THE MORPHOLOGY OF THE NARROW-LINE REGION OF MARKARIAN 3

摘要

We present the results of Hubble Space Telescope observations of the Seyfert 2 galaxy Mrk 3. Images were taken with the Faint Object Camera and Wide Field Planetary Camera with filters centered on the emission lines [O II] λ3727, [O III] λ5007, Hα, and Hγ. The narrow-line region (NLR) of Mrk 3 has a striking S-shaped morphology extending over more than 2″ and comprising a large number of resolved knots. Its small-scale structure is remarkably well reproduced in all the emission lines observed, with a close knot-to-knot correspondence. The ratios between [O Ⅱ] and [O III] and between [O III] and Hγ emission-line fluxes are constant. The photon budget confirms that the nucleus of Mrk 3 is heavily obscured along the line of sight. Radio maps of Mrk 3 shows two highly collimated radio jets and the optical line and radio emissions are very closely associated. We propose a scenario where the line-emitting gas is compressed by the shocks created by the passage of the supersonic jet. The increase in the density due to the shocks causes the line emission to be highly enhanced in the region where this interaction occurs. On each side of the S, the line emission is, on average, displaced in opposite directions with respect to the radio emission as expected from the finite cooling time of the shocked gas which follows the galaxy rotation. The Faint Object Spectrograph spectrum west of the nucleus shows that the line widths are very large, FWHM ～ 1200 km s~(-1), and that the [O III] lines are clearly double-peaked as would be expected in this interpretation. The range of shock velocity obtained from the cooling length argument is consistent with the measured line widths. The prominent western radio lobe has a high-excitation emission-line core, surrounded by a diffuse halo of relatively lower ionization. We interpret this as a consequence of the sweeping up of gas by the expanding radio lobe. The narrow linear ridge of the S-shaped NLR is misaligned with respect to the symmetry axis of the larger scale biconical emission. If there is a wide angle ionization cone, as in NGC 1068, it must be partly filled by gas and its opening angle must be larger than 110°. It appears that, at least on the small scale, the dominant factors in determining the NLR morphology are either the gas distribution or the effects of the radio outflow rather than the geometry of the ionizing field. A small-scale bar (≈0.35″ x 1″) of continuum emission is also observed, possibly due to free-free emission from the hot shocked gas or scattered nuclear light.