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Astron. Astrophys. 347, 55-62 (1999)

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1. Introduction

In a recent paper, Donzelli & Pastoriza (1997, hereafter DP97) have studied the spectroscopic properties of 27 physical pairs consisting of a main galaxy (component A) and a companion (component B) which has about half or less the diameter of component A. We confirmed for this sample an earlier result that physical pairs have higher mean star formation rates than isolated galaxies (Bushouse 1986; Kennicutt et al. 1987). However, it was not found the excess of AGN which was reported in previous studies of similar galaxy samples (e.g. Keel et al. 1985; Veilleux, Kim & Kron 1995; Liu & Kennicutt 1995). In fact, we found in our sample of 54 galaxies only 4 LINER and 1 Seyfert 2 nuclei, which represent 9.2% of the sample. In a recent paper Ho, Filippenko & Sargent (1997) show that in a magnitude-limited sample of 486 galaxies with [FORMULA], half of the objects have H II star-forming nuclei, and the other half have some form of AGN or composite spectrum.

Enhancement of the star formation activity in interacting galaxies, like that observed in the physical pairs of our sample, can be accounted for by merger between a gas-rich disk and a less-massive dwarf galaxy (Hernquist & Mihos 1995). As a consequence of the interaction, the disk gas is forced to fall into the inner regions of the disk. The gas accumulates in the nuclear region forming high-density knots. Intense star formation can take place in these condensations, which can be identified as those observed in galaxies with circumnuclear star formation like NGC 613, NGC 1672 and NGC 1097 (Sersic & Pastoriza 1967; Osmer, Smith & Weedman 1974). The circumnuclear star-formation can mask a low luminosity LINER or even a Seyfert 2 nucleus (e.g. Storchi-Bergmann, Wilson & Baldwin 1996b; Philips, Charles & Baldwin 1983).

Recently Veron, Goncalves & Veron-Cetty (1997, hereafter V97) reported a number of galaxies having composite spectra. When observed with projected apertures larger than 2 kpc, these galaxies show spectra intermediate between those of an H II region and an AGN , due to the simultaneous presence on the slit of a Seyfert or LINER nucleus and an H II region. In this paper we argue that the observed deficiency of Seyfert nuclei in the DP97 sample, as compared to Liu & Kennicutt (1995) sample, might be explained by a masking of the AGN spectrum by strong circumnuclear star formation activity.

The goal of this work is to study the stellar population and nuclear emission in the sample of 27 physical galaxy pairs presented in DP97. The galaxy sample and observations are presented in Sect. 2. The nuclear stellar population is analyzed in Sect. 3. In Sect. 4 we subtract the stellar population templates from the galaxy spectra in order to obtain pure-emission line ratios; diagnostic-diagrams are used to classify the emission line spectra. In Sect. 5 the nature of the composite spectra is studied. Conclusions are summarised in Sect. 6.

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999