Galaxy Harbors Unusual Dark Matter Concentration
When viewed with optical telescopes, the galaxy appears to be a mundane, fuzzy dot. But when observed with a radio telescope, a dramatic feature is revealed: an expansive disk, and spiral arms of gas, is spreading out from the central visible core.
"Normally, when you see spiral arms, you see stars in the arms, but there is nothing there," said Gerhardt Meurer, a postdoctoral fellow in the Department of Physics and Astronomy at The Johns Hopkins University.
No stars are mixed with the gas, which is called neutral hydrogen, since it has not been ionized by radiation or shock waves from exploding stars. Meurer and other astronomers observed the galaxy with the Australia Telescope Compact Array, a collection of radio telescopes near Narrabri, Australia. The galaxy, called NGC2915, harbors an unusually high proportion of gas to stars -- about 2.7 times more neutral hydrogen than the combined mass of its stars. In comparison, Earth's galaxy, the Milky Way, has only about one-quarter as much gas as the mass of its stars.
"You can say this galaxy is largely unevolved because it still has so much gas compared to stars," Meurer said. The only part of the galaxy that is formed is its center.
The findings are detailed in a paper published this month in The Astronomical Journal. Also involved in the research were Claude Carignan, an astronomer at the University of Montreal, and Sylvie Beaulieu and Ken Freeman, both astronomers at the Mount Stromlo and Siding Springs observatories in Australia. NGC2915 is in the southern constellation Chamaeleon.
The astronomers discovered that at least 95 percent of NGC2915's mass is in the form of dark matter, and the density of dark matter in the galaxy's center is more than 10 times greater than that typically found in normal spiral galaxies. The findings are based on observations the astronomers made between May 1992 and March 1993. Because it is difficult to resolve small objects with radio telescopes, they are not commonly used to observe galaxies like NGC2915, which are called blue compact dwarf galaxies.
"In some ways it's like a first step, a sign to astronomers to start paying attention to these blue compact dwarf galaxies to try to understand dark matter," Meurer said.
The existence of dark matter in the universe has been inferred from a number of astronomical observations. According to many theories and observations, the universe contains far more matter than is directly observed using current technology. Astronomers think that at least 90 percent of the mass in the universe is derived from dark matter. The possible constituents of the dark matter range from gases, to stars that are too faint to be detected by standard methods and exotic particles that do not emit visible light.
The dark matter's gravity in NGC2915 may act like a sort of "glue" that concentrates the gas toward the center of the galaxy so that stars can form, Meurer noted. When stars die, they explode as supernovas, driving gases outward with tremendous force. Something must keep the gas together long enough for it to form new stars. In normal galaxies the gravitational attraction of the stars and gas is sufficient to hold the gas together under these explosive actions. In NGC2915, dark matter provides the dominant gravitational attraction, and the unusual concentration of dark matter may result in a higher intensity of star formation than normally seen in galaxies, Meurer said.
Dark matter cannot be seen directly, but it can be detected and measured through its gravitational force, which causes much more rapid orbital motions of gas and stars than can be attributed to the visible matter. Meurer's team found that the neutral hydrogen gas in NGC2915 had the curious property of mimicking the distribution of dark matter. This result confirmed earlier findings that neutral hydrogen often has the uncanny ability to trace dark matter's presence.
The hydrogen gas also holds vital secrets about the formation of stars and the evolution of galaxies, since stars are created from gas.
One reason for the galaxy's unusual makeup could be its isolated place in the cosmos: normally galaxies exist in groups or clusters, but NGC2915 is off by itself. The gravitational interactions of neighboring galaxies would have torn the fragile disk of neutral hydrogen apart, Meurer said.
When viewed with optical telescopes, blue compact dwarf galaxies appear much smaller than giant elliptical and spiral galaxies, such as the Milky Way. The diameter of NGC2915's small core of stars is only one-fifth the diameter of the Milky Way's stellar spiral. It has only 11 percent of the Milky Way's mass, and it is only 1.7 percent as bright, in the visible spectrum, as the Milky Way. But the radio observations are striking because they show NGC2915 to be a much larger galaxy, with a spiral disk about 75 percent as large as the Milky Way's disk. The findings mark the first time that a blue compact dwarf galaxy has been used to calculate the mass distribution of dark matter.
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