December 19, 2008
Water in the Early Universe
The discovery of water in the early Universe was possible only due to the chance alignment of a foreground galaxy and the distant quasar MG J0414+0534. The foreground galaxy acts like a cosmic telescope, magnifying and distorting the light from the quasar forming four distinct images of the quasar. Without this gravitational lensing effect, 580 days of continuous observations with the 100 m telescope would have been needed instead of the 14 hours used to make this remarkable discovery. "Others have tried and failed to find water, and we knew we were looking for a very faint signal", says Violette Impellizzeri, "so we thought of using a foreground galaxy like a cosmic magnifying glass to observe at a far greater distance and had to be persistent, and sure enough the line emission of water popped up."
The detection of water from MG J0414+0534 with the Effelsberg radio telescope also occurred thanks to a touch of fortune. The object is within just the right redshift interval to stretch the line emission of the H2O molecule from its original frequency of 22 GHz to 6 GHz and so within the tuning range of the 6 GHz receiver installed at the telescope.
"It is interesting that we found water in the first gravitationally-magnified object we observed from the distant Universe", says co-author John McKean. "This suggests that the water molecule may have been much more abundant in the early Universe than first thought, and can be used for further research into supermassive black holes and galaxy evolution at high redshift."
The water emission was seen in the form of a maser, that is, beamed radiation similar to a laser, but at microwaves wavelengths. The signal corresponds to a luminosity of 10,000 times the luminosity of the Sun. Such astrophysical masers are known to originate in regions of hot and dense dust and gas. With the detection of water from MG J0414+0534 it is the first time such a dense gas component has been observed in the early Universe and shows that the conditions for the water molecule to form and survive already existed only 2.5 billion years after the Big Bang.
Water masers have been found in a number of galaxies at closer distances. Typically, they are thought to arise in the hot gas and dust closely orbiting a supermassive black hole at the galaxy's core. This amplified radio emission is more often observed when the orbiting disk is seen nearly edge-on. However, the astronomers say MG J0414+0534 is oriented with the disk almost face-on as seen from Earth. "This may mean that the water molecules in the masers we're seeing are not in the disk, but in the superfast jets of material being ejected by the gravitational power of the black hole," explains John McKean.
For the future, the detection of water in distant galaxies may still be challenging due to the sensitivity limitations of current day telescopes. Of the nearby galaxies within half a billion light years from Earth only about one hundred galaxies show detectable water vapour emission, and almost all of them are relatively nearby. "In 2003 I participated in the detection of H2O megamaser emission in the galaxy 3C 403", says Christian Henkel, co-author of the study. At that time it was the most distant galaxy where water had been detected. Later on, this record went to a galaxy with water emission at redshift 0.66, (light travel time of 6 billion years). "Now, MG J0414+0534 at redshift 2.64, is by far the most distant galaxy to show water vapour emission" he continues.
"Because water masers arise near the cores of galaxies, our result opens new interesting possibilities for studying supermassive black holes at a time when galaxies were forming", concludes Violette Impellizzeri. "It will also generate further searches for water in other distant galaxies with the telescopes we have at our disposal today and with the next generation of radio telescopes; we now know water is out there."
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