ScienceWeek

ASTROPHYSICS: ON THE MOST ANCIENT STELLAR EXPLOSION

The following points are made by Enrico Ramirez-Ruiz (Nature 2006 440:154):

1) A trio of new contributions[1-3] presents observations of the most distant stellar explosion ever seen: a gamma-ray burst (GRB) that took place when the Universe (currently accepted age, roughly 13.7 billion years) was only about 900 million years old. Thus, for the first time, the most distant objects that can be identified spectroscopically are not just galaxies -- and therefore huge agglomerates of stars, gas and dust -- but also individual stars.

2) The hunt for cosmic structures that ignited when the Universe was in its infancy takes us to the frontier of our current observational capabilities. Not too long ago, the most distant objects known were quasi-stellar objects, or quasars[4] --glaringly luminous objects powered by gas falling into massive black holes at the centers of galaxies. Over the past few years, however, extremely sensitive surveys with space- and ground-based telescopes have allowed us to observe ordinary galaxies beyond the farthest quasars[5].

3) Since then, the race has been on to find the most distant star. As most stars lead relatively unexciting lives, they remain far less luminous than galaxies, and so distant stars are generally too faint to be detected with current technology. Some heavyweight stars, however, end their lives violently and spectacularly. These stars send out bursts of radiation so luminous that they appear bright even when viewed across vast stretches of the Universe.

4) On 4 September 2005, such a flash of gamma-rays, lasting for 80 seconds, hit NASA's purpose-built GRB-detection satellite, Swift. Swift's gamma-ray monitor established the position of the burst -- prosaically labelled GRB 050904 -- in the constellation Pisces. As Cusumano et al[1] describe, within seconds Swift turned around to direct its X-ray telescope at the region where the burst occurred, pinpointing the location of a rapidly fading source of radiation to within a hundredth of a degree. This in turn allowed powerful optical and infrared telescopes around the world to search for the decaying "afterglow" signal within their wavelength ranges. Kawai et al[2] report measurements of the afterglow of GRB 050904 at optical wavelengths. And Haislip et al[3] detail the tightly choreographed sequence of observations necessary to detect the barely visible afterglow at even longer, near-infrared wavelengths.

References (abridged):

1. Cusumano, G. et al. Nature 440, 164 (2006)

2. Kawai, N. et al. Nature 440, 184 186 (2006)

3. Haislip, J. B. et al. Nature 440, 181 183 (2006)

4. Fan, X. et al. Astron. J. 125, 1649 1659 (2003)

5. Kodaira, K. et al. Publ. Astron. Soc. Jpn 55, L17 L21 (2003)

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ASTROPHYSICS: ON BLACK HOLES IN THE EARLY UNIVERSE

The following points are made by Xiaohui Fan (Science 2003 300:752):

1) When did the first generation of galaxies and quasars form? How did these first sources of light end the cosmic "dark ages"? And what is the relation between the star formation in the first galaxies and the initial growth of supermassive black holes in the first quasars?

2) The most distant galaxies and quasars with confirmed redshifts are at approximately z = 6.5. At these high redshifts, the Universe was less than 1 billion years old and the first generations of galaxies and black holes were forming. At the other end of cosmic history, Hubble Space Telescope observations have shown that most, if not all, galaxies contain supermassive black holes. The masses of the black holes are tightly correlated with the velocity dispersions and masses of their host galaxies. This result suggests that the evolution of the black holes and the galaxies are connected, such that the process responsible for the assembly of the galaxy also feeds the growth of the black holes.

3) Optically bright quasars represent the critical phase of black hole evolution when it is acquiring most of its mass. The luminous quasars at redshifts of >6 likely represent black holes with several billion solar masses (M) residing in a halo of ~10^(13) M -- an amazing feat of early structure formation. A growing body of evidence suggests that high-redshift quasars are accompanied by intense star-formation activities on a galaxy scale.

4) With the help of gravitational lensing, the system studied by Carilli et al (Science 2003 300:773) provides the best case-study to date of the simultaneous formation of a supermassive black hole in a luminous quasar and a young star-forming galaxy at high redshift. Future high-resolution observations of high-redshift quasars, combined with more detailed understanding of black hole population from local systems, will help us to eventually understand the relation of black hole and quasar formation to star formation, and the role of black holes in the formation of galaxies.

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ON THE FIRST STARS

The following points are made by Timothy C. Beers (Nature 2003 422:825):

1) Late last year, the discovery of the most iron-deficient star yet identified, HE0107 5240, was announced. This star has a measured abundance of iron less than 1/200,000 that of the Sun. Its significance is that it seems to be a relic from the early Universe, and astronomers are now busy considering how to interpret it.

2) Researchers have presented various interpretations of HE0107 5240. Each of these interpretations centers on whether this star exhibits properties that might reveal the likely range of mass that should be associated with the so-called "population III" stars -- objects that are presumed to have formed shortly after the Big Bang, and which are thought to have produced the first elements heavier than H, He and Li, as well as the "first light" in the Universe. "Population II" stars are objects that formed after population III stars, and which incorporated the metals created by this previous generation. Our Sun, and other (younger) metal-rich stars in the Galactic disk, are referred to as "population I" objects.

3) To the astronomer, metals include all elements heavier than He, and they are thought to be produced only by nuclear reactions that take place during the lifetimes, or at the deaths, of stars. Stars such as our Sun have inherited the net production of metals by all of the previous generations that lived (and died) before it. Stars with the lowest observed abundances of heavy elements, such as HE0107 5240, must therefore have been born before other stars, because the gas clouds from which they formed had only the slightest traces of these heavy elements. Thus, regardless of the outcome of debate about the nature of the very first stars, HE0107 5240 remains chemically the most primitive object yet discovered, and is a crucial "laboratory" for tests of the origins of the first elements in the Universe.

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