Star Formation in Active Galactic Nuclei: the Cases of NGC 5135, NGC 6221 and NGC 7130

Summary

We present long-slit spectroscopy of the composite Seyfert 2 starburst nuclei of the galaxies NGC 5135, NGC 6221 and NGC 7130 (IC 5135). Extended emission is detected in all three galaxies, reaching about 1 kpc from the nuclei. We study the spatial variation of the stellar population and emitting gas properties over the central regions. We compare our observed emission-line ratios with those obtained using composite photoionization models, which include ionization by a power law and hot stars, to find the gaseous abundance and the HII region parameters.

Several Seyfert galaxies exhibit the observational characteristics of vigorous star formation, either around the nucleus or in the galaxy disk, which is evident from extranuclear low excitation optical emission, diffuse non-thermal radio emission and very steep infrared spectra between 25 and 60 µm (Wilson 1988).

To investigate the connection of star formation and nuclear activity we have selected three Seyfert 2 galaxies with composite (Seyfert + HII) spectra – NGC 5135, NGC 6221 and NGC 7130. The nuclear spectrum of NGC 6221 presents a low excitation kinematical component and another, which is blueshifted, with high excitation (Pence & Blackman 1984). NGC 7130 also has 2 kinematical components, and the emission line ratios vary from values typical of active nuclei at the nucleus, to values characteristic of HII regions further out (Shields & Filippenko 1990).