Mysterious Stellar Dust in the Early Universe

23. Dezember 2004

Since last year some astronomers thought they finally knew the origin of the large amount of dust surrounding the earliest quasars: It would have formed during supernova explosions of the first stellar generation after the big bang. This notion was inferred from observations of the allegedly "heavily smoking" supernova remnant Cassiopeia A. However, new observations obtained with the ISO and SPITZER infrared satellite observatories now show that this important result is no longer tenable.

The issue of the origin of the first dust grains in the universe is of fundamental significance. As is generally known, in the beginning there was mostly hydrogen, a gas consisting of the simplest atoms. Heavier elements (such as carbon, oxygen, silicon etc. up to iron) were then first synthesized in the interiors of stars of the first generation. Still heavier elements, though, can only be created in supernova explosions.

Heavier elements therefore were only available when the stars of later generations formed. Dust grains – the first solid bodies in the universe – consist of these heavier elements. They are formed either within the cool winds emanating from several-billion-year-old stars similar to our sun or, after already a few million years, in supernova explosions. Only then was dust available for the formation of the stars of later generations and – particularly important from a human point of view – their possible planetary systems.

In recent years, astronomers have detected large quantities of interstellar dust in the vicinity of the most distant quasars which are observed in the early universe, only about 700 million years after the big bang. So the problem arose: How could all this dust be formed so rapidly? The only possibility obviously were supernova explosions as the universe at that time was much too young for old solar-like stars emitting cool winds. But could supernovae really be so productive?

A first answer to this question was given in an often cited paper [1] published in 2003. So far supernova remnants were considered to be poor in dust since only small amounts of warm dust were detected within them in the infrared regime at short wavelengths. However, in the direction of the supernova remnant Cassiopeia A (Cas A for short) the authors observed a strong thermal emission in the sub-millimeter spectral region that is characteristic for large quantities of cold interstellar dust. They attributed this dust to the environment of Cas A and consequently thought to have found a solution to the puzzle of the large amounts of dust existing in the early universe: Apparently Type II supernovae (to which Cas A belongs) indeed produce enough dust. A Type II supernova occurs when the core of an extremely short-lived massive star collapses at the end of its evolution, thereby releasing enormous amounts of gravitational energy which rips the star apart in a huge explosion.

But the supernova remnant Cas A was also observed by the ISO infrared satellite within a sky survey in the far-infrared regime – the so-called ISO Serendipity Survey at 170 μm. At this wavelength very cold dust (T ≈ 10 ... 20 Kelvin or -250 ... -260°C) emits its "thermal radiation". In this way, Krause and his colleagues at the Max Planck Institute for Astronomy in Heidelberg discovered a cold interstellar cloud covering Cas A. They assumed that the sub-millimeter radiation measured by Dunne et al. actually comes from this cloud, which is located in the direction of the supernova remnant, but lies much nearer in the foreground and is not associated with the supernova remnant. Now they were able to confirm this assumption by observations obtained with the SPITZER infrared space telescope. Their results have recently been published in Nature [2].

Cas A is the youngest known supernova remnant in our Milky Way. It lies at a distance of about 11 000 light years, beyond the dusty Perseus spiral arm. Krause and his colleagues presume that this very dust clouds lying in the foreground have made it impossible for astronomers of the late 17th century to detect the supernova explosion whose remnant we now see as Cas A. Cas A is so close to Earth that the supernova would have appeared as the brightest star in the whole sky, if it were not obscured by the dust cloud in the Perseus arm.

The German-American team mapped Cas A at a wavelength of 160 μm using the SPITZERspace telescope and its imaging photometer and then compared the results with a map of the same celestial region created in the radio range. The comparison shows that the dust within the interstellar clouds virtually produces the entire infrared radiation. So there is no significant quantity of dust to be associated with the Cas A supernova remnant.

Now astronomers will have to resume their search in order to identify the first sources of dust in the universe. If they succeed, we will know how and where the very first stars were formed and if there are other, hitherto unknown mechanisms besides the stellar ones that are able to create dust. The answer will deepen significantly our knowledge of the earliest evolution of galaxies.




[1] Loretta Dunne, Stephen Eales, Rob Ivison, Haley Morgan, Mike Edmunds: »Type II supernovae as a significant source of interstellar dust«, Nature, 424 , pp. 285-287 (2003)
[2] Oliver Krause, Stephan M. Birkmann, George H. Rieke, Dietrich Lemke, Ulrich Klaas, Dean C. Hines, Karl D. Gordon: »No cold dust within the supernova remnant Cassiopeia A«, Nature, 432 , pp. 596-598 (2004)


Authors:
S. Birkmann, D. Lemke, U. Klaas
Max Planck Institute for Astronomy
D-69117 Heidelberg


O. Krause, G. H. Rieke, K. D. Gordon
Steward Observatory
University of Arizona


D. C. Hines
Space Science Institute
Boulder, Colorado

Zur Redakteursansicht