  We report a helicity analysis of sub-threshold rho^0 production on 2H, 3He
and 12C at low photo-production energies. The results are indicative of a large
longitudinal rho^0 polarization (l=1, m=0) and are consistent with a strong
helicity-flip production mechanism. This signature is used to extract in-medium
rho^0_L invariant mass distributions for all three nuclei in a manner which is
less model-dependent than previous measurements. The results are compared to
kinematic and phenomenological models of the rho^0 spectral function. The 2H
and 3He data distributions support the role of N*(1520) excitation in shaping
the in-medium rho^0_L invariant mass distribution, while the 12C distributions
are consistent with quasi-free rho^0_L production. The data support an
in-medium modification of the rho^0_L invariant mass distribution.
