Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208
Yerkes Observatory, University of Chicago, P.O. Box 258, Williams Bay, WI 53191
Department of Astronomy and Astrophysics, and Enrico Fermi Institute, University of Chicago, 5630 South Ellis Avenue, Chicago, IL 60637
Published in 1995, Vol.73 of the ASP Conference Series: Airborne Astronomy Symposium on the Galactic Ecosystem: From Gas to Stars to Dust, ed. M.R. Hass, J.A. Davidson, and E.F. Erickson (San Francisco:ASP)
We discuss proposed observations of the polarization of submillimeter continuum emission from the extended Galactic Center region. The observations will be made from the South Pole, using the VIPER 2 meter telescope.
On the largest scales, magnetic fields in the disks of spiral galaxies have spiral configurations. This is true for both the hot, ionized regions and the cooler, predominantly neutral regions. However, for the nucleus of the Galaxy, observations of ionized regions show evidence for a large scale field that has a vertical orientation (i.e. the field is perpendicular to the Galactic Plane). Large quantities of neutral gas are also found in the Galactic nucleus: it contains about one tenth of the Galaxy's total supply, forming a layer roughly 40 pc thick and 450 pc across (Güsten 1989). The large scale configuration of the magnetic field within this neutral gas layer is unknown.
The last decade has witnessed the development of a new technique for observing magnetic fields in neutral regions: polarimetry of far-infrared/submillimeter thermal emission from magnetically aligned dust grains (Hildebrand 1988, Davidson et al.1994). Polarimetry of 100 micron dust emission, obtained using the KAO, has provided magnetic field maps for several relatively small (~10 pc) regions within the Galactic nucleus, one of which is the circumnuclear ring in Sgr A (Werner et al.1988, Hildebrand et al.1993).
Due to recent advances in instrumentation and techniques for far-infrared/submillimeter polarimetry (Platt et al. 1994) it is now feasible to map the large scale magnetic field within the entire Galactic Center neutral gas layer, which stands out clearly at the center of the 240 micron DIRBE map of Fig.1. It is highly advantageous, however, to use a telescope that permits chopper throws of about a degree, and to observe from a location that allows sensitive measurements over some portion of the far-infrared/submillimeter wavelength range. Such a telescope at such a location will be available when the VIPER telescope is installed at the South Pole in the Austral Summer of 1995-6. VIPER is a 2 meter submillimeter telescope that is being built by the Center for Astrophysical Research from Antarctica (CARA). We are building a 32 beam submillimeter polarimeter for operation on this telescope during the Antarctic winter, and will use it to map the large scale magnetic field in the neutral gas at the Galactic Center.
Figure 1. 240 micron DIRBE map of the central 1.5 kpc of the Galaxy. The array polarimeter for VIPER will operate at 450 microns, and will have an angular resolution of four arcminutes. The brightness of the Galactic Center is sufficient for us to make a polarimetric map of the area within the heavy contour, to an accuracy of sigma(P) = 0.3%, during a few weeks of winter-over observations. The footprint of the array polarimeter is shown in the top right corner of the figure. The Northern and Southern symbols show the positions of Sgr B2 and Sgr A, respectively.
Our observations will consist of hundreds of individual polarization measurements (see Fig. 1), and we will use the resulting magnetic field map to answer the following questions:
The discovery of a magnetic field in the circumnuclear ring has stimulated investigators to model it using the theory of self-similar magnetic accretion disks -- a theory that was originally developed by Blandford and Payne (1982) to account for extragalactic jets. One of the assumptions in this model is that the magnetic field far from the disk is axial (Wardle and Konigl, 1990). Our observations will determine whether or not the field in the neutral gas that surrounds the ring has an axial configuration, thus providing an important test for the magnetic accretion disk model of the ring.
The discovery of non-thermal radio filaments in the Galactic Center (Yusef-Zadeh, Morris, and Chance 1984) provided the first evidence for large-scale vertical magnetic fields in the hot, ionized regions of the Galactic Center. There is now evidence, from radio morphology and polarization, for organized vertical fields extending out to Galactocentric radii of several hundred pc (Haynes et al.1992, Tsuboi et al.1986). Various hypotheses for the origin of these fields have been proposed. For example, they could be generated by a dynamo mechanism, or they could be of primordial origin. A strong vertical field will influence cosmic ray confinement and transport (Blitz et al. 1985), star formation (Morris 1990), and gas dynamics and accretion.
Our observations will determine the extent to which these vertical fields extend into the neutral gas layer. Because most of the nuclear gas is in this layer, this information will have relevance for understanding the origin of the vertical fields and their role in the gas dynamics at the Galactic Center.
The formation of massive stars occurs in relatively high-mass molecular clouds, and the Sgr B2 molecular cloud represents an extreme case of this general behavior (Goldsmith et al. 1990). The mean density of Sgr B2 is comparable to that of high-density gas found in starburst nuclei. Polarimetry using VIPER represents the best method for obtaining complete maps of magnetic field configurations within Sgr B2 and other Galactic Center GMC's, and thus for determining the possible importance of magnetic fields for star formation in starburst galaxies.
Our observations will provide an opportunity to estimate the strength of the Galactic Center magnetic field, using the method of Chandrasekhar and Fermi (1953).
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