The ultraviolet spectrum of the liner NGC 4579

We present a Hubble Space Telescope ultraviolet spectrum of the LINER nucleus of NGC 4579, spanning the wavelength range 1160-3270 Å. The data reveal a low-excitation spectrum of narrow (FWHM ≈ 1200 km s^-1) and broad (FWHM ≈ 6600 km s^-1) emission lines, which provide new diagnostics of the physical conditions within the emission-line regions. A featureless ultraviolet continuum is detected unambiguously, with f_ν ∝ ν^-1.0 over the range 1300-2200 Å and ν^-2.4 over 2200-3000 Å. The 2200 Å continuum flux is a factor of 3.3 lower than the value reported by Maoz et al. (1995) based on an HST image taken 19 months earlier; this is the first time that UV variability has been discovered in a LINER. The predictions of fast shock models are incompatible with the narrow-line spectrum, because the high-excitation lines produced in fast shocks are weak or absent in NGC 4579. Models of a single slab of gas photoionized by a power-law spectrum do not reproduce the observed line ratios well either, and it appears that the narrow-line region cannot be characterized by a unique value of the ionization parameter. We argue that the narrow-line region is photoionized by the central UV/X-ray source, and that more sophisticated photoionization calculations, incorporating the effects of density and/or ionization stratification and different continuum shapes, can yield results in better agreement with the data.