The Fe9+ region in active galactic nuclei

Spectroscopic data at resolutions of 1 Å or better, including the [Fe X]λ 6374 Å line, are presented for 29 Seyfert and related galaxies. [Fe X] λ 6374 has certainly or probably been detected in 12 galaxies. The strength of both this line and [Fe XI] λ 7892 is correlated with that of the [Fe VII] and [O III] lines, suggesting the Fe⁹⁺ and Fe¹⁰⁺ ions are photoionized. A possible relation between [Fe X] and Hβ intensities is consistent with this view, although current photoionization models do not agree well with the observed relative intensities of the highly ionized iron lines. Alternatively, the line ratios may be reproduced by collisional ionization in a gas of temperature near 10⁶ K, although a range of temperatures is needed. The probable variability of [Fe X] λ 6374 in NGC 4151 and the apparent absence of collisional de-excitation limit the radius of the Fe⁹⁺ zone to between 1 × 10⁻⁴ and 0.5 pc and its density to between 4 × 10⁴ and 6 × 10¹¹ cm⁻³. The [Fe X] and [Fe XI] lines are blueshifted with respect to the systemic velocity of the galaxy, typically by $$\approx120 \text{km s}^{-1}$$ and are also broader than lines from lower ionization species. We ascribe these effects to a combination of higher outflow/inflow velocities closer to the central source and obscuration.