Do LINER 2 galaxies harbour low‐luminosity active galactic nuclei?

We use ROSAT HRI spatial data and ASCA spectral measurements for a sample of seven nearby, early‐type spiral galaxies, to address the question of whether a low‐luminosity active galactic nucleus (LLAGN) is present in galaxies that have a LINER 2 classification. The brightest discrete X‐ray source in the ROSAT HRI observations is invariably found to be positionally coincident with the optical galactic nucleus, and in most cases its flux dominates the X‐ray emission from the central region of the galaxy. All seven galaxies have X‐ray spectra consistent with a two‐component, soft thermal plus hard power‐law, spectral form. If we exclude the two galaxies with relatively hard X‐ray spectra, NGC 3628 and NGC 4594, for which there is supporting evidence for a LLAGN (or alternatively in the case of NGC 3628 a dominant ultraluminous X‐ray binary), then the remaining galaxies show surprisingly similar X‐ray spectral properties. Specifically the flux ratio F X (0.5–1)/F X (2–5) , which measures the relative strengths of the thermal and non‐thermal emission components, shows little scatter about a mean of 0.66, a value very similar to that measured in the classic starburst galaxy NGC 253. As there is no obvious reason why the luminosity of the hard power‐law continuum emanating from a putative LLAGN should be very closely correlated with the thermal emission of the surrounding region, this suggests that that the broad‐band (0.5–5 keV) X‐ray emission from these LINER 2 galaxies may originate in a common set of processes probably associated with the starburst phenomenon. Conversely, it appears that in many LINER 2 galaxies and perhaps the majority, the nuclear X‐ray luminosity does not derive directly from the presence of a LLAGN.