  We present UV imaging polarimetry data of the Seyfert 2 galaxy Mrk 3 taken by
the Hubble Space Telescope. The polarized flux is found to be extended to ~1
kpc from the nucleus, and the position angles of polarization are
centrosymmetric, confirming that the polarization is caused by scattering. We
determine the location of the hidden nucleus as the center of this
centrosymmetric pattern. From the polarization images taken in two broad bands,
we have obtained the color distribution of the polarized flux. Some regions
have blue polarized flux, consistent with optically-thin dust scattering, but
some bright knots have a color similar to that of Seyfert 1 nucleus. Also, the
recent Chandra X-ray observation suggests that the ratio of scattered UV flux
to scattered X-ray flux is rather similar to the intrinsic UV/X-ray ratio in a
Seyfert 1 nucleus, if the observed extended X-ray continuum is scattered light.
While the scattered X-ray would be essentially from electron scattering, the UV
slope and UV/X-ray ratio both being similar to Seyfert 1's would lead to two
possibilities as to the nature of the UV scatterers. One is that the UV may
also be scattered by electrons, in which case the scattering gas is somehow
dust-free. The other is that the UV is scattered by dust grains, but the
wavelength-independent UV scattering with low efficiency indicated by the UV
slope and UV/X-ray ratio would suggest that the grains reside in UV-opaque
clouds, or the dust might be mainly composed of large grains and lacks
small-grain population.

Astrophysics