22 November 2011

Galaxies are the ultimate recyclers

New observations have shed light on the ways galaxies continuously recycle immense volumes of hydrogen gas and heavy elements, allowing them to build successive generations of stars.
A spiral galaxy

A spiral galaxy like our Milky Way acquires gas for star formation and recycles it through a massive gaseous halo. Credit: NASA; ESA; A. Feild, STScI

MARYLAND: New observations have shed light on the ways galaxies continuously recycle immense volumes of hydrogen gas and heavy elements, allowing them to build successive generations of stars.

This ongoing recycling keeps some galaxies from emptying their ‘fuel tanks’ and stretches their star-forming epoch to over 10 billion years. Three studies published in the current issue of Science have investigated the different aspects of the gas-recycling phenomenon, basing their conclusions on a series of NASA Hubble Space Telescope observations. Data from large ground-based telescopes in Hawaii, Arizona and Chile also contributed to the studies by measuring the properties of the galaxies.

“In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope, we detected ubiquitous, large halos of ionised oxygen surrounding star-forming galaxies; we found much less ionised oxygen around galaxies with little or no star formation,” said the researchers of one of the papers led by Jason Tumlinson of the Space Telescope Science Institute in Baltimore.

Halos of hot gas

The COS observations of distant stars demonstrate that a large mass of clouds is falling through the giant halo of our Milky Way, fueling its ongoing star formation.

These clouds of hot hydrogen reside within 20,000 light-years of the Milky Way disk and contain enough material to make 100 million suns. Some of this gas is recycled material that is continually being replenished by star formation and the explosive energy of novae and supernovae, which kicks chemically enriched gas back into the halo.

The COS observations also show halos of hot gas surrounding vigorous star-forming galaxies. These halos, rich in heavy elements, extend as much as 450,000 light-years beyond the visible portions of their galactic disks.

The amount of heavy-element mass discovered far outside a galaxy came as a surprise. COS measured 10 million solar masses of oxygen in a galaxy’s halo, which corresponds to about one billion solar masses of gas – as much as in the entire space between stars in a galaxy’s disk.

“With the new spectrograph we can see galaxy halos out to at least 150,000 parsecs (one kiloparsec is about 19 trillion miles),” said lead author of one of the studies, Todd Tripp of the University of Massachusetts in the U.S. “Where once we saw only the framework we are now getting a more complete picture, including the composition and movement of gases in the envelope, varying temperatures in different locations and the chemical structure, all in incredible detail.”

Gas escapes forever

The researchers also found that this gas is nearly absent from galaxies that have stopped forming stars. In these galaxies, the ‘recycling’ process ignites a rapid firestorm of star birth, which can blow away the remaining fuel, essentially turning off further star-birth activity.

This is evidence that gas pushed out of a galaxy, rather than pulled in from intergalactic space, determine a galaxy’s fate. The Hubble observations demonstrate that those galaxies forming stars at a very rapid rate, perhaps a hundred solar masses per year, can drive two-million-degree gas very far out into intergalactic space at speeds of up to two million miles per hour. That’s fast enough for the gas to escape forever and never refuel the parent galaxy.

Carrying huge mass

While hot gas ‘winds’ from galaxies have been known for some time, the new COS observations reveal that hot outflows extend to much greater distances than previously thought and can carry a tremendous amount of mass out of a galaxy.

Some of the hot gas is moving more slowly and could eventually be recycled. The observations show how gas-rich star-forming spiral galaxies can evolve to elliptical galaxies that no longer have star formation.

The light emitted by this hot plasma is invisible, so the researchers used COS to detect the presence of the gas by the way it absorbs certain colors of light from background quasars. Quasars are the brightest objects in the universe and are the brilliant cores of active galaxies that contain active central black holes.

Distant lighthouse beacons

The quasars serve as distant lighthouse beacons that shine through the gas-rich ‘fog’ of hot plasma encircling galaxies. At ultraviolet wavelengths, COS is sensitive to the presence of heavy elements, such as nitrogen, oxygen, and neon.

COS’s high sensitivity allows many galaxies to be studied that happen to lie in front of the much more distant quasars. The ionised heavy elements are markers for estimating how much mass is in a galaxy’s halo.

“In detecting the neon ions we find that there’s a lot of gas at several hundred thousand degrees Kelvin, which we’ve never been able to see unambiguously before,” said Tripp. “It means we can characterise the total mass distribution in the envelope, setting more precise constraints on the temperatures overall. We can now access more diverse ions, and we have new leverage on determining whether stuff is heating up or cooling off. We’re gaining new insights.”

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