Artist concept of IRAS

Astronomers studying very faint objects originally detected by the Infrared Astronomical Satellite (IRAS) have discovered a new, very distant object which they say is the most luminous object ever seen in the universe.

The observations, published today (June 27) in the British science journal Nature, show that the luminous object is a massive dust cloud which radiates 99 percent of its light in the infrared part of the spectrum.

The team of astronomers believe that this mysterious cloud may be a massive galaxy in the process of formation -- or, alternatively, a quasar embedded in the dust of a massive galaxy.

Infrared light -- or, more simply, heat radiation -- is invisible to the human eye, but can be detected by electronic sensors such as those on the IRAS satellite. IRAS discovered hundreds of thousands of infrared objects that astronomers are now observing more closely in order to determine what they are. Often these sources turn out to be dusty objects, because dust particles are very efficient emitters of infrared radiation.

Astronomers define the luminosity of an object as the total amount of energy emitted at all wavelengths. The luminosity of this newly found object is an incredible 300 trillion times that of the sun, or 30,000 times that of our entire Milky Way Galaxy, which itself consists of hundreds of billions of stars like the sun. This tremendous energy output is greater even than the most luminous known quasars, which were discovered in visible or ultraviolet light. Quasars are thought to derive their immense power from the presence of massive black holes in their centers.

Unlike typical quasars, this new source is emitting most of its light in the infrared region of the spectrum of light. The dust cloud which the team believe is responsible for the far-infrared radiation has a mass of between 400 million and 1 billion times that of the sun. The lower figure exceeds the mass of interstellar dust in any previously known galaxy. The higher figure is comparable to the whole mass of heavy elements in the Milky Way Galaxy, most of which is today locked up in stars.

The object, in the constellation of Ursa Major, is approximately 16 billion light-years from Earth. When astronomers study distant objects they are essentially looking back in time; in this case the object is seen as it was 16 billion years ago, or more than 80 percent of the way back in time to when the universe is thought to have originated in the Big Bang.

The team of astronomers, from four British universities and two U.S. institutions, is led by Prof. Michael Rowan-Robinson of Queen Mary and Westfield College, London.

The team first observed the mystery object as part of a program to identify infrared sources detected by IRAS.

The luminous dust cloud was discovered when the team was trying to locate a visible light counterpart for one of the new faint IRAS sources (the luminous source appears faint due to its tremendous distance from us) at the United Kingdom's 4.2-meter (13.7-foot) William Herschel Telescope in the Canary Islands. Using a spectrometer, they were able to detect the signatures of elements such as carbon and hydrogen in the faint mystery source.

The redshift -- or shift of the characteristic element lines towards the red end of the spectrum caused by the expansion of the universe -- is 2.236, showing the object to be a very distant galaxy. (The universal expansion causes more distant parts of the universe to appear to be receding from us more rapidly; thus, the more distant the object, the larger the measured redshift of the spectral lines.)

More detailed images and spectra were later obtained at Caltech's 200-inch telescope at Palomar Observatory, near San Diego, and the William Herschel Telescope.

A joint mission between the United States, Netherlands and United Kingdom, IRAS was launched by NASA in 1983. For 10 months it surveyed the entire sky, providing our first ever comprehensive view of the universe at four different infrared wavelengths.

In 1985 the U.S. Infrared Processing and Analysis Center (IPAC), funded by NASA and based at the Jet Propulsion Laboratory and the California Institute of Technology, produced a catalog of sources detected by IRAS. From 1985-1988 IPAC re-analyzed the satellite data to produce a more sensitive catalog of infrared sources.

The powerhouse that heats the massive dust cloud and causes it to glow so brightly in infrared light is hidden from view, but the astronomers theorize that it may be due to about a billion extremely hot, luminous young stars formed in the early stages of the birth of a galaxy. If their theory is correct, this discovery marks the first time astronomers have witnessed the birth of a galaxy.

An alternate theory is that the powerhouse is itself a quasar, more luminous than ever seen before, shrouded from our view by the cloud of dust. Astronomers from other observatories around the world have begun to train their telescopes on the new object to shed further light on how galaxies and quasars formed during the early history of the universe.

Joining Rowan-Robinson on the team of astronomers involved in the discovery are Dr. Tom Broadhurst, Dr. Andy Lawrence, Seb Oliver and Andy Taylor of Queen Mary and Westfield College, London; Dr. Richard McMahon of the Institute of Astronomy, University of Cambridge, England; Dr. Carol Lonsdale, Dr. Perry Hacking and Tim Conrow of the JPL/Caltech IPAC; Prof. George Efstathiou and Dr. Will Saunders of the University of Oxford, England; Prof. Richard Ellis of the University of Durham, England; and Dr. Jim Condon of the National Radio Astronomy Observatory, Charlottesville, Virginia.

JPL contributions to the project, as well as the U.S. portion of the IRAS mission, are funded by NASA's Office of Space Science and Applications.

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