The sensitivity of the enhanced VLA, combined with the good
frequency coverage provided by the proposed receivers (particularly
the 2.4 GHz band), makes it possible to make polarization measurements of
much fainter and more diffuse sources. The experiments described
below all require good sensitivity and good resolution at the low
frequencies, and polarization capabilities at least as good as the
current systems (1%) at noise levels of a few microJy. We also
need to achieve dynamic ranges of 
:1 routinely.
No polarization has ever been detected in an external SNR/RSN.
Since galactic remnants are highly polarized but often symmetric,
this is presumably a combination of poor resolution and
insufficient sensitivity. The enhanced VLA would solve both
problems: the extended A+ configuration provides 
images at
2.4 GHz, corresponding to sub-parsec resolution out to 2.6 Mpc, at
sensitivities of 50 microJy in 2 hrs. This is sufficient to
image 1% polarization in remnants as strong as Cas A out to that
distance. For RSNe, resolution is obviously impossible, but much
more rigorous limits could be placed on the integrated
polarization, and consequently on the symmetry of the supernova.
Only a few radio halos have been reliably imaged in polarization, those of NGC 4631 and NGC 891 being the best examples; the strong and well-ordered magnetic fields inferred far above the disks of these galaxies are intriguing, but may indicate the peculiarities of those systems rather than any general trend. Knowledge of magnetic field structures at high-z is important both for understanding diffusion of cosmic rays above the disk, and for arguments about the relative strength of the pressure/energy in magnetic fields, cosmic rays, and thermal particles. With the enhanced VLA, one could go a factor 10 deeper than the best current 1.4 GHz images of NGC 891, and start looking at normal rather than actively star-forming galaxies.
The evolution of large-scale magnetic fields in galaxies is at
best poorly understood, with simple models (e.g.,
axisymmetric/bisymmetric fields) that observations already show to
be woefully inadequate. With the theorists frustrated, the hope
is that more numerous and more detailed observations will make the
physics of what's going on a bit clearer. The enhanced VLA will
be able to
investigate magnetic fields down to much smaller scales, and in a
much larger sample of galaxies than the 
already imaged.
A 1-kpc beam at Virgo (15 Mpc) would be about 
, and would
(for a typical disk) give a flux density (I) of 300 microJy/beam.
Assuming a polarization of 10%, one requires a noise level of
4.3 microJy/beam to see this at 
, an experiment which
would currently require 24 hrs, but which could be done in under
3 hrs with the enhancement. More local galaxies could be imaged at
much higher resolution, showing the magnetic field structure down
to 75 pc (at 1 Mpc) in a similar time.