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From p181bck@mpifr-bonn.mpg.de Wed Dec 2 09:07:45 1998 Date: Wed, 2 Dec 1998 16:59:13 +0100 From: p181bck@mpifr-bonn.mpg.de (Rainer Beck) To: ccarilli@NRAO.EDU Subject: VLA upgrade Content-Length: 3399 X-Lines: 76 Status: RO Proposal of a VLA upgrade for polarization observations ------------------------------------------------------- The VLA is an excellent instrument to observe extended polarization of galaxies. In recent years we learnt a lot about the structure and strength of interstellar magnetic fields. HGowever, our knowledge is still very limited, and some of the major questions are still unsolved: 1. Are interstellar fields primordial or generated by dynamos? This can be decided by determination of patterns in the Faraday rotation measures. To avoid beam depolarization, high angular resolution is required. 2. If dynamos work, what drives them? This needs high-resolution observations of fields near possible sources of ISM turbulence. 3. How strong and how regular are magnetic fields in young galaxies? How fast can the magnetic field develop? This needs polarization observations of distant galaxies with high resolution. 4. Is the pitch angle of the field in spiral galaxies determined by the dynamo or by the density wave? This needs accurate observations of polarization angles, i.e. high signal-to-noise ratios, especially in smooth, low-brightness interarm regions. 5. What is the diffusivity of interstellar fields? Under which conditions is the field frozen into a gas flow? This needs high-resolution comparison between gas flows as determined from CO or HI data and the field structure, not only in spiral galaxies, but also in barred and irregular ones. 6. Which component of the ISM gas is mainly responsible for anchoring the magnetic fields? This needs high-resolution comparisons with CO, HI, HII, etc. 7. Are interstellar fields deformed by star-formation processes, and do they influence star formation? This needs high-resolution observations. All these questions need high-sensitivity and (mostly) high-resolution polarization observations. While the resolution of the VLA is fully sufficient even in its present configurations, SENSITIVITY is what limits us at the moment. We detected significant polarized intensities in D-array at 3cm and 6cm in all spiral galaxies and some irregulars. In C-array, only bright galaxies like M51 are visible in polarized emission. With higher resolution, intensities per beam become too low. To achieve the aims as listed above, a significant increase in sensitivity is ESSENTIAL, such that we gain MORE THAN A FACTOR OF 10 in observation time. We don't need a continuous frequency coverage because we concentrate on frequencies where the missing spacings can be filled by our Effelsberg single-dish data, i.e. X- and C-band. L-band polarization data are generally corrupted by Faraday depolarization. To obtain accurate rotation measures, a large frequency separation is required in the Faraday-thin regime. RM(3/6cm) is often insufficient, thus another, longer wavelength (e.g. 11cm, also available at Effelsberg) is of high importance for our work. More telescopes near the center of the array would help to see the weak, extended emission. Filling the missing spacings with Effelsberg data only helps for large galaxies. The Effelsberg beam e.g. at 6cm is 2.45' and can fill all missing spacings only for the D-configuration. We are looking forward to the future glory of the VLA ! Rainer Beck, on behalf of the MPIfR continuum group
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