Charging in at #9 is a study that could be a dark horse in terms of its long-term impact on our understanding of galaxies. Galaxies are the basic building blocks of the universe at large. The Hubble Space Telescope has shown that galaxies were around remarkably quickly once the universe was cold enough for atoms to form. In the Hubble Deep Field we see galaxies already fully formed, 12 billion years ago, with vigorous star formation and cannibalistic activity already in progress. Back then galaxies were smaller affairs. Today the giants in our backyard, represented by our home galaxy the Milky Way and its companions such M31 and M32 in Andromeda, are the result of mergers and acquisitions driven by the gravitational agglomeration of smaller galaxies.

   Mature galaxies in the local neighborhood contain more than stars, gas, and dust according to paper #9, a study of 36 nearby galaxies which concludes that 32 of them have massive compact objects in their cores. For this analysis John Magorrian (University of Toronto) and colleagues selected galaxies with dust-free nuclear bulges. They took photometry of the galactic bulges from the Hubble Space Telescope and supplemented those data with ground-based observations of velocity distributions within the galaxies.

   The velocity dispersion and light distribution in a galactic bulge is influenced by its mass distribution and the mass-to-light ratio. For 32 galaxies in the sample the kinematic fits throw up huge central concentrations of matter which is not showing up in the photometry measurements. So the dense concentrations cannot be hot stars. Typically the central clump of dark matter tips the scales at a billion solar masses. These massive dark objects are probably black holes, since star clusters of the required mass and size are hard to make and are unstable.

   Much of the interest in #9 stems from the ordinariness of the local galaxies in the survey, and the implication that supermassive black holes are common in galactic cores. The act of building a large galaxy must frequently involve a quasar phase in which a central massive object grows rapidly by accretion, a process which creates an extremely luminous galactic nucleus, for a while. Today the velocity signature tells of the fossil black hole at the heart of a galaxy.

   A study of the dust in our own galaxy, Paper #5, has been bobbing around in the Top Ten for almost two years. Dust is the stuff that gets in the way of other galaxies, making them look redder and fainter than they really are. This dust can be detected by anyone, without needing a telescope. Go to a dark place and look at the sky, where you can see our galaxy the Milky Way stretching across the sky from horizon to horizon. Against this background, first resolved into stars by Galileo, you will see dark areas with no stars. These voids are where cold dust blocks the light from more distant stars and galaxies. The dust grains are mainly graphite, silicates, and polycyclic aromatic hydrocarbons.

   Paper #5 gives a prescription for correcting the effects of dust in contaminating observations of distant galaxies. Its maps are derived from data gathered in space by the Cosmic Background Explorer and the Infrared Astronomy Satellite. On these maps a wealth of filamentary detail is apparent and corrections are made for dust in the solar system, left by comets, which is seen reflected as the zodiacal light. The primary use of the maps is in estimations of galactic extinction when more distant galaxies are observed.

   As David Schlegel, currently of Princeton University, tells Science Watch, "This paper mostly gets cited by astronomers studying extragalactic sources where they need to measure reliable intrinsic brightnesses or colors. Quite a few citations are also for the detection of the cosmic infrared background. This was presumed to come from distant, star-forming and dusty galaxies. The background is stronger than expected and implies that fully two-thirds of the starlight in distant galaxies is first absorbed by dust and then re-emitted in the far-infrared." Ignoring this emission from warm dust would imply overlooking the majority of the energetics of the early universe. The new maps have also thrown up dozens of individual sources of infrared emission. "We know very little about them, " says Schlegel. "They may be star-forming galaxies or something more exotic like active galactic nuclei."