6 Summary and conclusions

We present the results from the most extensive study as yet of optical 2D velocity fields of luminous blue compact galaxies (BCGs) utilising Fabry-Perot interferometry, targeting the H$\alpha$ emission line. The velocity fields and rotation curves for a sample of six luminous BCGs and two companions were presented in a previous paper (Östlin et al. 1999, Paper I). The velocity fields present large scale peculiarities, and secondary dynamical components (e.g. counter-rotation) are common.

In this paper we have analysed the ionised gas dynamics together with optical/near-infrared surface photometry and spectral synthesis models to constrain the dynamical and photometric masses of the galaxies. Moreover, we construct multi-component mass models including a dark halo component.

The BCGs in this study are starbursts in the sense that the time-scale for building up the observed stellar mass with the current star formation rate, derived from the H$\alpha$ luminosities, is much smaller than a Hubble time. We find that the young burst population dominates the integrated optical luminosities, while only contributing 1 to 5% of the total stellar mass, which ranges from a few times 10⁸ to more than $10^{10}~M_\odot$. The mass is dominated by an older underlying population and the integrated (burst + old population) mass-to-light ratios are $M/L_V \sim 1$.

In about half the cases, the observed rotational velocities are too small to allow for pure rotational support. A possible explanation is that velocity dispersion dominates the gravitational support. This is consistent with the observed line widths ( $\sigma_{{\rm H}\alpha} = 35$ to 80 kms^-1), but does not explain the strange shape of many of the rotation curves. Another possibility is that the galaxies are not in dynamical equilibrium, e.g. because they are involved in mergers, explaining the peculiar kinematics. It is also possible that gas and stars are dynamically decoupled and the H$\alpha$ velocity field does not trace the gravitational potential. A way to distinguish between these alternatives would be to obtain the rotation curve and velocity dispersion for the stellar component. In two cases, we find evidence for the presence of dark matter within the extent of the H$\alpha$ rotation curves, and in two other cases we find marginal evidence.

We have further analysed the morphology of the BCGs, and in general we find strong large-scale asymmetries down to the faintest isophotal levels, revealing large scale asymmetries in the distribution of stars. In most cases we see clear signatures of merging/interaction.

We have discussed the possible trigger mechanisms for the strong starbursts in this sample of luminous BCGs. When considering also the kinematics and morphologies we are lead to the conclusion that dwarf galaxy mergers is the favoured explanation for the starbursts. The two companion galaxies appear largely unaffected by the presence of their brilliant neighbours, but their star formation activity may have increased due to the tidal drag imposed by the BCGs. The dynamics of the studied galaxies fall in two broad classes: one with well-behaving rotation curves at large radii and one with very perturbed dynamics. This may indicate a distinction of the fate of these galaxies, once the starbursts fade. Alternatively, depending on the state in which we see the interaction/merger, we will detect more or less chaotic velocity fields. These local dwarf galaxy mergers may be the best analogues of hierarchical buildup of more massive galaxies at high red-shifts.

In 1999 and 2000 new Fabry-Perot observations were obtained for another 15 BCGs, extending down to fainter luminosities. Together with the present data set, we will have high quality velocity fields for two dozen BCGs. This will allow us to obtain a more comprehensive picture of the evolution of BCGs.

Acknowledgements

Thanks to T. Karlsson for reducing a CCD image of ESO185-IG13, and to E. van Groningen for obtaining additional data on this galaxy. Thanks also to M. Dahlgren for obtaining the CCD image of Tololo0341-407. C. Carignan is thanked for fruitful discussions on the dynamics of BCGs and for making specific adaptations of his mass model code for the present study. A. Bosma is thanked for stimulating discussions on the interpretation of velocity fields. A. Westman is thanked for her great help in preparing the figures. An anonymous referee is thanked for many useful comments on the manuscript. G. Östlin acknowledges financial support from the Swedish Foundation for International Cooperation in Research and Higher Education (STINT). This work was partly supported by the Swedish Natural Science Research Council. This work is based on observations collected at the European Southern Observatory La Silla, Chile.