Telescope spies a galactic satellite

A huge gas cloud once considered a remnant from when the Milky Way or nearby galaxies formed is, in fact, a satellite of our galaxy, new radio telescope observations indicate. Unlike most satellites of the Milky Way, the body is orbiting in the direction opposite that of the galaxy’s rotation.

The object, known as Complex H, is crashing through the outer parts of the galaxy, reports Felix J. Lockman of the National Radio Astronomy Observatory in Green Bank, W. Va., in the July 1 Astrophysical Journal Letters.

Previous studies of Complex H were limited because the cloud is passing behind the galaxy’s outer disk, where gas and dust block visible light emitted by the cloud.

Astronomers had assumed the body was a so-called high-velocity cloud, a type of fast-moving mass of atomic hydrogen commonly found far from the galaxy. Such objects may ultimately be incorporated into the galaxy.

Complex H’s radio emissions show that the object is much closer than high-velocity clouds and that its motion is tied to that of the Milky Way.

Observations with the Green Bank Telescope reveal that a diffuse tail trails Complex H’s core. This structure supports a model in which the cloud is a Milky Way satellite whose outermost layers are being torn apart by our galaxy’s gravity, says Lockman.

Complex H lies about 108,000 light-years from the Milky Way’s core, stretches 33,000 light-years across, and contains 6 million suns’ worth of hydrogen gas.


If you have a comment on this article that you would like considered for publication in Science News, send it to Please include your name and location.

More Stories from Science News on Astronomy