4.4.3 Ongoing Star Formation, Starbursts and Star Clusters
The present star formation process is another important aspect of BCGs, since they offer fine laboratories for studying vigorous star formation in metal-poor environments with relatively small extinction problems. The ongoing star formation has been studied by means of spectral synthesis of either integrated colours (Thuan 1983, Bergvall 1985) or spectral energy distributions (Lequeux et al. 1981, Thuan 1986, Fanelli et al. 1988, etc.)or both. This gives information on the current star formation event and in general short bursts are found to best match the data (e.g. Mas-Hesse and Kunth 1999). However, this does not exclude longer star formation episodes as the following example illustrates: if star formation propagates through a galaxy, but typically takes place in individual luminous short lived H II regions, from studying the most luminous H II region one might get the illusion to witness a sudden starburst event although the average SFR might have been continuous. With this in mind, we can use spectral synthesis to investigate the duration of individual star-forming events, the initial mass function (IMF), the stellar content (e.g. WR stars), and other interesting properties.
That most BCGs are not necessarily efficient star formers was shown by Sage et al. (1992) who investigated the neutral and molecular gas, infrared and optical properties of a small sample of BCGs and dIs. Most galaxies were found to be not more efficient than normal spirals in forming stars, the star formation efficiencies being high only when compared to other dwarfs. However compact star clusters (see below) require high SFRs to be gravitationally bound and there are indeed BCGs which appear to be very efficient star formers, and true dwarf analogues of giant starbursts.
Detailed studies have revealed that star formation in BCGs and dwarf
starbursts often take
place in dense ``super star clusters'' (e.g. NGC 1569,
Arp and Sandage 1985;
NGC 1705,
Melnick et al. 1985a).
Super star clusters (SSCs) are comparable in luminosity to R136
(the central cluster of 30 Doradus in the LMC), and are
sometimes much more luminous. It has been
proposed that these may be newly born globular clusters, although it is
still quite uncertain
whether they are massive enough and if they are gravitationally bound.
Conti and Vacca (1994)
studied He2-10 and Meurer et al. (1995) nine
starbursts galaxies, both using the HST/FOC, to find bright SSCs with UV
luminosities
close to those expected for proto globular clusters. Östlin et al. (1998)
studied the luminous metal-poor BCG ESO 338-IG04 (= Tol1924-416) with
the HST/WFPC2 and found more than hundred
luminous star clusters whose ages and masses were estimated from
multicolour photometry.
It would be interesting to know to what extent it is a general feature of
BGCs to reveal young star clusters when studied with enough spatial
resolution. Given that
special conditions may be required for SSCs to form, their presence and
abundance can yield
important insights into the starburst mechanism. Similar objects are
found in massive
starbursts (e.g. M82,
O'Connell et al. 1995),
merging galaxies (e.g. the Antennae,
Whitmore & Schweizer
1995),
and in the circum-nuclear regions of giant barred spirals (e.g.
Barth et al. 1995).
Ho and Filipenko
(1996)
managed to determine the velocity dispersion of SSCs in NCG 1569 and NGC 1705 and showed that they have masses on the
order of 105 M
, comparable to old Galactic GCs.
Bound massive star clusters offer an alternative way to probe the SFH in BCGs (and other galaxies). Thuan et al. (1996) found old GCs around Mrk996, and Östlin et al. (1998) in addition to old ones a rich population of intermediate age (~ 2.5 Gyr) GCs in ESO 338-IG04, revealing a former starburst event. In view of the many young SSCs at least a fraction could survive to become GCs. Moreover, young SSCs may be used to investigate the recent evolution of the starburst because they represent true ``simple stellar populations'' (coeval on a time scale of around one million year) and should be chemically homogeneous. In the case of ESO 338-IG04 it is clear from the age distribution of young SSCs that the present burst has been active for at least 30 Myrs (Östlin et al. 1998, 1999c).