Hubble Rules Out a Leading Explanation for Dark Matter
Until now, the dim, small stars were considered ideal candidates for dark matter. Whatever dark matter is, its gravitational pull ultimately will determine whether the universe will expand forever or will someday collapse. Picking a region in our Milky Way Galaxy, astronomers predicted that Hubble should have spied 38 red dwarf stars if this class of objects harbored most of the dark matter. The diamond-shaped symbols in the left-hand image illustrate what scientists expected to see. Instead, they saw no stars.
Two teams of astronomers, working independently with NASA's Hubble Space Telescope (HST), have ruled out the possibility that red dwarf stars constitute the invisible matter, called dark matter, believed to account for more than 90 percent of the mass of the universe.
Until now, the dim, small stars were considered ideal candidates for dark matter. Whatever dark matter is, its gravitational pull ultimately will determine whether the universe will expand forever or will someday collapse.
"Our results increase the mystery of the missing mass. They rule out a popular but conservative interpretation of dark matter," said Dr. John Bahcall, professor of natural science at the Institute of Advanced Study, Princeton, NJ, a leader of one of the teams.
The group led by Bahcall and Andrew Gould of Ohio State University, Columbus, Ohio, (formerly of the Institute for Advanced Study) showed that faint red dwarf stars, which were thought to be abundant, actually are sparse in the Milky Way, Earth's home galaxy, and in the universe by inference.
The team, led by Dr. Francesco Paresce of the Space Telescope Science Institute in Baltimore, MD, and the European Space Agency, determined that the faint red stars rarely form and that there is a cutoff point below which nature does not make this type of dim, low-mass star.
The pair of HST observations involved accurately counting stars and gauging their brightness. The observations overturn several decades of conjecture, theory and observation about the typical mass and abundance of the smallest stars in the universe.
PREVIOUS GROUND-BASED RESULTS INCONCLUSIVE
In our own stellar neighborhood, there are almost as many red dwarfs as there are all other types of stars put together. The general trend throughout our galaxy is that small stars are more plentiful than larger stars, just as there are more pebbles on the beach than rocks. This led many astronomers to believe that they were only seeing the tip of the iceberg and that many more extremely faint red dwarf stars were at the limits of detection with ground-based instruments.
According to stellar evolution theory, stars as small as eight percent of the mass of our Sun are still capable of shining by nuclear fusion processes.
Over the past two decades, theoreticians have suggested that the lowest mass stars also should be the most prevalent and, so, might provide a solution for dark matter. This seemed to be supported by previous observations with ground-based telescopes that hinted at an unexpected abundance of what appeared to be red stars at the faintest detection levels achievable from the ground.
However, these prior observations were uncertain because the light from these faint objects is blurred slightly by Earth's turbulent atmosphere. This makes the red stars appear indistinguishable from the far more distant, diffuse-looking galaxies.
PINNING DOWN THE LONG-SOUGHT HALO POPULATION
Hubble's capabilities made it possible for a team of astronomers led by Bahcall and Gould to observe red stars that are 100 times dimmer than those detectable from the ground - a level where stars can be distinguished easily from galaxies. Hubble Space Telescope's extremely high resolution also can separate faint stars from the much more numerous galaxies by resolving the stars as distinct points of light, as opposed to the "fuzzy" extended signature of a remote galaxy.
Bahcall and Gould, with their colleagues Chris Flynn and Sophia Kirhakos (also of the Institute for Advanced Study, Princeton) used images of random areas in the sky taken with the HST Wide Field Planetary Camera 2 (in WF mode) while the telescope was performing scheduled observations with other instruments. By simply counting the number of faint stars in the areas observed by HST, the scientists demonstrated that the Milky Way has relatively few faint red stars.
The HST observations show that dim red stars make up no more than six percent of the mass in the halo of the Galaxy, and no more than 15 percent of the mass of the Milky Way's disk. The Galactic halo is a vast spherical region that envelopes the Milky Way's spiral disk of stars, of which Earth's Sun is one inhabitant.
FAINT RED STARS MISSING FROM A GLOBULAR CLUSTER
By coincidence, Paresce pursued the search for faint red dwarfs after his curiosity was piqued by an HST image taken near the core of the globular cluster NGC 6397. He was surprised to see that the inner region was so devoid of stars, he could see right through the cluster to far more distant background galaxies. Computer simulations based on models of stellar population predicted the field should be saturated with dim stars - but it wasn't.
HST's sensitivity and resolution allowed Paresce, and co-investigators Guido De Marchi (ST ScI, and the University of Firenze, Italy), and Martino Romaniello (University of Pisa, Italy) to conduct the most complete study to date of the population of the cluster (globular clusters are ancient, pristine laboratories for studying stellar evolution). To Paresce's surprise, he found that stars 1/5 the mass of our Sun are very abundant (there are about 100 stars this size for every single star the mass of our Sun) but that stars below that range are rare. "The very small stars simply don't exist, " he said.
A star is born as a result of the gravitational collapse of a cloud of interstellar gas and dust. This contraction stops when the infalling gas is hot and dense enough to trigger nuclear fusion, causing the star to glow and radiate energy.
"There must be a mass limit below which the material is unstable and cannot make stars," Paresce emphasizes. "Apparently, nature breaks things off below this threshold."
Paresce has considered the possibility that very low-mass stars formed long ago but were thrown out of the cluster due to interactions with more massive stars within the cluster, or during passage through the plane of our Galaxy. This process would presumably be common among the approximately 150 globular clusters that orbit the Milky Way. However, the cast-off stars would be expected to be found in the Milky Way's halo, and Bahcall's HST results don't support this explanation.
THE SEARCH FOR DARK MATTER
The HST findings are the latest contribution to a series of recent, intriguing astronomical observations that are struggling to pin down the elusive truth behind the universe's "missing mass."
Models describing the origin of helium and other light elements during the birth of the universe, or "Big Bang," predict that less than 5% of the universe is made up of "normal stuff," such as neutrons and protons. This means more than 90% of the universe must be some unknown material that does not emit any radiation that can be detected by current instrumentation. Candidates for dark matter include black holes, neutron stars and a variety of exotic elementary particles.
Within the past year, astronomers have uncovered indirect evidence for a dark matter candidate called a MACHO (MAssive Compact Halo Objects). These previous observations detected several instances of an invisible object that happens to lie along the line of sight to an extragalactic star. When the intervening object is briefly aligned between Earth and a distant star, it amplifies, or gravitationally lenses, the light from the distant star.
The new HST finding shows that faint red stars are not abundant enough to explain the gravitational lensing events attributed to MACHOs. Bahcall cautions, however, that his results do not rule out other halo objects that could be smaller than the red stars such as brown dwarfs - objects not massive enough to burn hydrogen and shine in visible light.
Additional circumstantial evidence for dark matter in the halo of our galaxy has been inferred from its gravitational influence on the motions of stars within the Milky Way's disk.
Recently, this notion was further supported by ground-based observation, made by Peggy Sachett of the Institute for Advanced Study, that show a faint glow of light around a neighboring spiral galaxy that is the shape expected for a halo composed of dark matter. This could either be light from the dark matter itself or stars that trace the presence of the galaxy's dark matter.
The reality of dark matter also has been inferred from the motions of galaxies in clusters, the properties of high-temperature gas located in clusters of galaxies and from the relative amounts of light elements and isotopes produced in the Big Bang.
The ultimate fate of the universe will be determined by the amount of dark matter present. Astronomers have calculated that the amount of matter - - planets, stars and galaxies - observed in the universe cannot exert enough gravitational pull to stop the expansion which began with the Big Bang. Therefore, if the universe contains less than a critical density of matter it will continue expanding forever, but if enough of the mysterious dark matter exists, the combined gravitational pull someday will cause the universe to stop expanding and eventually collapse.
Bahcall stresses, "The dark matter problem remains one of the fundamental puzzles in physics and astronomy. Our results only sharpen the question of what is the dark matter."
Bahcall's results appeared in the November 1, 1994 issue of the Astrophysical Journal. Paresce's paper will appear in the February 10, 1995, issue of the Astrophysical Journal.
All visible celestial objects known today account for only 10% of the mass in the universe. The rest of this "missing mass," also known as "dark matter," is presumably invisible because it does not emit or reflect visible light or other forms of electromagnetic radiation. Or perhaps its light is so feeble that current astronomical instruments are unable to detect it. However, dark matter can be indirectly detected due to its gravitational influence on other nearby visible objects.
The presence of dark matter was first discovered in 1932 by astronomer Jan Oort, who measured the perpendicular motions of nearby stars relative to the disk of our Milky Way. He studied the gravitational influence of the galactic disk on these stars, and so, was able to measure the mass of the disk (just as the mass of Earth can be calculated from the acceleration of a falling object). To his surprise, this calculated mass was twice the amount of mass seen as stars and nebulae. A year later, Fritz Zwicky examined the dynamics of clusters of galaxies, and also came to the startling conclusion that the observed galaxies only accounted for 10% of the mass needed to gravitationally bind the galaxies in the cluster.
One widely-used method to deduce the amount of missing mass involves measuring the rotation speed of a spiral galaxy. Spectroscopic and radio observations have obtained the rotation velocities of hundreds of spiral galaxies. These experiments have revealed that, in most cases, a galaxy's mass continues to increase toward the edge of its visible disk of stars. This implies that spiral galaxies are surrounded in haloes of matter that cannot be seen. Observations of elliptical galaxies, groups, and clusters of galaxies also indicate the presence of dark matter interacting gravitationally with the visible objects.
The nature of dark matter, and its abundance, are among the most important questions in modern cosmology today. What is it made of? Some astronomers believe that dark matter is composed of protons and neutrons, called baryonic or simply "normal" matter. Baryonic dark matter candidates include extra-solar planets, remnants of stellar evolution such as comets, objects not massive enough to ignite hydrogen fusion called brown dwarfs, dying embers of stars such as cold white dwarfs and neutron stars, as well as interstellar and intergalactic gases.
Non-baryonic dark matter, on the other hand, could be elementary particles that do not interact strongly with normal matter. Except for the neutrino particle, many such elementary particles are still in the realm of theory and have not been detected.
Since all visible matter is only a small fraction of the total mass in the universe, the amount of dark mass that is present will determine the evolutionary future of the universe. If there is not enough dark matter to gravitationally bind the universe together, it could continue expanding forever. If there is enough mass in the universe to gravitationally hold it together, the universe may slow down its expansion, come to a halt, and begin to contract and eventually collapse.
The temperature of dark matter in the early universe also may have determined the early evolution of the universe. Not long after the Big Bang and prior to the formation of galaxies, matter began to aggregate under the influence of gravity. Dark matter might have provided the "seeds," a lumpy background in which ordinary matter could congregate to form galaxies and stars. If this "cold dark matter" were present, where particles had a negligible random motion, galaxy formation would begin on small scales. Matter would gather in sizes comparable to current galaxies or smaller, and eventually build to become clusters and superclusters due to the gravitational attraction of the galaxies.
If, however, "warm dark matter" was present, it would erase the small galaxy-sized "seeds" that initially formed. Instead, enormous gaseous pancake-like structures as large as superclusters and clusters, are created, subsequently condensing into individual galaxies.
Globular star clusters are among the oldest objects in our galaxy. Their beauty is easily discerned through amateur telescopes that resolve tightly-packed swarms of glistening stars, suspended in the night sky like Christmas ornaments. More than 150 globular star clusters are known to be associated with the Milky Way Galaxy. Each cluster contains hundreds of thousands to a million stars within a volume of 10 to 30 light-years across.
In 1918, Harlow Shapley recognized the existence and structure of globular clusters. By studying the clusters' distribution in the sky and measuring their distances, he was able to deduce the location of the center of the Milky Way Galaxy and the Sun's distance from it. In the 1930s, Edwin P. Hubble discovered globular clusters in the neighboring Andromeda Galaxy, and since then globular star clusters have been found surrounding many other galaxies.
Globular clusters reside within a spherical volume of space called the "galactic halo," which surrounds the disk of our galaxy. The clusters orbit around the galactic center, taking millions of year to complete their highly elongated, randomly oriented orbits. Most globular clusters wander as far as 90 to 120 thousand light-years from the galactic center, and some extend as far as 300 thousand light-years out. The motions of these distant objects, influenced by the gravitational pull of the entire galaxy, allows astronomers to calculate the amount of mass in the galaxy. Some recent estimates reveal that the galaxy is 500 billion times the mass of the Sun. This estimate is significantly higher than the mass contributed by visible stars and nebulae alone, indicating that there is a great amount of unseen dark matter in the galaxy.
When compared to the Sun and other stars of the galactic disk, globular cluster stars appear to be deficient in heavy elements. This indicates that they are ancient objects, made from the pristine gas that condensed to form the galaxy long ago. However, about 20% of globular clusters are slightly richer in heavy elements compared to their counterparts, and are, therefore, presumably younger.
Although chemical composition differs from one cluster to the next, all member stars within a given cluster have a similar composition, indicating that they were born from the same cloud. This provides a unique opportunity for the study of stellar evolution. Yet each star began life with a different mass. By observing the luminosity and temperatures of their current states, astronomers are learning a great deal about the life cycles of stars.
Globular clusters contain mostly low-mass stars that are so tightly packed together that the density of stars near the center is about 2 stars per cubic light-year. In comparison, our solar neighborhood has about one star per 300 cubic light-years. If you were looking into the sky from a hypothetical planet in the middle of a globular cluster, like 47 Tucanae, you would be surrounded in a perpetual twilight cast by the light of thousands of nearby stars.