. Such explosions occur approximately once per century per galaxy.

Again to improve the detection odds we shall observe a large number of galaxies per frame. We have selected eight clusters of galaxies each containing around 100 galaxies with diameters that are approximately one degree. All the clusters are visible on any clear night from Mt John as, being close to the south pole of the sky, they never set. In addition to the cluster galaxies, there are several hundred fainter field galaxies also visible in each frame. For the cluster galaxies alone the probability is that a new supernova should be discovered every few weeks!

A careful study of the way in which the light from the supernova fades can lead to important information about the distance of the parent galaxy and as supernovae are amongst the brightest objects observable in the universe they can be seen to its limits. Supernovae may well prove vital in resolving the current cosmological difficulty caused by some theoretical models predicting a universe that is younger than its oldest stars! A measurement by MOA of the expansion of the universe in the southern direction can confirm the rate of expansion is the same as in other directions.

A detailed investigation of the morphology (shapes) and photometry (brightnesses) of the individual galaxies in the eight clusters to be monitored is underway using output from photographic plate scans made by the COSMOS automated measuring machine at the Royal Observatory Edinburgh. This will be of value in checking for possible relationships between how bright a supernova becomes and what shape and brightness its parent galaxy has.

So two out of three of our monitoring programmes have cosmological significance. The search for dark matter will provide data on the large scale structure of the universe and may even yield answers to the questions: Is the universe open or closed and will it expand forever or eventually begin to contract? The supernova study will help determine the size and age of the universe.

The planet search programme will, it is hoped, produce information on the planetary formation process, and a comparison of the frequency of planets in close and distant regions of the Galaxy.

The MOA project is funded by the Marsden Fund of the Royal Society of New Zealand, the NZ/Japan Foundation, the Department of Education of Japan, the Lottery Science Board of NZ, and various Universities and Laboratories in NZ and Japan.

Richard Dodd is the Principal Astronomer at the Carter Observatory and is the Principal Investigator for the supernova programme of the MOA project.
John Hearnshaw is Professor of Astronomy at the University of Canterbury and Principal Investigator for the planet search programme of the MOA project.
Denis Sullivan is Reader in Physics at the Victoria University of Wellington and Principal Investigator for the variable star search programme of the MOA project
Associate Professor Phil Yock is a particle physicist at the University of Auckland, and the overall group leader for the MOA project in New Zealand.