The extended emission-line region of the Seyfert galaxy Mrk 573

Abstract

We present the results of observations of the extended emission-line region of the Seyfert 2 galaxy Mrk 573, obtained using the WFPC2 on board HST and the TIGER integral field spectrograph on the CFHT. We describe and use a new ‘guided’ deconvolution technique in which the spatial resolution of the TIGER data cubes is improved, allowing us to obtain the [O iii] gaseous kinematics with an unprecedented spatial resolution of ≃0.35 arcsec. The HST images outline the detailed complex structure of the central ∼3 kpc of this galaxy in [O iii] and Hα+[N ii], with strings of knots and a system of arcs straddling the nucleus. The [O iii]/(Hα+[N ii]) map shows that, taken together, all these individual features define a double wedge form with sharp edges, consistent with an ionization cone. The ‘linear’ radio structure is closely associated with the strings of emission-line knots, with the north-west radio lobe lying inside one of the arcs. Strong velocity perturbations are found in the vicinity of all of the radio components, witnessing the interaction between the radio ejecta and the ambient material. The spectral and kinematical properties of the different components of the emission-line region of Mrk 573 are discussed. The emission-line knots, associated with the radio knots and the velocity perturbations, probably trace the deflection of the radio jet by clouds. The inner arcs may represent radiative bow shocks driven by the weakened radio jets. The similar structure of each of these arcs in the different emission lines, and the absence of any strong kinematic perturbation in the arcs themselves, indicates that they are not photoionizing shocks. Instead, the arcs are photoionized by an external source of radiation. The same conclusion is reached for the outer arcs, which mark the transition between the narrow-line and extended narrow-line regions of Mrk 573. We use the measured variation of the ionization parameter and gas density to derive the flux of ionizing photons as a function of distance from the nucleus. We find that, unless the ionizing photon flux of the compact central source has decreased by a factor of 10 over the last 4000 yr, a model in which all the ionizing photons originate in the central source is excluded. Instead, we speculate that fast shocks, possibly associated with the jet/cloud interactions, may provide the required spatially extended source of ionizing photons.