Carl Heiles is professor emeritus of astronomy. He is interested in the diffuse interstellar medium. Itsmorphology and dynamics are dominated by supernova explosions, which tend to cluster and form huge bubbles in the interstellar gas. The nearest of these is the Eridanus superbubble, which was formed by the explosions of previous generations of Orion cluster stars. He is currently embarked on a very detailed and comprehensive study of this superbubble---the X-ray emission from the hot interior gas, the H-alpha and 21-cm line emission from the cold gas in the surrounding shell, and the molecular line emission from the denser portions of this shell.
One of the dominant forces on the interstellar gas is magnetic. Interstellar magnetic fields are notoriously difficult to observe. The new Arecibo and Green Bank Telescope (GBT) are opening up many important opportunities using Zeeman splitting of the 21-cm line. Professor Heiles is a major user of Arecibo and will be of the GBT, which will begin operations during the next year. At Arecibo, he has embarked on a large project to Measure magnetic fields in cold atomic clouds, ongoing over the next year or two. At this point, early in the project, they have two surprising results: one, magnetic field strengths in cold dense gas are much weaker than we had thought; two, a by-product of these measurements is accurate kinetic temperatures for the gas, and these are much different from previous results---with some significant departures from the established theoretical paradigm.
Zeeman splitting is revealed by exceedingly weak polarization in the spectral lines. In the past few years he has developed the technique of cross correlation single-dish spectropolarimetry so that this is now the most accurate and versatile method. Along the way he has become very interested in characterizing the details of the telescope response pattern, because these can produce systematic effects that affect the accuracy of Zeeman splitting measurements. In dealing with the new telescopes, this aspect has become an increasingly interesting and important part of the measurement process.