Recycling hydrogen: A galactic enterprise

Skip to Navigation

Ezine

  • Published: Dec 1, 2011
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Recycling hydrogen: A galactic enterprise

Green galaxies

Galaxies have been recycling immense volumes of hydrogen gas and heavy elements for billions of years. In so doing they have helped to build successive generations of stars in processes that stretch out across deep time and deep space, according to data obtained using the ultraviolet-sensitive Cosmic Origins Spectrograph.

Some galaxies can ignite a rapid firestorm of star birth that ultimately blows away their fuel in a final flash of glory, extinguishing more star-birth activity early in their lives. However, new spectrographic observations suggest that other types of galaxy initiate an ongoing recycling process that prevents them from emptying their "fuel tanks" and allows them to sustain their star-forming epoch for more than 10 billion years. It's might just be the last word in sustainable, renewable energy.

Three back-to-back papers published in the 18th November 2011 issue of Science, provide the evidence for this galactic recycling in the form of a series of Hubble Space Telescope observations that used the special capabilities of the instrument's comparatively new Cosmic Origins Spectrograph (COS). COS was able to detect otherwise invisible mass in the halo of our Milky Way galaxy and to sample more than forty other galaxies. Data from large ground-based telescopes in Hawaii, Arizona, and Chile were also added to the studies to allow the researchers involved to measure the properties of this collection of galaxies and to come to the conclusions presented.

The invisible mass in these galaxies is not some exotic and previously unknown form of matter, it is simply hydrogen, helium, and heavier elements such as carbon, oxygen, nitrogen, and neon. The matter remained hidden until the new generation instrument was available to leaders of the three studies, Nicolas Lehner of the University of Notre Dame in South Bend, Indiana, Jason Tumlinson of the Space Telescope Science Institute in Baltimore, Maryland and Todd Tripp of the University of Massachusetts at Amherst and their respective teams.

Halo flow

The shape and colour of any galaxy is largely dependent on the flow of gas that moves through the extended halo that surrounds it. Computer simulations of the formation of galaxies have shown astronomers that the observed properties of galaxies cannot arise without some kind of complex accretion and "feedback" mechanisms being invoked. These mechanisms allow galaxies to acquire gas and subsequently to expel it, through star formation and expiration. The three teams have now presented different evidence of the various aspects of this galactic gas-recycling phenomenon.

"Our results confirm a theoretical suspicion that galaxies expel and can recycle their gas, but they also present a fresh challenge to theoretical models to understand these gas flows and integrate them with the overall picture of galaxy formation," explains Tumlinson.

The COS observations of distant stars suggest that a vast cloud mass is falling through the giant corona halo of our Milky Way galaxy and so providing the fuel for the ongoing formation of new stars. The clouds of ionized hydrogen lie within a distance of just 20,000 light-years from disc of the Milky Way and, the team suggests, contain enough material to make the equivalent of 100 million suns. Some of the observed gas is recycled material that has already been used by earlier stars that exploded as novae and supernovae in the distant past. However, the material ejected by a supernova is chemically enriched gas containing heavier elements than hydrogen. Some of the gas, however, is the virginal first pressing and is being accreted into new stars for the first time.

This enormous reservoir of matter fuels the Milky Way with the equivalent of about a solar mass per year, which is comparable to the rate at which our galaxy makes stars. The worrying implication for the long-term future of our galaxy being that at the current rate the Milky Way will be able to sustain star formation through this recycling process for a mere one billion years longer. The almost ubiquitous halos of hot gas surrounding vigorous star-forming galaxies extend as far as 450,000 light-years beyond the visible portions of the galactic discs. The gas is almost absent from galaxies that have stopped forming stars.

"Only with COS can we now address some of the most crucial questions that are at the forefront of extragalactic astrophysics," Tumlinson says. "We now know where is the missing fuel for galactic star formation," Lehner adds. "We now have to find out its birthplace."


The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

 Galaxies have been recycling immense volumes of hydrogen gas and heavy elements for billions of years. In so doing they have helped to build successive generations of stars in processes that stretch out across deep time and deep space, according to data obtained using the ultraviolet-sensitive Cosmic Origins Spectrograph.

Social Links

Share This Links

Bookmark and Share

Microsites

Suppliers Selection
Societies Selection

Banner Ad

Click here to see
all job opportunities

Copyright Information

Interested in separation science? Visit our sister site separationsNOW.com

Copyright © 2017 John Wiley & Sons, Inc. All Rights Reserved