The origin of the narrow-line region of Markarian 3 : an overpressured jet cocoon

Abstract

We have obtained Hubble Space Telescope (HST ) Faint Object Camera (FOC) f/48 long-slit optical spectroscopy of the inner 2" of the narrow-line region (NLR) of the Seyfert 2 galaxy Mrk 3 with a spatial resolution of 0".06. Spectra were taken in six locations with the slit approximately perpendicular to the radio axis. In the region cospatial with the radio jet, where the brightest emission line knots are located, the velocity field is highly perturbed, showing two velocity systems separated by as much as 1700 km s-ˡ. In several locations the split lines form almost complete velocity ellipsoids, implying that we are seeing an expanding shell of gas. The diameter of this shell (~200 pc) is much larger than the width of the radio jet (d<15 pc). 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 cocoon mediates the energy exchange between jets and the line-emitting gas. The gas motions within the NLR of Mrk 3 are therefore clearly dominated by the interaction between the jets and the interstellar medium; the NLR itself is essentially a cylindrical shell expanding supersonically. With its current size of 200 pc, the cocoon has expanded to several disk scale heights. Because of the external gas density stratification, the hot gas located above the plane of the disk blows out into the halo, puncturing the bubble and fracturing the velocity ellipsoids. The system is e†ectively momentum driven. From the size and velocity of the expanding region, we derive an upper limit to the age of the radio source of <~1.5x10 5 yr, and a lower limit for the jet power of >~2x10 42 ergs s-ˡ, required to inflate the cocoon; we estimate that the jet minimum advance speed is 3x10-³ pc yr-ˡ. The total kinetic energy of the high-velocity gas associated with the radio jet can be estimated as ~6x10 54 ergs, comparable to the total energy carried by the jet over its lifetime ; this quantitatively supports the idea that the NLR gas is accelerated by the jet. Radio outflows are associated with at least 50% of Seyfert galaxies with typical sizes smaller than a few kpc. If the advance speed of Mrk 3 is representative of the Seyfert population, then these sources must also be short lived and are probably recurrent. Evidence that this is indeed the case is provided by the fact that the expansion timescale derived for NGC 1068 is comparable to that seen in Mrk 3. 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 active galactic nuclei can be ascribed to a di†erence in jet power rather than to a di†erent efficiency in synchrotron emission production.