4 Presence of nuclear activity

8 (1 uncertain) out of 22 shell galaxies nuclei show emission lines, most of which (7 out of 22) are [OII] lines. In E 2890150 (T=-2) is visible only H$\beta$.

Among pair galaxies, 10 objects out of 30 show emission lines. Together with [OII] lines as in the case of shell galaxies, [OIII] (4958 and 5007 Å) and Balmer lines (H$\beta$,H$\gamma$ and H$\delta$) are also frequently detected. The detection includes all galaxies with latest morphological types i.e. RR 24b (T=3), RR 24b (T=-0.3), RR 62a (T=-2) and RR 331a (T=-0.4) for which the gas presence is reasonably expected.

Phillips et al. (1986) found, among early-type galaxies, a large presence of nuclei which are indinstingushable from a LINER. Ho (1996) suggests that it is not convenient to classify activity only on the base of ([N II] $\lambda 6583$)/ H$\alpha$) lines ratio, as in the case of Phillips et al. (1986), since the latter cannot give useful informations about the excitation level in galaxies. The original definition of a LINER, given by Heckman (1980), requests that the emission line ratios ([O II $\lambda$ 3727)/([O III] $\lambda$ 5007) is $\ge$ 1 and that ([O I $\lambda$ 6300)/([O III] $\lambda$ 5007) is $\ge$ 1.3. Veilleux & Osterbrok (1987) proposed other criteria for the LINER definition, among which the request that the ratio of ([O III] $\lambda$ 5007)/H$\beta$ is $\le$ 3.

Our spectral range does not extend enough in order to compute several lines ratios and to use diagnostic diagrams (e.g. Veilleux & Osterbrok 1987). We use ([O III]$\lambda$ 5007)/H$\beta$ line ratio in order to measure the nucleous ionization level and for 6 objects, belonging to the shell galaxies sample we attempt a first classification of the nuclear activity using ([N II]/ H$\alpha$) line ratio reported in Phillips et al. (1986). 4 shell galaxies (NGC 1316, NGC 1553, NGC 6776 and NGC 6958) out of 6 may be classified as LINERs on the basis of ([N II]/ H$\alpha$) vs. ([O III]$\lambda$ 5007)/H$\beta$ diagnostic diagram. The remaining two objects (NGC 1549, companion of NGC 1553, and NGC 1571) do not present LINER properties. E 2890150 shows ([O III]$\lambda$ 5007)/H$\beta$ line ratio that suggests the presence of a low excitation region, probably typical of the HII regions.

RR 24a/b are in the regime of low excitation. The ([O III] $\lambda$5007)/H$\beta$ ratio for RR 331a suggests that the nucleous is in the ionization regime of Seyfert galaxies. The ([O III] $\lambda$ 5007) line profile as function of the radius, shown in Fig. 5 adds further evidence to this conclusion showing a double component. In a recent study of Aoki et al. (1996), [OIII] emission line shows the same behaviour in the nuclear region of the Seyfert galaxy NGC 7319.

We conclude that a small fraction of the studied objects may be classified as LINERs.

  

Figure 5: Variation of the ([O III] $\lambda$ 5007) line profile as function of the distance from the centre (r=0) of RR 331a. Notice the double component in the central lines

 

Table 5: Rotational support parameters for the stellar component for galaxies with unperturbed curve along their major axis