A heavy form of hydrogen made shortly after the big bang is more plentiful than expected in our galaxy, observations by a NASA satellite suggest. This could mean that the Milky Way has collected much more primordial material through galactic mergers than previously believed.
Atoms of heavy hydrogen, also called deuterium, contain both a proton and a neutron in the nucleus, making them heavier than regular hydrogen atoms. Astronomers believe virtually all the deuterium in the universe was produced in nuclear reactions in the first 2 minutes after the big bang.
In the 1970s, measurements by NASA鈥檚 Copernicus satellite suggested that the ratio of deuterium to regular hydrogen varies from place to place in our galaxy. This was puzzling because astronomers believe material in the galaxy is well mixed and therefore the same percentage of deuterium should be found everywhere.
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In 2003, Bruce Draine of Princeton University in New Jersey, US, suggested the deuterium may indeed be distributed evenly across the galaxy, but that it may simply be difficult to detect in some places.
Unobservable form
Deuterium can only be detected by telescopes when it is freely floating in the form of a gas, so Draine argued it may become stuck to dust grains in dusty regions of space. In this form, it would not be observable.
Now, observations with NASA鈥檚 Far Ultraviolet Spectroscopic Explorer (FUSE) satellite have found evidence in support of this theory. The study was led by Jeffrey Linsky of the JILA research institute in Boulder, Colorado, US, and will appear in the 20 August issue of the Astrophysical Journal.
FUSE looked for the signature of deuterium in ultraviolet light coming from various places in the galaxy.
The researchers identified dusty, relatively undisturbed regions by searching for low levels of gaseous silicon and iron 鈥� these elements had presumably condensed into solid dust grains. The deuterium to hydrogen ratio in these dusty regions was as low as 5 parts per million (ppm).
By contrast, observations found deuterium levels as high as 23 ppm in environments where the dust was likely to vaporise. These disturbed areas include the vicinity of hot stars and supernovae.
Model rethink
The researchers argue that the galaxy鈥檚 overall ratio is the higher figure of 23 ppm, with the lower values of 5 ppm only an illusion caused by deuterium hiding out on dust grains.
Astronomers think the big bang left the universe with a deuterium to hydrogen ratio of 27 ppm. That figure is considered very reliable and comes from both theoretical calculations and measurements by NASA鈥檚 Wilkinson Microwave Anisotropy Probe (WMAP), which measured the microwave 鈥渁fterglow鈥� of the big bang.
Deuterium is quickly converted into heavier elements inside stars, and previous models of the formation and evolution of our galaxy predict the ratio should now have fallen to 10 or 15 ppm, Linsky says. But the persistence of deuterium suggests 鈥渟omething is wrong in those models鈥�, he told New 杏吧原创.
If the ratio derived by Linsky鈥檚 team is correct, then 85% of the gas in our galaxy has never been inside a star, a figure that is unexpectedly high.
Cannibalising newcomers
According to Linksy, the most likely explanation for the higher ratio is that the galaxy has absorbed a lot more 鈥減ristine鈥� gas 鈥� which has not been altered much by stars 鈥� than previously believed.
This could be due to the Milky Way gobbling up a higher number of small galaxies, which do not process their gas very quickly, than thought.
There is already plenty of evidence that our galaxy has absorbed new material over its lifetime, says Brian Fields of the University of Illinois in Urbana, US.
鈥淲e know that we鈥檙e cannibalising baby galaxies as we speak,鈥� Fields told New 杏吧原创. Even so, the new results 鈥渨ould suggest that there鈥檚 a lot more raining on our head than we would have thought鈥�.
Journal reference: Astrophysical Journal (in press)
