Evidence for low-level AGN activity in the nucleus of the LINER galaxy NGC 4594

We investigate the properties of the LINER galaxy, NGC 4594, using data taken with the Faint Object Spectrograph on the HST and with the ASCA X-ray observatory. The ultraviolet spectrum shows only narrow-line emission with no evidence for broad emission-line components. The emission-line data are compared with shock and photoionization model predictions. Fast shock models are incompatible with the UV emission-line spectrum and the best fit is found with a model, devised by Shull and McKee (1979), where slow shocks are incident on a relatively dense medium. Among photoionization models the best agreement is obtained with a CLOUDY model which has an ionization parameter of U = 3.2 x 10 exp -4. The hard X-ray ASCA data on NGC4594 yield a spectral index of alpha(X) = 0.62 +/- 0.03 which extrapolates, within the errors, to the soft X-ray spectrum measured with the ROSAT PSPC. When the ASCA and ROSAT data are combined, the best-fitting column density is N(H) = 5.3 x 10 exp 20/sq cm, slightly higher than the Galactic value in this direction. However, allowing for contamination by soft extended emission close to the nucleus, the column density to the nuclear source may be as high as 2.9 x 10 exp 21/cu cm. The multiwavelength spectrum of the NGC 4594 differs from those of luminous AGN in the relative weakness of the UV continuum. This is consistent with the Siemiginowska model for a low-state accretion disk. (Author)