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We present the results of HST observations of the Seyfert 2 galaxy, Mrk 3. Observations were taken with the Faint Object Camera and Wide Field Planetary Camera on board of the {\sl Hubble Space Telescope}. Filters centered on different emission lines ([O II]$\lambda$3727,[O III]$\lambda$5007, H$\alpha$, H$\beta$ and H$\gamma$) were used.
The emission line region of Mrk 3 has a striking spiral-like shape. It is formed by a large number of resolved knots embedded in a more extended lower brightness emission region. The brightest part of the spiral is confined to a narrow cone with full opening angle of $\approx$ 40$^o$. The ionization parameter is constant along the spiral, independently on reddening. All these results are naturally explained with an outflow from the nucleus of Mrk 3.
The line and radio emission morphology are very similar in extension and in shape. The dynamics of the ionized gas seems to be dominated by entrainment of the radio jets. There is a change in the emissivity and ionization of NLR gas in correspondence with the maximum of the radio emission. this feature is likely to be the working surface of the radio jet, shocking the photoionized gas.
The asymmetry of the emission-lines profiles of Mrk 3 corresponds to an asymmetry of the emission line flux in the innermost region, with the receding side being fainter than the approaching side. This result is in agreement with the interpretation that asymmetric line profiles are due to absorbtion by dust surrounding the NLR clouds of an otherwise symmetric outflow.
The nuclear emission is very heavily absorbed along the line of sight. Notwithstanding the biconical structure on the large scale, the analysis of the innermost region shows that the ionizing flux is not strongly collimated. These results are best explained by an obscuring torus which is optically thick but geometrically thin ( $ a/h {< \atop\sim} 0.2 $).
