4.4 Blue Compact and H II Galaxies
The concept of ``compact galaxies'' was introduced by Zwicky (1965) to denote galaxies barely distinguishable from stars on the Palomar Sky Survey plates. Originally, most studies of blue compact galaxies (BCGs) concerned objects from the lists of compact/emission line/UV-excess galaxies produced by Zwicky (1966), Haro (1956) and Markarian (1967). However, only a fraction of the objects in these lists are BCGs, the others being AGNs, normal spirals with nuclear star formation, H II regions in the outskirts of nearby spirals etc. Later, many apparently similar objects have been added, mostly from emission line surveys (cf. Sect. 6). This type of galaxy is sometimes also referred to as H II galaxies (Melnick et al. 1985b, Hazard 1986, Terlevich et al. 1991), since they have spectra reminiscent of Galactic H II regions (and were often discovered because of this property). Other types include ``blue amorphous galaxies'' (Sandage and Brucato 1979, Gallagher and Hunter 1987). Different notation reflects a focus on different physical aspects meaning that the classifications do not necessary overlap completely, and this loosely classified group may contain objects with different evolutionary history. Without arguing that any name is better than the other, we shall henceforth use the name BCGs, unless we want to draw attention to differences. Since many BCGs have been discovered by means of objective prism surveys, which are not very sensitive to the properties of the host galaxy, ``ordinary'' dIs with bright H II regions may ``contaminate'' samples selected in this way. Note also that not all galaxies denoted as BCGs in the literature are strictly compact according to a surface brightness criterion. BCGs have luminosities in the range MB -12 to MB -21. Those BCGs which are less luminous than MB -17 are commonly referred to as blue compact dwarfs (BCDs). Interestingly, even some luminous BCGs may be very metal-poor. In Fig. 2 we show the appearance of BCGs on the Digitized Sky Survey, and a LSBG and a normal spiral for comparison. In Fig. 5 and 9 we show high resolution images of the two most metal-poor BCGs, and in Fig. 6 a spectrum of one of them (IZw18).
The blue compact galaxies are the class of objects where most galaxies with low abundances as derived from H II regions have been found. The overrepresentation of BCGs is probably related to selection effects, since high surface brightness and prominent emission line spectra make it relatively easy to determine nebular abundances. Early studies (e.g. Arp 1965, Zwicky 1966) hinted that BCGs had dramatically different properties compared to ``normal'' dwarf and giant galaxies. The wide interest in low metallicity BCGs was triggered by the work of Sargent and Searle (1970) and Searle and Sargent (1972) where they showed two BCGs to be metal-poor and forming stars at high rates. They concluded that either these galaxies were young, now forming their first generation of stars, or that star formation occurred in short bursts separated by long quiescent periods (Searle et al. 1973). Today the latter explanation seems the correct one for the majority of BCGs, if not all.