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4.4.6 Starburst Triggers in BCGs

Searle and Sargent (1972) had already speculated on the reasons for a sudden increase of the star formation rate in BCGs, followed by long quiescent phases. The regulation of their star formation histories in general, is of course a very important question, intimately linking to the physical relations between BCGs and other dwarfs, which will be addressed in Sect. 7.2. Moreover, given the heterogeneity of BCGs as a class the possibility that different mechanisms operate is not excluded. The star formation rate in BCGs (even the mass averaged value, cf. 3.2) varies considerably, and some are rather star forming dwarfs than starburst ones. Given the increased central H I densities (van Zee et al. 1998c) it is evident that some mechanism that can concentrate gas in the centres is needed.

A scenario with statistical fluctuations proposed by Searle et al. (1973), has been further elaborated by Gerola et al. (1980). The basic ingredients are a positive feedback of star formation which in combination with the small masses of BCGs give rise to large fluctuations in the SFRs. A popular explanation is that supernova driven winds halt star formation by expelling the gas. Later, the lost gas might accrete back on the galaxy and create a new starburst (Tayler 1976, Dekel and Silk 1986, Silk et al. 1987, Babul and Rees 1992). A problem with models where the precursor accretes gas continuously is the time scale for onset of rapid star formation, - why would the galaxy wait to form stars for a long time, then suddenly (on time scales of the order of 107 years) burst into an unsustainable star formation rate?

It is well known that mergers between giant galaxies can produce impressive starbursts (Sanders et al. 1988), and it is possible that mergers are key mechanism in building up the galaxy population and regulating starburst activity (Lacey et al. 1993). Therefore this possibility is attractive also for explaining dwarf starbursts, i.e. BCGs. It is evident that there are at least a few BCGs that seem to be interacting/merging in some form (e.g. IIZw40, He2-10, and galaxies in Östlin et al. 1999a, b). Moreover, many of these BCGs show Super Star Clusters (SSCs) of the same kind as those found in giant mergers.

On the other hand several studies indicate that many BCGs are pretty isolated (cf. Sect. 6.3). However there might be low surface brightness or pure H I companions missing in present catalogues. A related question is whether tidal interactions are strong enough to ignite bursts in dwarfs that, like BCGs (Salzer 1989) are not accompanied by giant galaxies. First, tidal forces between dwarfs are too modest to trigger radial gas flows unless the galaxies are almost in contact (Campos-Aguilar et al. 1993). Moreover it is uncertain whether dwarfs are at all unstable against tidal perturbations (Mihos et al. 1997). Perhaps a direct contact is needed to trigger dwarf starburst explaining why pairs are rare since one would only pick up the burst once the merger is already in progress. While mergers would be sufficient for producing BCGs it is uncertain how common they are. The environments of isolated BCGs should be studied in more detail for this purpose.

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