In this paper we have studied the stellar population, nuclear emission and the presence of AGN in a sample of 27 physical galaxy pairs. The main conclusions can be summarised as follows:
(i) The stellar population analysis based on equivalent widths of absorption lines and continuum distribution shows that about half of the bulges of the A components have a very red stellar population and solar abundance, while more than 70% of the B components have an important flux contribution at Å from populations younger than years. From this we infer that the interaction induces strong star formation on the smaller component of the pairs. The effect of interaction is also apparent from the tidal distortions observed in the B components.
(ii) From the usual diagnostic-diagrams, we observed that almost all galaxies have emission line-ratios typical of H II region spectra. However, we noticed that many of these line-ratios are found very close to the transition zone between H II regions and LINERs. In fact, the diagram, which is a good indicator of LINER-like emission line spectra, shows that 4 LINERs and 1 Seyfert 2 galaxy are present in our sample. This number corresponds to 9.2% of the galaxies in our sample.
(iii) We investigated the possible nature of the galaxies with line ratios in the transition zone between H II regions and AGN assuming that these objects could have a composite spectrum due to the simultaneous presence on the slit of a Seyfert nucleus and an H II region. To test this hypothesis we built composite spectra contaminating emission-pure AGN spectra with H II regions of solar abundance. We found that this model applies to 15 objects (and possibly 5 others) in our sample, indicating that they might host a low-luminosity AGN . The true nature (LINER or type 1 and 2 Seyfert) of the AGN can only be revealed through high spatial resolution spectroscopy. It is interesting to point out that both components of the pairs AM 1118-350, AM 1304-333, AM 2229-735 and AM 2322-821 possibly host a low-luminosity active nucleus.
© European Southern Observatory (ESO) 1999
Online publication: June 18, 1999