We looked at galaxies with line ratios, which put them close to the borderline between starburst galaxies and LINERs/Seyferts in diagnostic diagrams. Comparison of the observed line ratios with line ratios from various models indicates the presence of a composite ionising mechanism: star formation in combination with either an active nucleus or with shocks. This is in agreement with the increasing number of observations of active nuclei with circumnuclear star formation and of starburst nuclei surrounded by shock-ionised shells. The far-infrared properties confirm the important role played by star formation in these galaxies.

Introduction

We searched the literature for galaxies, which, on the basis of their optical line ratios, could not be uniquely classified as either starburst galaxies or Seyferts/LINERs. The (arbitrary) selection criterion used was that the [NII]/Hα ratio should be within 0.3 from the empirical borderline between starbursts and Seyferts/LINERs in the log([NII] λ6583/Hα) vs. log([OIII] λ5007/Hβ) diagram (Veilleux & Osterbrock 1987). In all cases this also meant that the observed log([SII] λ6716+31/Hα) and log([OI] λ6300/Hα) ratios were close to their corresponding borderlines. In this way a total of 62 galaxies was selected. All selected galaxies are spiral galaxies, mostly late type: 54% are Sb and 27% Sc galaxies. The percentage of barred galaxies is 39%.

We used the photoionisation code CLOUDY to see if the observed line ratios can be explained by a single ionising mechanism, i.e. by a starburst or by an active nucleus.