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J. R. Forster, M. C. H. Wright (Radio Astronomy Lab., UC, Berkeley), R. Rao (Dept.of Physics, U. Chicago)
M87 is the dominant elliptical galaxy in the Virgo cluster, and it contains a massive black hole at its center. At high radio frequencies the galaxy appears as a bright, compact core with a one-sided jet extending ~20 arcsec from the nucleus. Extended synchrotron emission surrounds the nucleus, forming a double-lobed radio source 1 arcmin (~5 kpc) in extent. This 1 arcmin core is embedded in a much more extended (16 arcmin) steep-spectrum radio source, which is itself embedded in a halo of X-ray emission extending ~30 arcmin in diameter.
We present new polarization images of the radio source in M87 at 90 GHz. The data were obtained using multiple configurations of the 10-element BIMA array, and fully sample angular scales from 0.5 arcsec to 1 arcmin. Faraday rotation in the extended lobes is negligible at 90 GHz, so the position angles measured are the intrinsic angles produced by the transverse component of the magnetic field. The data show that the magnetic field structure is systematic on the large scale and is aligned with the boundaries of the radio lobes. The increase in percentage polarization near the lobe boundaries and the direction of the magnetic field lines suggest that the core is expanding in a high-pressure environment. Shear flow and compression cause the field lines to align circumferentially at the boundary. The percentage polarization is low along the jet, indicating a more complex magnetic field structure in the knots.
The author(s) of this abstract have provided an email address for comments about the abstract: rforster@astro.berkeley.edu