Mid-infrared polarization studies of SgrA: a three-dimensional study of the central parsec

Thermal emission from magnetically aligned dust grains produces the observed mid-infrared polarization in the northern arm and east-west bar of SgrA West; recent arcsecond-resolution imaging polarimetry at 12.5 mu m of these ionized filaments is presented, which confirms and extends previous studies, A lower limit similar to 2 mG is found for the magnetic field in the northern arm and the IRS 16 complex appears to be displaced from the northern arm by similar to 0.15 pc along the line of sight. It is shown that the physical conditions in the ionized filaments of the central parsec lead to a very uniform grain alignment that is directed along the local magnetic field. The position angle of polarized emission will then be at right angles to the projection of the field direction on the plane of the sky and its amplitude a measure of the component of field along the line of sight; this makes possible a partial reconstruction of the field in three dimensions. We present the first application of the use of polarimetry in this way. This partial reconstruction is compared with the H92 alpha observations of Roberts et al. and the implications are that the northern arm and east-west bar do not define either an orbital path or a spiral arm but rather represent a tidally stretched structure in free fall about SgrA* with significant deviations from a single plane, and most likely represent the inner ionized rim of a more extended neutral cloud.