Resolved spectroscopy of the narrow-line region in NGC 1068. II. Physical conditions near the NGC 1068 "hot spot"

The physical conditions near the optical continuum peak ("hot spot") in the inner narrow-line region of the Seyfert 2 galaxy NGC 1068 are examined usi ng ultraviolet and optical spectra and photoionization models. The spectra were taken with the Hubble Space Telescope/Space Telescope Imaging Spectrog raph (STIS) through the 0 ".1 x 52 ".0 slit, covering the full STIS 1200-10 000 Angstrom waveband, and are from a region that includes the hot spot, ex tending 0 ".2, or similar to 14 pc (for H-0 = 75 km s(-1) Mpc(-1)), in the cross-dispersion direction. The spectra show emission lines from a wide ran ge of ionization states for the most abundant elements, similar to archival Faint Object Spectrograph spectra of the same region. Perhaps the most str iking feature of these spectra is the presence of strong coronal emission l ines, including [S XII] lambda 7611, which has hitherto been identified onl y in spectra of the solar corona. There is an apparent correlation between ionization energy and velocity of the emission lines with respect to the sy stemic velocity of the host galaxy, with the coronal fines blueshifted, mos t other high-excitation lines near systemic, nd some of the low-ionization lines redshifted. From the results of our modeling, we find that the emissi on-line gas is photoionized and consists of three principal components: (1) one in which most of the strong emission lines, such as [O III] lambda 500 7, [Ne V] lambda 3426, and C IV lambda 1550, arise; (2) a more tenuous, hig hly ionized component, which is the source of the coronal-line emission; an d (3) a component, which is not coplanar with the other two, in which the l ow-ionization and neutral lines, such as [N II] lambda 6548 and [O I] lambd a 6300, are formed. The first two components are directly ionized by the EU V-X-ray continuum emitted by the central source, while the low-ionization g as is ionized by a combination of highly absorbed continuum radiation and a small fraction of unabsorbed continuum scattered by free electrons associa ted with the hot spot. The combination of covering factor and Thomson optic al depth of the high-ionization components is insufficient to scatter the o bserved fraction of continuum radiation into our fine of sight. Therefore, the scattering must occur in an additional component of hot plasma, which c ontributes little or no UV/optical line emission.