Evidence for low-level AGN activity in the nucleus of the LINER galaxy NGC4594

Abstract

We investigate the properties of the low-ionization nuclear emission-line region (LINER) galaxy, NGC4594, using data taken with the Faint Object Spectrograph on the Hubble Space Telescope (HST) and with the ASCA X-ray observatory. The ultraviolet spectrum shows only narrow-line emission (FWHM ~800-1000 km s^-1) 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 & 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 cloudy model which has an ionization parameter of U=3.2x10^-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 ASCAROSAT data are combined, the best-fitting column density is N_H=5.3x10^20cm^-2, 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 2.9x10^21cm^-2. The multiwavelength spectrum of the NGC4594 differs from those of luminous active galactic nuclei (AGN) in the relative weakness of the UV continuum. This is consistent with the Siemiginowska model for a low-state accretion disc. Advection-dominated disc models, which have been applied to low-luminosity AGN, also agree qualitatively with our data.