K. L. Nicholson
Astrophysical properties of LINER galaxies
1998 (supervisor: K. O. Mason)

Over a decade ago, Heckman (1980) described a class of objects whose optical spectra are quite distinct from those of both HII regions and classical active galactic nuclei (AGNs). These objects are named "low-ionization nuclear emission-line regions" (LINERs). There is a growing body of evidence to suggest that LINERs are a far from uniform class of objects and the origin of the ionising flux in these galaxies is controversial.

Hubble Space Telescope (HST) data are used to investigate the ultraviolet properties of the LINER 2 galaxies, NGC4594 and NGC404. Their UV spectra show narrow line emission only. The UV emission line ratios of NGC4594 fit well with slow shock models and photoionisation by a non-stellar continuum. The UV spectrum of NGC404 reveals absorption lines consistent with a starburst with an age of > 6 Myr. NGC4594 is also observed in the X-ray wavelengths and the data resemble that of a "low-luminosity" AGN. The multiwavelength spectrum of NGC4594 differs from those of luminous AGN in the relative weakness of the UV continuum. This is consistent with "low-state accretion" disk models and "advection-dominated" disk models. The results show that shocks, photoionization by an AGN and ionization by hot stars may all make energetically significant contributions to the physical nature of LINERs.

Data from the ROSAT PSPC is used to study the long-term X-ray variability characteristics of the LINER galaxy, Arp 185. None is detected at a level which suggests that the source of X-ray emission in this galaxy is not accretion onto a compact object. Arp 185 is investigated further by looking at its X-ray spectrum and radial profile. These data suggest that Arp 185 is a starburst-driven LINER.