Resolved Spectroscopy of the Narrow‐Line Region in NGC 1068. II. Physical Conditions near the NGC 1068 “Hot Spot”

Abstract

The physical conditions near the optical continuum peak ("" hot spot ÏÏ) in the inner narrow-line region of the Seyfert 2 galaxy NGC 1068 are examined using ultraviolet and optical spectra and photoioniza-tion models. The spectra were taken with the Hubble Space T elescope/Space Telescope Imaging Spectro-graph (STIS) through the slit, covering the full STIS 1200È10000 waveband, and are from a 0A .1 ] 52A .0 A Ž region that includes the hot spot, extending or D14 pc (for km s~1 Mpc~1), in the cross-0A .2, H 0 \ 75 dispersion direction. The spectra show emission lines from a wide range of ionization states for the most abundant elements, similar to archival Faint Object Spectrograph spectra of the same region. Perhaps the most striking feature of these spectra is the presence of strong coronal emission lines, including [S XII] j7611, which has hitherto been identiÐed only in spectra of the solar corona. There is an apparent correlation between ionization energy and velocity of the emission lines with respect to the systemic velocity of the host galaxy, with the coronal lines blueshifted, most other high-excitation lines near sys-temic, and some of the low-ionization lines redshifted. From the results of our modeling, we Ðnd that the emission-line gas is photoionized and consists of three principal components : (1) one in which most of the strong emission lines, such as [O III] j5007, [Ne V] j3426, and C IV j1550, arise ; (2) a more tenuous, highly ionized component, which is the source of the coronal-line emission ; and (3) a component, which is not coplanar with the other two, in which the low-ionization and neutral lines, such as [N II] j6548 and [O I] j6300, are formed. The Ðrst two components are directly ionized by the EUVÈX-ray continuum emitted by the central source, while the low-ionization gas is ionized by a combination of highly absorbed continuum radiation and a small fraction of unabsorbed continuum scattered by free electrons associated with the hot spot. The combination of covering factor and Thomson optical depth of the high-ionization components is insufficient to scatter the observed fraction of continuum radiation into our line of sight. Therefore, the scattering must occur in an additional component of hot plasma, which contributes little or no UV/optical line emission.