This is a research program to study magnetic fields in ultraluminous infrared galaxies (ULIRGs) using polarization and Very Long Baseline Interferometry observations of OH lines. The methods include a survey of Zeeman splitting in OH megamasers using Arecibo Observatory, Very Long Baseline Interferometry maps of masers in several ULIRGs, and measurement of magnetic fields outside megamaser regions using OH absorption lines. The main goal is to determine magnetic fields in the astrophysical plasmas residing in infrared galaxies, and to study their evolution with redshift.
Magnetic fields are one of the three major forces on diffuse gas in interstellar space. Current thinking is that their forces on the gas are absolutely required for star formation, in particular by providing a mechanism for the gas to shed its angular momentum. Without this shedding, centrifugal forces prevent the gas from collapsing to form a star; instead, it would form a rotating disk. Magnetic fields are hard to measure, because their observational signatures are verey weak. Thus, particularly in external galaxies (which are distant and faint), information on magnetic fields is very scarce. We see the manifestations of magnetic fields in external galaxies in the forms of magnetically-confined jets, synchrotron radiation, and polarization effects. While these phenomena are useful in telling us that the field exists, obtaining the crucial thing for its effect on gas---the strength of the field---is exceedingly difficult. Zeeman splitating of OH megamasers has resulted in the only direct measurements of the strrength of the interstellar magnetic field. This project measured field strengths in extragalactic megamasers. A total of 77 sources were observed with the Arecibo telescope. Of these, maser emission could not be detected for eight sources, and two sources were only ambiguously detected. Another 27 sources were detected at low signal-to-noise ratios or with interference that prevented placing any useful limits on the presence of magnetic fields. In 26 sources, it was possible to place upper limits on the magnitude of magnetic fields, typically between 10 and 30 mG. Nearly all of the remaining sources exhibited reliable detections, with line-of-sight magnetic field strengths between 6.1 and 27.6 milliGauss. The distribution of magnetic field strengths suggests the magnetic fields in OH masing clouds in OHMs are larger than those in Galactic OH masers. The results are consistent with magnetic fields playing a dynamically important role in OH masing clouds. Our work also contributed to the general ability of astronomers to measure polarization with single-dish telescopes, particularly the Green Bank Telescope and the Arecibo Telescope. Such measurements are, themselves, difficult. We have documented our procedures and provided temporary copies of the documentation to the GBT and Arecibo; these are used by all observers who measure polarization with those ttelescopes. In the near future, we hope to publish this documentation and make it more available to the community.
