5 Conclusion

We have conducted a spectroscopic survey for a complete sample of VLIRGs selected from the 2 Jy redshift survey catalogue. We have studied their optical properties and the spectral and morphological classifications for these objects. Important clues are found concerning the nature of VLIRGs and the possible evolution connection between starbursts and AGNs.

1

Spectral classification shows that over half (60%, 44/73) of VLIRGs present AGN-like spectra. As the infrared luminosity increases, this fraction increases from 56% ($\rm 11.5 \leq Log({\it L}_{IR}/{\it L}_{\odot}) \leq 12.0$) to 82% ($\rm Log({\it L}_{IR}/{\it L}_{\odot}) \geq 12.0$). If we exclude LINER-like galaxies, the fraction of AGNs is 13%, 36% for the two subsamples, respectively (see Tables 2a-c).

2

From HII galaxies to AGNs, there is no clear boundary in the diagnostic diagrams (see Figs. 1-3). We classified galaxies with both HII and LINER spectral features as a mixture type. Evidences show that this type of VLIRGs has similar properties as LINERs. We combine the LINERs and the mixture type into a single class as LINER-like galaxies.

3

Statistics show that there exist correlations among three dust indicators: E(B-V), EW(NaID) and C6563/C4861 in the nuclei of HII galaxies, but not in the nuclei of LINER-like galaxies and Seyfert 2s. This indicates complex nuclear structures of AGNs. The line and continuum emission could come from different nuclear or circumnuclear regions. Seyfert 1's show less reddening.

4

As separations between VLIRGs and their nearest neighbors decrease, infrared luminosities and $\rm H\alpha$ equivalent widths increase significantly. There exists a character separation, about 10 kpc, in ULIRGs, below which the infrared luminosity will be over $\rm \ge 2\;10^{12} {\it L}_{\odot}$.The histogram of the relative velocity shows that many of our sample galaxies have even smaller separations, which provides the favorable condition for triggering starbursts, and leads to the high infrared luminosities for these objects.

5

It is clear from the relationship between spectral and morphological types that the interaction between galaxies triggers massive starbursts as well as nuclear activities. VLIRGs have circumnuclear starburst regions around central AGN. As the spectra evolve from HII-like to AGN-like, the dominant energy output mechanism for VLIRGs changes from starbursts to AGNs.

Acknowledgements

We would like to thank Dr. J.Y. Wei for his support for the project. Many thanks are due to Profs. J.Y. Hu, J.H. Huang, H.J. Su and Dr. S. Mao for valuable suggestions. We are grateful to Prof. J.S. Chen and his group for their help in the data analysis. Finally, we acknowledge Dr. M.A. Strauss for his valuable comments on this paper. This work was partially supported by NSFC.