6 Conclusions

In this paper, we have presented the surface photometric study of a new sample of 21 Blue Compact Dwarf Galaxies, observed from the southern hemisphere, selected out from several lists. An atlas of isophotal maps and brightness profiles has been produced. From the profiles, we could classify the BCDGs into the same three groups as obtained in Paper I. The results of the Northern sample (Paper I) and of this Southern sample were put together, and we obtained:

• 23% (9 objects) of our BCDGs have brightness profiles consistent with a pure r^1/4 law valid for classical luminous spheroidal systems. These galaxies have generally smooth isophotes at all brightness levels (with two or three exceptions), and show one large unresolved star forming region. Many of them show obvious departures from axisymmetry as boxy isophotes, isophote rotation, off-centered "nuclei'', or in few cases, double central components.
• 25% (10 objects) of our BCDGs have brightness profiles that can be described by a pure exponential law characterizing disk systems. These systems seem closely related to Magellanic irregular galaxies because their morphology shows less smooth isophotes at least in the central regions, and usually multiple star forming knots. Kinematical data are necessary to draw conclusions on the flattening of these systems.
• 45% (20 objects) BCDGs could be "classified'' as composite because their brightness distribution profiles are neither described by a pure exponential law or a pure r^1/4 law only. Nonetheless, nine of them are dominated by an exponential law and seem to be low luminosity analogs of giant "disk+bulge'' systems; 11 of them are dominated by a r^1/4 law, and are possibly low luminosity analogs of "disky ellipticals''. But, three of them remain unclassified (Mk 1131, Mk 1499 and SBS 1331+493), showing irregular patterns in the inner parts surrounded by a regular envelope. Their brightness distribution is difficult to interpret in terms of standard laws.

The color distribution of the BCDGs is representative of a blue galaxy population with some very blue cases. Some BCDGs show very strong B-R color gradients indicating the presence of a stellar population distinct from the starburst region. We note a color discrepancy between the elliptical-like BCDGs and the disk-like BCDGs. The latter types show a tendency to be bluer in average than the pure r^1/4 BCDGs, but composite profile objects blur the picture.

The luminosity-radius relationship was investigated. The selection bias towards objects with a centrally located burst that provides excess of light in the B band is still obvious in the data. This bias itself takes origin in the selection mode of the original catalogs, basically done on prism-objective plates.

The high median values of the mean surface brightness inside the effective radius, the significantly large mean value of the concentration index, and the average short scale length of the brightness distribution as measured by the effective radii demonstrate that our sample indeed contains "compact'' dwarf galaxies in overwhelming majority. r^1/4 dominated objects have higher average $<\mu_{\rm eff}\gt$ compared to the exponential dominated ones, but the latter already are significantly brighter than normal disk galaxies. Exponential dominated BCDGs seem to have a concentration index slightly correlated with their asymptotic color.

The results of this study of a sample of more than 40 BCDGs are leading to a more general view of the BCDGs. The various relationships investigated in this article are together consistent with the hypothesis that the differences in the morphology relate to differences in the intrinsic properties. The elliptical BCDGs are redder but more compact than the disk BCDGs. A small, but not negligible fraction of the BCDGs show a very disturbed light distribution with bluer colors when compared to the whole sample of the BCDGs. Those BCDGs may have experienced a strong past interaction (but in this case, the other actor is not in the immediate neighborhood) or an on-going merging event, while the star formation trigger in the elliptical and in the disk galaxies may be less extreme and violent.

In the new sample, we did not find any new close pair of BCDGs, but many fields show faint extended objects with similar colors to those of the BCDGs. The most obvious case is Tololo 1924-416 with a blue nebulosity located 3' SE off the galaxy for which we present the surface photometry as well. Observations are planned to obtain the redshifts of candidate neighbours in order to identify any low luminosity physical companions. Moreover, a number of features observed in the BCDGs images may be linked to recent past interaction and/or on-going merging phenomena (boxy isophotes, asymmetric envelopes, double nuclei structures). It is still premature to draw a conclusion on the triggering causes of the starburst without additional spectro-imaging observations that are presently under progress.

Considering the photometric results obtained on the present sample, it appears more difficult to force all BCDGs back into the evolutionary perspective contemplated by different authors ([Lin & Faber1983]; [Davies & Phillipps1988] and [Silk et al.1987]; [Papaderos et al.1996a]; [Telles et al.1997]; [Telles1996]). Using B and R surface photometry only, there is no obvious and un-confusing argument allowing to set up a unique evolutionary scenario because the picture of the BCDGs is too much dominated by the light of the on-going starburst. Elliptical-like BCDGs may evolve into nucleated dwarf galaxies, while disk-like BCDGs may end as dwarf spheroidal galaxies, but for the composite profile BCDGs, there is no way to tell.

In our view, there is much evolution within the subclass of the Blue Compact Dwarfs and they may constitute a segregated category during the short time-scale events that are responsible for their properties. It is most probable that the ending fate of a BCDG does not lie in the fading of the starburst but in its final dynamical state.

Acknowledgements

The authors wish to thank the support staff at the La Silla Observatory for their help. V.D particularly wishes to thank G. Dudziak for obtaining the spectrum of Tol 0954-293 during his own observing run. The program was supported partly by the governmental grant of the Ministère de l'Enseignement Supérieur et de la Recherche in France, and by the Program of Pre-doctoral Fellowship at the European Southern Observatory.