MACHO Project

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In astronomy, the MACHO Project was an observational search during 1992-1999 for dark matter around our Milky Way galaxy in the form of hypothetical Massive Compact Halo Objects (MACHOs), using the method of gravitational microlensing. It was one of three first-generation microlensing searches started in the early 1990s, the others being the independent EROS and OGLE projects. The MACHO project was carried out by a team of US and Australian astronomers; observations used the 1.27-metre (50-inch) telescope at the Mount Stromlo Observatory near Canberra, which was dedicated to the project full-time from 1992 until 1999. The project did not solve the dark matter problem, but placed important upper limits on the fraction of dark matter in MACHOs across a wide range of masses, and achieved several notable discoveries in the field of microlensing, and new results on several classes of variable stars.

Contents

Microlensing

If a compact object (which may be dark or bright) is located very close to the line of sight to a background star, then gravitational lensing causes the star to appear to split into two images on opposite sides of the compact object (or a complete ring if the alignment is almost perfect). Microlensing refers to the special case of lensing where the two images are too close to be seen as separate objects in a telescope, but the time-varying geometry (as the source and lens move) causes the apparent brightness (the sum of the two images) to vary with time; this variation has a characteristic shape which can be calculated theoretically. Microlensing is predicted and observed to be very rare, typically less than 1 star per million microlensed at any given moment in time, but it is a powerful technique because the effect relies only on the gravity of the lensing object, rather than its light: therefore it is sensitive to completely dark or very faint objects including black holes, substellar brown dwarfs and remnants of dead stars (e.g. old white dwarfs and neutron stars). For these reasons, microlensing is very useful for testing whether dark matter is made of such objects.

Observations

The 1.27-metre (50 in) telescope at Mt. Stromlo was refurbished for the project, [1] and equipped with a prime-focus wide-field corrector, a dichroic beamsplitter and a pair of 16 megapixel CCD cameras (between 1992 and 1995, this was the largest CCD system in astronomical use). The cameras imaged a 42 by 42 arcminute square field of sky in two colours (blue-green and red light) simultaneously. Every clear night, the system monitored dense star fields, up to 80 fields in the Large Magellanic Cloud or 100 fields in the Galactic Bulge on a regular basis, with each field observed from twice per night to weekly, depending on field priority and season. From this image data, light curves (brightness vs time) were constructed for over 8 million stars in the LMC, and 15 million stars in the Bulge; large computer searches were then run to find brightening events characteristic of microlensing, and also variable stars.

Results

The project made a number of notable discoveries documented in around 35 scientific papers published between 1993 and 2003: the most important results were firstly, upper limits on the contribution of MACHOs to the dark matter in the Milky Way: no more than xx percent can be composed of MACHOs between xx solar masses () and xx solar masses;[ vague ] [2] [3] secondly, confirmation that microlensing occurs as expected, based on large samples of events with the theoretically predicted properties.

The project also achieved several "firsts" in microlensing, including the following:

Later developments

The MACHO project completed observations in December 1999; after 2000 the telescope+camera system was then used for a supernova search, but this was cut short when the telescope was destroyed by the January 2003 Canberra bushfires.

Several other microlensing surveys continued, including the OGLE and MOA projects, with the main focus after 2000 changing towards microlensing as a method for detection of exoplanets. Microlensing planet searches are especially sensitive to low-mass exoplanets and those in fairly wide orbits (many astronomical units, beyond the snow-line), and are also the only way of detecting "orphan planets" ejected from their parent systems; so microlensing planet searches are complementary to the better-known radial velocity and transit methods. As of 2017 the new Korean Microlensing Telescope Network (KMTNet) is a new-generation microlensing survey. The future NASA NGRST space mission (planned launch c. 2027) includes a substantial microlensing planet survey as one of its key projects.

Related Research Articles

<span class="mw-page-title-main">Large Magellanic Cloud</span> Satellite galaxy of the Milky Way

The Large Magellanic Cloud (LMC) is a spiral satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (163,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal (c. 16 kiloparsecs (52,000 light-years) away) and the possible dwarf irregular galaxy called the Canis Major Overdensity. Based on the D25 isophote at the B-band (445 nm wavelength of light), the Large Magellanic Cloud is about 9.86 kiloparsecs (32,200 light-years) across. It is roughly one-hundredth the mass of the Milky Way and is the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy (M31), the Milky Way, and the Triangulum Galaxy (M33).

<span class="mw-page-title-main">Gravitational lens</span> Light bending by mass between source and observer

A gravitational lens is matter, such as a cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's general theory of relativity. If light is treated as corpuscles travelling at the speed of light, Newtonian physics also predicts the bending of light, but only half of that predicted by general relativity.

<span class="mw-page-title-main">Black dwarf</span> Theoretical stellar remnant

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently to no longer emit significant heat or light. Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe, no black dwarfs are expected to exist in the universe at the present time. The temperature of the coolest white dwarfs is one observational limit on the universe's age.

<span class="mw-page-title-main">Cepheid variable</span> Type of variable star that pulsates radially

A Cepheid variable is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period and amplitude.

A MAssive Compact Halo Object (MACHO) is a kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body that emits little or no radiation and drifts through interstellar space unassociated with any planetary system. Since MACHOs are not luminous, they are hard to detect. MACHO candidates include black holes or neutron stars as well as brown dwarfs and unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs. The term was coined by astrophysicist Kim Griest.

<span class="mw-page-title-main">Stardome Observatory</span> Astronomical observatory in New Zealand

Stardome Observatory is a public astronomical observatory situated in Maungakiekie/One Tree Hill Domain in Auckland, New Zealand.

<span class="mw-page-title-main">Mount Stromlo Observatory</span> Astronomical observatory of the Australian National University in Canberra

Mount Stromlo Observatory located just outside Canberra, Australia, is part of the Research School of Astronomy and Astrophysics at the Australian National University (ANU). Australia's oldest telescope and several others at the observatory were destroyed by bushfire in 2003.

<span class="mw-page-title-main">Dark matter halo</span> Theoretical cosmological structure

In modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matter halo may contain multiple virialized clumps of dark matter bound together by gravity, known as subhalos. Modern cosmological models, such as ΛCDM, propose that dark matter halos and subhalos may contain galaxies. The dark matter halo of a galaxy envelops the galactic disc and extends well beyond the edge of the visible galaxy. Thought to consist of dark matter, halos have not been observed directly. Their existence is inferred through observations of their effects on the motions of stars and gas in galaxies and gravitational lensing. Dark matter halos play a key role in current models of galaxy formation and evolution. Theories that attempt to explain the nature of dark matter halos with varying degrees of success include cold dark matter (CDM), warm dark matter, and massive compact halo objects (MACHOs).

<span class="mw-page-title-main">Gravitational microlensing</span> Astronomical phenomenon due to the gravitational lens effect

Gravitational microlensing is an astronomical phenomenon caused by the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light (stars) or large objects that block background light. These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light.

<span class="mw-page-title-main">South African Astronomical Observatory</span> Observatory

South African Astronomical Observatory (SAAO) is the national centre for optical and infrared astronomy in South Africa. It was established in 1972. The observatory is run by the National Research Foundation of South Africa. The facility's function is to conduct research in astronomy and astrophysics. The primary telescopes are located in Sutherland, which is 370 kilometres (230 mi) from Observatory, Cape Town, where the headquarters is located.

<span class="mw-page-title-main">RV Tauri</span> Star in the constellation Taurus

RV Tauri is a star in the constellation Taurus. It is a yellow supergiant and is the prototype of a class of pulsating variables known as RV Tauri variables. It is a post-AGB star and a spectroscopic binary about 4,700 light years away.

<span class="mw-page-title-main">Optical Gravitational Lensing Experiment</span> Long-term variability sky survey

The Optical Gravitational Lensing Experiment (OGLE) is a Polish astronomical project based at the University of Warsaw that runs a long-term variability sky survey (1992–present). The main goals are the detection and classification of variable stars, discovery of microlensing events, dwarf novae, and studies of the structure of the Galaxy and the Magellanic Clouds. Since the project began in 1992, it has discovered a multitude of extrasolar planets, together with the first planet discovered using the transit method (OGLE-TR-56b) and gravitational microlensing. The project has been led by professor Andrzej Udalski since its inception.

<span class="mw-page-title-main">Microlensing Observations in Astrophysics</span>

Microlensing Observations in Astrophysics (MOA) is a collaborative project between researchers in New Zealand and Japan, led by Professor Yasushi Muraki of Nagoya University. They use microlensing to observe dark matter, extra-solar planets, and stellar atmospheres from the Southern Hemisphere. The group concentrates especially on the detection and observation of gravitational microlensing events of high magnification, of order 100 or more, as these provide the greatest sensitivity to extrasolar planets. They work with other groups in Australia, the United States and elsewhere. Observations are conducted at New Zealand's Mt. John University Observatory using a 1.8 m (70.9 in) reflector telescope built for the project.

<span class="mw-page-title-main">Christopher Stubbs</span>

Christopher Stubbs is an experimental physicist currently on the faculty at Harvard University in both the Department of Physics and the Department of Astronomy. He is the current Dean of Science at Harvard University and a former Chair of Harvard's Department of Physics.

<span class="mw-page-title-main">Stellar pulsation</span>

Stellar pulsations are caused by expansions and contractions in the outer layers as a star seeks to maintain equilibrium. These fluctuations in stellar radius cause corresponding changes in the luminosity of the star. Astronomers are able to deduce this mechanism by measuring the spectrum and observing the Doppler effect. Many intrinsic variable stars that pulsate with large amplitudes, such as the classical Cepheids, RR Lyrae stars and large-amplitude Delta Scuti stars show regular light curves.

<span class="mw-page-title-main">Time-domain astronomy</span> Study of how astronomical objects change with time

Time-domain astronomy is the study of how astronomical objects change with time. Though the study may be said to begin with Galileo's Letters on Sunspots, the term now refers especially to variable objects beyond the Solar System. Changes over time may be due to movements or changes in the object itself. Common targets included are supernovae, pulsating stars, novas, flare stars, blazars and active galactic nuclei. Visible light time domain studies include OGLE, HAT-South, PanSTARRS, SkyMapper, ASAS, WASP, CRTS, GOTO and in a near future the LSST at the Vera C. Rubin Observatory.

<span class="mw-page-title-main">R Sagittae</span> Star in the constellation Sagitta

R Sagittae is an RV Tauri variable star in the constellation Sagitta that varies from magnitude 8.0 to 10.5 in 70.77 days. It is a post-AGB low mass yellow supergiant that varies between spectral types G0Ib and G8Ib as it pulsates. Its variable star designation of "R" indicates that it was the first star discovered to be variable in the constellation. It was discovered in 1859 by Joseph Baxendell, though classified as a semi regular variable until RV Tauri variables were identified as a distinct class in 1905.

<span class="mw-page-title-main">U Monocerotis</span> Variable star system in the constellation Monoceros

U Monocerotis is a pulsating variable star and spectroscopic binary in the constellation Monoceros. The primary star is an RV Tauri variable, a cool luminous post-AGB star evolving into a white dwarf.

The Eridanus II Dwarf is a low-surface brightness dwarf galaxy in the constellation Eridanus. Eridanus II was independently discovered by two groups in 2015, using data from the Dark Energy Survey. This galaxy is probably a distant satellite of the Milky Way. Eridanus II contains a centrally located globular cluster; and is the smallest, least luminous galaxy known to contain a globular cluster. Crnojević et al., 2016. Eridanus II is significant, in a general sense, because the widely accepted Lambda CDM cosmology predicts the existence of many more dwarf galaxies than have yet been observed. The search for just such bodies was one of the motivations for the ongoing Dark Energy Survey observations. Eridanus II has special significance because of its apparently stable globular cluster. The stability of this cluster, near the center of such a small, diffuse, galaxy places constraints on the nature of dark matter.

Dr. Sun Hong Rhie was a Korean–American astrophysicist best known for her foundational contributions to the theory of gravitational microlensing, a technique for the discovery of exoplanets.

References

  1. ↑ Hart, J.; van Hermelen, J.; Hovey, G.; Freeman, K. C.; Peterson, B. A.; Axelrod, T. S.; Quinn, P. J.; Rodgers, A. W.; Allsman, R. A.; Alcock, C.; Bennett, D. P.; Cook, K. H.; Griest, K.; Marshall, S. L.; Pratt, M. R.; Stubbs, C. W.; Sutherland, W. (1996). "The Telescope System of the MACHO Program". Publications of the Astronomical Society of the Pacific. 108: 220. Bibcode:1996PASP..108..220H. doi: 10.1086/133713 .
  2. ↑ Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T. S.; Becker, A. C.; Bennett, D. P.; Cook, K. H.; Dalal, N.; Drake, A. J.; Freeman, K. C.; Geha, M.; Griest, K.; Lehner, M. J.; Marshall, S. L.; Minniti, D.; Nelson, C. A.; Peterson, B. A.; Popowski, P.; Pratt, M. R.; Quinn, P. J.; Stubbs, C. W.; Sutherland, W.; Tomaney, A. B.; Vandehei, T.; Welch, D. (10 October 2000). "The MACHO Project: Microlensing Results from 5.7 Years of Large Magellanic Cloud Observations". The Astrophysical Journal. 542 (1): 281–307. arXiv: astro-ph/0001272 . Bibcode:2000ApJ...542..281A. doi:10.1086/309512. S2CID   15077430.
  3. ↑ Alcock, C.; Allsman, R. A.; Alves, D.; et al. (20 May 1998). "EROS and MACHO Combined Limits on Planetary-Mass Dark Matter in the Galactic Halo". The Astrophysical Journal. 499 (1): L9–L12. arXiv: astro-ph/9803082 . Bibcode:1998ApJ...499L...9A. doi:10.1086/311355. S2CID   123466020.
  4. ↑ Alcock, C.; Akerlof, C. W.; Allsman, R. A.; Axelrod, T. S.; Bennett, D. P.; Chan, S.; Cook, K. H.; Freeman, K. C.; Griest, K.; Marshall, S. L.; Park, H-S.; Perlmutter, S.; Peterson, B. A.; Pratt, M. R.; Quinn, P. J.; Rodgers, A. W.; Stubbs, C. W.; Sutherland, W. (14 October 1993). "Possible gravitational microlensing of a star in the Large Magellanic Cloud". Nature. 365 (6447): 621–623. arXiv: astro-ph/9309052 . Bibcode:1993Natur.365..621A. doi:10.1038/365621a0. S2CID   4233746.
  5. ↑ MACHO Collaboration. "IAU Circular 6068".
  6. ↑ Alcock, C.; Allsman, R. A.; Alves, D.; Axelrod, T. S.; Bennett, D. P.; Cook, K. H.; Freeman, K. C.; Griest, K.; Guern, J.; Lehner, M. J.; Marshall, S. L.; Peterson, B. A.; Pratt, M. R.; Quinn, P. J.; Rodgers, A. W.; Stubbs, C. W.; Sutherland, W. (1 December 1995). "First Observation of Parallax in a Gravitational Microlensing Event". The Astrophysical Journal. 454 (2): L125. arXiv: astro-ph/9506114 . Bibcode:1995ApJ...454L.125A. doi:10.1086/309783. S2CID   119035972.
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