Graphic: Max Planck Institute for Astrophysics / Spruit |
Where does iron come from? According to astrophysicists, iron, like all other heavy elements, is created in the center of massive stars and is expelled into space once these stars explode as supernovae at the end of their lives. The material then mixes with the interstellar matter and may form new stars and planetary systems. Our solar system was formed after several generations of stars and therefore contains enough heavy elements like iron, oxygen etc. to form Earth-like planets which can sustain life. Prof. Günther Hasinger and Dr. Stefanie Komossa of the Max-Planck-Institute for Extraterrestrial Physics in Germany and Dr. Norbert Schartel of the European Space Agency ESA in Spain made a startling discovery: spectral observations carried out with the X-ray observatory XMM-Newton demonstrate that the young quasar APM 08279+5255 contains a three times larger iron fraction than our own solar system although it is much older. We observe the quasar at a time when the universe was only about 1.5 billion years old; in contrast, our sun was formed 9 billion years after the Big Bang. This is significant in that the center of this young quasar already contains a larger fraction of iron than our much older solar system. Either there is a previously unknown much more efficient way of producing iron, or, at the time when the quasar emitted its light, the universe was already older than expected (ApJ Letters Vol. 573, L77, July 10, 2002).
The quasar APM 08279+5255 is one of the most luminous objects in the universe. Its energy output exceeds that of our sun by more than a quadrillion. This is the only reason why we can still detect intense radiation from this quasar, despite its enormous distance. The quasar's luminosity is mainly powered by gaseous matter sucked in by a giant super massive black hole at the center of the quasar. The material becomes super-heated during this process and emits X-rays before it disappears forever into the black hole. Part of this matter, however, is not swallowed but instead forced out by the intense radiation pressure of the central object (figure 1). In the case of APM 08279+5255, we are looking down the stream of this outflowing material. In addition to being intrinsically luminous, the quasar's light is further magnified by a so-called gravitational lens. These properties make APM 08279+5255 an excellent "laboratory" in which to study the physical conditions in the early universe and in the immediate vicinity of super massive black holes.
Analyzing the quasar's X-ray light, detected with the European X-ray satellite XMM-Newton, Günther Hasinger, Stefanie Komossa and Norbert Schartel noticed that the material streaming away from the center of the quasar contains huge amounts of iron. From the "dip" in the quasar spectrum (figure 2) the scientists could determine the amount of iron located in the central region of the quasar, and thus in the early Universe. Interestingly, iron appears to be the only element clearly showing up in the spectrum; other elements, like oxygen, are barely detectable. The estimated ratio of iron to oxygen is about 3 to 5 times higher than in our solar system. All the heavy elements, which planets and humans are also comprised of, were created inside stars billions of years ago. This is also the case for the element iron, which is mainly created by a special type of supernova (type I): supernovae are suns at the end of their lives which pass away in giant explosions, blowing the elements produced in their interior out into interstellar space. Some fraction of this "star dust" is used to build new stars, another fraction is ultimately sucked into super massive black holes at the centers of galaxies. Since, however, stars which pass away as type I supernovae have rather long lifetimes (about one billion years), large quantities of iron in the early universe are quite remarkable.
The iron abundance is of such significance because iron represents a kind of "cosmic clock": all heavy elements were produced after the Big Bang in the interior of stars by the processes described previously. The creation of iron took considerable time: at least 1.5 billion years to produce the metal abundances of our sun. It is therefore highly surprising that an object as young as the quasar APM 08279+5255 already contained a larger fraction of iron than our sun which is much older. Either there is a more efficient way of producing iron - some kind of "iron factory" - or the universe, at a redshift of 4, is already older than previously expected.
What is the meaning of the redshift "z" ? The light which astronomers receive from distant objects has been travelling for an extremely long time. Viewing objects at large distances is therefore equivalent to viewing them in the distant past, when they were still young. Telescopes thus resemble "time machines". Accessing ever greater distances in astronomy allows unique insights into the early phases of the universe. During the time the light needs to travel from a distant galaxy to Earth, the whole cosmos is expanding, stretching the light wave. This increases the wavelength of light, equivalent to a 'redshift' (z), which is a measure of the distance and therefore the age of a galaxy or a quasar: the higher the z-value of an object, the larger its distance, and the smaller its age. At the distance of APM 08279+5255 (z = 3.91) the universe was at an age of only 1/10 of its present age of 15 billion years; the quasar's light originated in the early days of the cosmos.
The new observations presented here paint an extreme picture of the center of the quasar APM 08279+5255: there must have been a whole series of supernovae at the quasar's center to produce the large amount of iron observed. In addition, in order to explain the high luminosity of APM 08279+5255 and the huge outflow of matter from its center many solar masses of stardust have to be swallowed, and partly blown out again, every year (figure 1).
Even a very high rate of type-I supernovae can only partly explain the observations of large amounts of iron. It is also likely that more time is needed to produce the iron, i.e., a larger age of the universe at the redshift of APM 08279+5255. In this way we find independent evidence for the existence of the recently discovered "cosmological constant", a kind of "dark energy" which still pushes the universe apart.
The observations of APM 08279+5255 carried out with XMM-Newton provide important new information on nucleosynthesis and the chemical evolution of the early universe, on the new 'unified models' for different types of quasar-activity, and on the measurement of cosmological parameters like the cosmological constant. Presently, at these high redshifts we can only study very few, particularly luminous objects, like APM 08279+5255. In the future however, scientists hope to use XEUS, the future large X-ray observatory of ESA, to routinely analyze X-rays from many faint distant objects, in order to answer the questions emerging from the present discovery.
Journal
The Astrophysical Journal Letters