Weekly Science Update

Wednesday, August 01, 2007
Measuring the Distances to Unseen Objects
One of the single most important measurements of a celestial object is its distance, because distance enables astronomers to calculate key intrinsic properties like mass and luminosity. But distance is also one of the most difficult parameters to determine accurately. The most direct method is called parallax. When a celestial body is viewed from different, widely separated points, its angular position with respect to background stars appears different. Parallax is used (for example) to triangulate the distances to nearby stars by measuring their apparent angles six months apart, at two opposite sides of the Earth's orbit around the sun.

In a paper written more than forty years ago, scientists predicted that if it ever became possible to obtain simultaneous parallax measurements -- with one of the measurements from space (i.e., from a large separation) -- then the distance to very short-lived celestial phenomenon could be measured. In particular, that early paper was thinking about measuring the distances to dark bodies that create microlensing events, short flashs of light that are produced when an unseen body's gravitational field, acting like a lens (hence the name), changes the intensity of visible light rays from a more distant, background star as the unseen body slowly moves in space. About fifteen microlensing events have been found so far using techniques that CfA Director, Charles Alcock, and his team helped to pioneer. He has argued that most of these events were produced by very dim bodies in a halo around the Milky Way, but others have argued that the events were produced by stars in the Milky Way's disk, or even that they are associated with stars in the Magellanic Clouds, the small irregular galaxies adjacent to the Milky Way.

SAO astronomers Giovanni Fazio and Brian Patten were members of a team of twenty-three scientists who have -- for the first time -- successfully used parallax to measure the distance to a microlensing event. The space-based measurement was made by the Infrared Array Camera (IRAC) on the Spitzer Space Observatory: Fazio is the PI of the SAO team that led the development of IRAC. About twenty-three days before the microlensing flash, a network of telescopes set up to search for this phenomenon saw some preliminary twinkling which they suspected was from the distortions in a gravitational lens, and issued an alert that a bright event was likely to occur. IRAC was directed to stare at the spot, as did several ground-based telescopes. The star did brighten -- by over a factor of two in a month. A meticulous analysis of all the complex results strongly favors the location of the lensing body as being in the Milky Way's halo, indicating that there is probably a large quantity of previously undetected dark material associated with our galaxy.

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