Spiral Galaxies Far Bigger Than Previously Thought: A Study

Spiral galaxies may be much larger and more massive than previously believed, researchers at the University of Colorado Boulder reported in a new study conducted using the Hubble Space Telescope.

Study leader John Stocke said new observations with Hubble's $70 million Cosmic Spectograph (COS) designed by the university show that normal spiral galaxies are in fact surrounded by halos of gas that can extend to over 1 million light-years in diameter.

In comparison, the current estimated diameter of the Milky Way is approximately 100,000 light-years.

The material for the galaxy halos was originally ejected from galaxies by exploding stars, a product of the star formation process, Stocke explained.

"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," he said, adding that, 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."

Building on earlier studies identifying oxygen-rich gas clouds around spiral galaxies by other scientists, Stocke and his colleagues determined that such clouds contain almost as much mass as all the stars in their respective galaxies.

"This was a big surprise," Stocke said. "The new findings have significant consequences for how spiral galaxies change over time."

Additionally, the team of researchers discovered giant reservoirs of gas estimated to be millions of degrees Fahrenheit enshrouding the spiral galaxies and halos under examination. The halos of spiral galaxies, in contrast, were just tens of thousands of degrees.

Shull, a professor at the university's astrophysical and planetary sciences department and a member of the school's Center for Astrophysics and Space Astronomy emphasized that the study of such "circumgalactic" gas is still in its infancy.

"But given the expected lifetime of COS on Hubble, perhaps another five years, it should be possible to confirm these early detections, elaborate on the results and scan other spiral galaxies in the universe," he said.

Prior to the installation of COS in 2009, theoretical studies showed that spiral galaxies should possess about five times more gas than astronomers were detecting, meaning that the new observations are much more in line with scientific expectations, Stocke explained.

In order to come to these conclusions, the team used distant quasars - the swirling centers of supermassive black holes - as flashlights to track ultraviolet light as it passed through the extended gas halos of foreground galaxies.

The light absorbed by the gas was then broken down by the spectrograph, much like a prism does, into characteristic color "fingerprints" that revealed temperatures, densities, velocities, distances and chemical compositions of the gas clouds.

"This gas is way too diffuse to allow its detection by direct imaging, so spectroscopy is the way to go," Stocke said.

However, while astronomers hope Hubble keeps on chugging for years go to come, there are no more servicing missions scheduled. Meanwhile, the James Webb Space Telescope, heralded as Hubble's successor, has no UV light-gathering capabilities, meaning that studies like this one will not be possible.

"Once Hubble ceases to function, we will lose the capability to study galaxy halos for perhaps a full generation of astronomers," Stocke said. "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."