The polarized spectral energy distribution of NGC 4151
  NGC 4151 is among the most well-studied Seyfert galaxies that does not suffer
from strong obscuration along the observer's line-of-sight. This allows to
probe the central active galactic nucleus (AGN) engine with photometry,
spectroscopy, reverberation mapping or interferometry. Yet, the broadband
polarization from NGC 4151 has been poorly examined in the past despite the
fact that polarimetry gives us a much cleaner view of the AGN physics than
photometry or spectroscopy alone. In this paper, we compile the 0.15 -- 89.0
$\mu$m total and polarized fluxes of NGC 4151 from archival and new data in
order to examine the physical processes at work in the heart of this AGN. We
demonstrate that, from the optical to the near-infrared (IR) band, the
polarized spectrum of NGC 4151 shows a much bluer power-law spectral index than
that of the total flux, corroborating the presence of an optically thick,
locally heated accretion flow, at least in its near-IR emitting radii. Specific
signatures from the atmosphere of the accretion structure are tentatively found
at the shortest ultraviolet (UV) wavelengths, before the onset of absorption
opacity. Otherwise, dust scattering appears to be the dominant contributor from
the near-UV to near-IR polarized spectrum, superimposed onto a weaker electron
component. We also identify a change in the polarization processes from the
near-IR to the mid-IR, most likely associated with the transition from Mie
scattering to dichroic absorption from aligned dust grains in the dusty torus
or narrow-line region. Finally, we present and dicuss the very first
far-infrared polarization measurement of NGC 4151 at 89 $\mu$m.
