ScienceWeek

ASTROPHYSICS: ON THE LIGHT OF THE FIRST STARS

The following points are made by Piero Madau (Nature 2006 440:1002):

1) New work [1] reports the detection of copious high-energy gamma-ray emission from two "blazars" -- a class of active galaxy -- around 2 billion light years from Earth. This observation indicates that such radiation can travel largely unimpeded through the Cosmos, and implies that the infrared glow of the first stars in the Universe and their remnants is fainter than previous measurements had led us to believe. If true, that could influence our ideas of how and when the first structures in the Universe evolved.

2) The formation of structure in the Universe is believed to proceed hierarchically, with smaller galaxies merging, through the action of gravity, to build more massive ones. But the timing and sequence of the events through which the very first galaxies and stars formed remain largely unknown. According to current theories, the first dwarf galaxies hosted metal-free stars over a hundred times more massive than the Sun. These stars shone intensely for only a few million years and then either blew themselves apart in gigantic supernova explosions, or collapsed to form the first massive black holes.

3) Astronomers have long been rummaging through the Universe for tell-tale signs of these dramatic beginnings. When the first stars ignited, they emitted large numbers of photons at ultraviolet wavelengths. These photons "reionized" the surrounding atomic hydrogen gas that had formed as the Universe cooled. Recently, astronomers using NASA's Wilkinson Microwave Anisotropy Probe (WMAP) reported the latest detection of photons produced soon after the Big Bang. Their data show that these "cosmic microwave background" photons became polarized (tending to oscillate in only one direction perpendicular to their line of travel) by scattering on free electrons in the early Universe. The level of polarization allows the era of reionization to be pinpointed to some 400 million years after the Big Bang, when the Universe was just 3% of its present age [2].

4) So how much of the background light that we see comes from the first stars? As the Universe aged and expanded, part of the ultraviolet radiation emitted by these stars was absorbed again by re-formed atomic hydrogen. Lower-energy ultraviolet light escaped this fate, but was stretched to longer, redder wavelengths. Therefore, although the early stellar populations were twinkling so long ago that current telescopes cannot detect them, their combined energy output is recorded in diffuse light that reaches Earth in the near-infrared region of the electromagnetic spectrum, at wavelengths of a few micrometers. Resolving this infrared glow is, however, a daunting task, because many other celestial sources -- among them older stars in closer galaxies, active galactic nuclei known as quasars, and the bright foreground sources in the Milky Way and the Solar System -- emit radiation at similar wavelengths.[3-5]

References (abridged):

1. Aharonian, F. et al. Nature 440, 1018-1021 (2006)

2. Spergel, D. N. et al. Astrophys. J. (submitted); preprint available at http://www.arxiv.org/astro-ph/0603449 (2006).

3. Dwek, E. & Arendt, R. G. Astrophys. J. 508, L9-L12 (1998)

4. Gorjian, V. , Wright, E. L. & Chary, R. R. Astrophys. J. 536, 550-560 (2000)

5. Cambresy, L. , Reach, W. T. , Beichman, C. A. & Jarrett, T. H. Astrophys. J. 555, 563-571 (2001)

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