Measuring dark matter.

Measuring dark matter

By analyzing the X-ray spectra of a halo of hot gas gravitationally bound by the elliptical galaxy NGC 1399, astronomers using the Broad Band X-ray Telescope inferred the galaxy's amount of dark matter -- mass hidden from view because it does not radiate at any observed wavelength. The preliminary finding marks the first use of X-ray spectral data to estimate the amount of dark matter in an elliptical galaxy, says Goddard astronomer Richard F. Mushotzky.

Goddard researchers measured the width of characteristic spectral lines detected by the X-ray telescope and radioed to Earth. Those measurements enabled them to determine the temperature, and thus the average speed, of the gas particles surrounding NGC 1399.

The researchers then calculated the amount of mass the galaxy must contain in order to hold the speedy particles in orbit. They found that NGC 1399 should have six times as much mass as its light-emitting portions appear to hold. The missing mass, they assert, must be dark matter.

The early findings, notes Mushotzky, suggest that elliptical galaxies contain more than six times as much dark matter as visible mass -- a ratio, he says, that agrees with previous estimates by other researchers of the amount of dark matter in spiral galaxies.