6 February 2006

Hot Halo Find Confirms Theory

by Kate Melville

NASA's Chandra X-ray Observatory has detected an extensive halo of hot gas around the spiral galaxy known as NGC 5746. There's a sense of relief among scientists, as the discovery supports the theory that galaxies like our Milky Way are still accumulating matter from the gradual inflow of intergalactic gas.

The halo encompassing the immense spiral galaxy is comprised of hot gas that extends more than 60,000 light years on either side of the disk of the galaxy. "We targeted NGC 5746 because we thought its distance and orientation would give us the best chance to detect a hot halo caused by the infall of intergalactic gas," said Jesper Rasmussen of the University of Birmingham, a coauthor of the report.

Spiral galaxies are thought to form from enormous clouds of intergalactic gas that collapse to form giant, spinning disks of stars and gas. It was predicted that massive spiral galaxies should be immersed in halos of hot gas left over from the galaxy formation process. Hot gas has previously been detected around spiral galaxies in which vigorous star formation is ejecting matter from the galaxy, but until now hot halos due to the inflow of intergalactic matter have not been detected.

The discovery of a hot halo around NGC 5746 was welcome news to astronomers because it shows that the "missing" hot halos predicted by computer models do actually exist. "Our observations solve the mystery of the missing hot halos around spiral galaxies," said Kristian Pedersen of the University of Copenhagen, lead author of the report.

The scientists can confirm, with a reasonable amount of certainty, that the gas forming the halo is not produced by unusual star formation or by the galaxy's own nuclear region. "What we found is in good agreement with computer simulations in which galaxies are built up gradually from the merger of smaller clouds of hot gas and dark matter," explained Pedersen. Computer simulations and Chandra data show that the likely origin of the hot halo is the gradual inflow of intergalactic matter left over from the formation of the galaxy. "What we are likely witnessing here is the ongoing galaxy formation process," said Pedersen.

Source: Chandra X-ray Observatory