  We have obtained HST FOC long-slit optical spectroscopy of the Narrow Line
Region of the Seyfert 2 galaxy Mrk 3. In the region cospatial with the
radio-jet the velocity field is highly perturbed and shows two velocity systems
separated by as much as 1700 km/s. We interpret this to be the consequence of
the rapid expansion of a cocoon of hot gas, shocked and heated by the
radio-emitting outflow, which compresses and accelerates the ambient gas. The
NLR itself is essentially a cylindrical shell expanding supersonically. From
the size and velocity of the expanding region, we derive an upper limit to the
radio-source age, ~< 1.5 E5 years, and a lower limit for the jet power, ~> 2
E42 erg/s required to inflate the cocoon and estimate that the jet minimum
advance speed is 3 E-3 pc per year. The total kinetic energy of the high
velocity NLR gas can be estimated as ~6 E54 erg, comparable to the total energy
carried by the jet over its lifetime and this quantitatively supports the idea
that the NLR gas is accelerated by the jet. If the advance speed of Mrk 3 is
representative of the Seyfert population then these sources must also be short
lived and probably recurrent. The jet kinetic luminosity of Mrk 3 is between 2
and 3 orders of magnitude smaller than that derived for radio-loud AGNs with
similar emission-line luminosity. On the other hand, the fraction of jet power
dissipated in radio-emission is similar. We speculate that the main distinction
between radio-quiet and radio-loud AGN is ascribed to a difference in jet power
rather than to a different efficiency in synchrotron emission production.
