"FIELDS OF DREAMS"
AAS Pierce Prize Lecture
January 9, 1997
Alyssa A. Goodman
Harvard-Smithsonian Center for Astrophysics
WASHINGTON, DC -- On Earth, when one wants to measure a magnetic field,all one needs are a few simple hand-held instruments. Even a 10-centcompass will suffice to measure the field's direction to very highprecision! However, in interstellar space, where the use of adime-store compass is at present very impractical, it is a good dealharder--and more expensive--to measure magnetic fields accurately.
Painstaking measurements of fields in our own Galaxy carried out overthe past thirty years have shown that the fields are often strongenough to affect the structure and evolution of the gaseous materialbetween the stars known as the "interstellar medium." For example,measurements of a relatively obscure quantum mechanical phenomenonknown as the "Zeeman Effect" have shown that magnetic fields in cloudsof gas surrounding new stars are absolutely critical to thestar-formation process. If not for the constraints imposed by themagnetic fields, new stars would form too quickly --and our Galaxywould begin to get very crowded.
The Zeeman Effect, as well as other measurements, all seem to point to a dynamic, vital role for magnetic fields in most interstellar dramas. Unfortunately, it is very difficult to get a clear picture of this key player. Magnetic fields, like gravitational or electric fields, are invisible, so we can map their structure only by recording their effects on a surrounding environment. In classic laboratory experiments, for example, iron filings on a flat surface react to and trace out a magnetic field's presence. Interstellar space is inherently three dimensional, however; so, even if it were somehow filled with iron filings, we would still have to disentangle the intersecting and overlapping effects of many fields lying along any line of sight.
Despite the difficulties, remarkable progress has been made recentlytoward revealing the structure of interstellar magnetic fields. Many of the techniques that have proved most successful rely on anidea related to the laboratory "iron filings" method. Interstellarspace is filled with tiny, solid particles known as "dust grains,"which may become aligned in the presence of a magnetic field. Thisalignment can be detected through astronomical observations whichmeasure polarization similar to the polarization produced by"Polaroid" sunglasses. In sunglasses, tiny iodine molecules arelined up in rows, which causes the electric vector of transmittedlight to pass through more easily when it is parallel to the rowsthan when it is perpendicular. In space, when the light from abackground star passes through a sea of elongatedmagnetically-aligned dust grains, its electric vector will passthrough the dusty region most easily when it is parallel to the shortaxis of the aligned grains, which is the direction of the magneticfield. Thus, the "polarization of background starlight" can give the(projected) direction of an interstellar magnetic field.
Background starlight polarimetry has been used to map interstellarmagnetic fields for nearly half a century. Unfortunately, theauthor's recent research has shown that this kind of polarimetry isnot always a reliable probe of magnetic fields in the cold, denseregions of interstellar space where stars like our Sun are born. Apparently, these regions are filled with grains which are unable topolarize background starlight. Nonetheless, since magnetic fieldsare believed to play a key role in star formation in these regions,new techniques for mapping fields must be sought out and used.
The polarization of background starlight is usually described as"selective absorption," that is aligned grains block out certainorientations of the light's electric vector. The new mappingtechniques utilize the grains as "selective emitters" rather than as"selective absorbers," with the emission produced as the dustradiates away its own heat at far-infrared through millimeterwavelengths. Because the Earth's atmosphere is opaque over much ofthis wavelength range, the new observations must be carried out abovemost or all of the atmosphere from very high mountains, airplanes,and satellites. For example, observations of polarized "thermal"radiation from magnetically-aligned dust, carried out primarily fromNASA's Kuiper Airborne Observatory, have revealed field structuresvery different from those implied by the old background starlightpolarimetry maps of star-forming regions. The same structures arejust now beginning to be understood theoretically, but they do appearto confirm the magnetic field's "starring" (or, at least,"supporting") role in the interstellar drama.
To make significant progress toward understanding the fascinating andvaried roles interstellar magnetic fields can play, a "Milky Way Magnetic Field Mapping Mission" is being proposed to NASA. This three-month satellite experiment known as "M4" would map out polarized thermal emission over a large fraction of the Milky Way, as well as in several star-forming regions of interest. The "dream plan" of M4 and/or its equivalents could provide us with the first clear and comprehensive view of the Galaxy's biggest "stars"--its interstellar magnetic fields!