Reading M87's DNA: A Double Helix revealing a large scale Helical
  Magnetic Field
  We present unprecedented high fidelity radio images of the M87 jet. We
analyzed Jansky Very Large Array (VLA) broadband, full polarization, radio data
from 4 to 18 GHz. The observations were taken with the most extended
configuration (A configuration), which allow the study of the emission of the
jet up to kpc scales with a linear resolution $\sim$10 pc. The high sensitivity
and resolution of our data allow to resolve the jet width. We confirm a
double-helix morphology of the jet material between $\sim$300 pc and $\sim$1
kpc. We found a gradient of the polarization degree with a minimum at the
projected axis and maxima at the jet edges, and a gradient in the Faraday depth
with opposite signs at the jet edges. We also found that the behavior of the
polarization properties along the wide range of frequencies is consistent with
internal Faraday depolarization. All these characteristics strongly support the
presence of a helical magnetic field in the M87 jet up to 1 kpc from the
central black hole although the jet is most likely particle dominated at these
large scales. Therefore, we propose a plausible scenario in which the helical
configuration of the magnetic field has been maintained to large scales thanks
to the presence of Kelvin-Helmholtz instabilities.
