EVIDENCE FOR LOW-LEVEL AGN ACTIVITY IN THE NUCLEUS OF THE LINER GALAXY NGC-4594

We investigate the properties of the low-ionization nuclear emission-l ine region (LINER) galaxy, NGC 4594, using data taken with the Faint O bject Spectrograph on the Hubble Space Telescope (HST) and with the AS CA X-ray observatory. The ultraviolet spectrum shows only narrow-line emission (FWHM approximate to 800-1000 km s(-1)) with no evidence for broad emission-line components. The emission-line data are compared wi th shock and photoionization model predictions. Fast shock models are incompatible with the UV emission-line spectrum and the best fit is fo und with a model, devised by Shull & McKee, where slow shocks (100 km s(-1)) are incident on a relatively dense medium (n(0) = 100 cm(-3)). Among photoionization models the best agreement is obtained with a CLO UDY model which has an ionization parameter of U = 3.2 x 10(-4). The h ard X-Pay 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 s pectrum 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(20) cm(-2 ), slightly higher than the Galactic value in this direction. However, allowing for contamination by soft extended emission close to the nuc leus, the column density to the nuclear source may be as high as 2.9 x 10(21) cm(-2). The multiwavelength spectrum of the NGC 4594 differs f rom those of luminous active galactic nuclei (AGN) in the relative wea kness of the UV continuum. This is consistent with the Siemiginowska m odel for a low-state accretion disc. Advection-dominated disc models, which have been applied to low-luminosity AGN, also agree qualitativel y with our data.