LOW-LUMINOSITY ACTIVE GALAXIES - ARE THEY SIMILAR TO SEYFERT-GALAXIES

A careful X-ray study of five low-luminosity active galactic nuclei (LLAGNs) was conducted to address specifically the issue of whether the dominant X-ray production mechanism is the same at all luminosities in AGNs. The sample consists of three Seyfert 1 galaxies (NGC 4639, NGC 5033, and NGC 5273), and two low-ionization nuclear emission-line regions (NGC 3642 and NGC 4278) having a weak, broad component of Ha emission. We find that the X-ray emission (ROSAT HRI) is mostly or entirely nuclear (less than or similar to 500 pc) despite the low X-ray luminosities (similar to 9 x 10(40) ergs s(-1)) of the sample. The correlation between X-ray luminosity and the broad H alpha emission-line luminosity observed in high-luminosity active galaxies continues down to the low-luminosity range; the mean L(X)/L(H alpha) ratio of LLAGNs is approximately 14, while that of AGNs is approximately 29. ROSAT PSPC observations of three LLAGNs in our sample indicate that the 0.2-2.2 keV X-ray spectral energy distributions are similar to those seen in Seyfert nuclei but do not have high intrinsic absorbing columns. Using existing ultraviolet data for four LLAGNs, we find that the ''optical'' (2500 Angstrom) to X-ray spectral index (alpha(OX)) has an average upper limit of 1.6; for comparison, the measured value in AGNs is typically 1.4, while that in M81 (the prototypical LLAGN) is 1.0.