Spiral Galaxies More Massive Than Previously Thought: Halo of Gas Surrounds Milky Way

Spiral galaxies, like our own Milky Way, form spectacular pinwheels of stars, planets and debris in space. Now, it turns out that these galaxies may be a lot more massive than previously believed. Scientists have discovered that halos of gas can extend outward for over one million light-years.

Detecting these halos wasn't easy. The scientists discovered them after using Hubble's $70 million Cosmic Origins Spectograph (COS). More specifically, the team used distant quasars, the swirling centers of supermassive black holes, as "flashlights" in order to track ultraviolet light as it passed through the extended gas haloes of the foreground galaxies. The light absorbed by the gas was broken down by the spectrograph, much like a prism does, in order to reveal temperatures, densities, velocities, distances and chemical compositions of the gas clouds.

Stretching out from spiral galaxies, the gas gave galaxies far more mass than previously thought. Our own Milky Way, for example, has a diameter that's about 100,000 light-years. The scientists also discovered giant reservoirs of gas estimated to be in the millions of degrees Fahrenheit that were enshrouding the spiral galaxies and halos.

"This gas is stored and then recycled through an extended galaxy halo, falling back onto the galaxies to reinvigorate a new generation of star formation," said John Stocke, the study leader, in a news release. "In many ways this is the 'missing link' in galaxy evolution that we need to understand in detail in order to have a complete picture of the process."

The astronomers found that the giant clouds of gas around spiral galaxies contained as much mass as all the stars in their respective galaxies. In addition, they discovered that these gas halos probably have significant consequences for how spiral galaxies change over time.

Yet scientists may not be able to see this gas for much longer. The successor to the Hubble Space Telescope, the James Webb Space Telescope, has no UV light-gathering capabilities. This will prevent astronomers from undertaking studies like those done with the COS.

"Once Hubble ceases to function, we will lose the capability to study galaxy halos for perhaps a full generation of astronomers," said Stocke in a news release. "But for now, we are fortunate to have both Hubble and its Cosmic Origins Spectrograph to help us answer some of the most pressing issues in cosmology."