Tim Pearson

Senior Research Associate, Astronomy, Caltech


My research interests include radio interferometry and its application to observations of active galactic nuclei and the cosmic microwave background radiation.

The Cosmic Microwave Background Radiation

Together with Professor Tony Readhead and Dr Steven Padin, I am working on a project to design, construct and operate a ground-based radio interferometer array, called the Cosmic Background Imager, to measure the very small brightness fluctuations in the 2.7 K cosmic microwave background radiation, a relic of the early, hot universe. In the standard models for the formation of structure (galaxies and clusters) in the universe, the angular power spectrum of these fluctuations carries information that should allow us to measure the major cosmological parameters (such as the density parameter, the baryon fraction, and the expansion rate of the universe) with high precision.

Active Galaxies and Quasars

Very-Long-Baseline Interferometry (VLBI) is a technique that can make images of astronomical radio sources with milli-arcsecond resolution - the highest resolution obtainable in any wave band - corresponding to scales of only a few light-years in high-redshift quasars. With such high resolution, we can see changes in the structure of the emitting regions on time-scales of a year or less. VLBI was pioneered at Caltech using ad hoc networks of antennas around the world, including the 40-meter antenna at the Owens Valley Radio Observatory, and special-purpose correlators in Pasadena to process the signals recorded on magnetic tape. The ad hoc networks are now superseded by the Very Long Baseline Array (VLBA), operated by the National Radio Astronomy Observatory. This dedicated array provides data of high quality and much more observing time, and makes possible many exciting new observations such as the imaging of polarization from the jets of active galaxies and quasars. In collaboration with colleagues at Caltech, Jodrell Bank (England), and elsewhere, I am using VLBI to make images in several radio wave bands of the structure of the nuclei of active galaxies and quasars. We have concentrated on large, complete samples so that we can make a useful statistical study. Among the discoveries we have made are that most active nuclei contain well-collimated, one-sided radio jets and that many show apparent ``superluminal'' expansion. Both these phenomena are consistent with highly relativistic ejection of material along a jet pointed almost directly toward the observer. These discoveries have led to the widespread acceptance of the idea that the ``central engine'' in these objects is associated with a very massive black hole, and to the development of ``unified theories'' of active galaxies and quasars, in which the primary difference between quasars and radio galaxies is in their orientation relative to the observer. A recent discovery is a group of compact symmetrical objects which are apparently not dominated by relativistic motion; these are almost certainly young objects (about 1000 years old) and they illuminate the early stages of the evolution of radio galaxies and quasars.

Gravitational Lensing

The gravitational field of intervening galaxies can distort or split the images of distant galaxies and quasars. I am part of a world-wide group conducting the Cosmic Lens All-Sky Survey (CLASS): a search for new such ``gravitational lenses.'' Observations of gravitational lenses can place strong constraints on the Hubble constant and other cosmological parameters. We have made images of about 10,000 flat-spectrum radio sources, using the Very Large Array and other telescopes, and we identify lenses by looking for achromatic splitting of the images into double or more complex structures. This project has already found several new lensed systems and many promising candidates.


In my spare time, I develop and maintain a subroutine library for creation of scientific graphics which is used by astronomers and other scientists worldwide.


A complete list of my published papers, including links to online abstracts or text where available.