Our 12 BCDGs follow tightly the "color-metallicity'' relation
described by the Galactic globular clusters (Aaronson et al. 1978) as
shown in Fig. 9a, where the colors J-K and B-K versus the
metallicity index [O/H] =
are
plotted. The [O/H] abundances are derived from the spectroscopic data
taken from Terlevich et al. (1991); Pagel et al. (1992);
Izotov et al. (1994); Masegosa
et al. (1994); Sortchi-Bergmann
et al. (1995) and Stasinska et al. (1996). This contrasts with the results obtained
by Thuan (1983) and Hunter & Gallagher
(1985), who both find that their BCDGs and
H II galaxies are "younger'' than the globular clusters. But as
shown in Fig. 9b, if we plot the same relation for a simulated 8''aperture photometer centered on the brightest knot, we also find that
the colors are "bluer'' than the globular clusters, which is expected
since the aperture of 8'' accounts only for the starburst. The lower
panel of the figure shows how the ratio between the aperture size
equal to 8'' and the radius r0.75 at 3/4th of the total light
of the BCDGs. In the near infrared, it varies with the metal content
in our BCDGs. The observed 8'' aperture relation, is thus
somewhat spurious, and only shows the relative importance of the
starburst emission within that aperture.
The large scatter in our data indicates the presence of an intermediate age stellar population, possibly AGB stars (as discussed above), whose contribution depends on the star formation history of the BCDGs. For comparison, our BCDGs' behaviour resembles of the LMC globular clusters whose ages vary from 108 to 1010 years for the same range of colors.
Again, the question of the degeneracy between age and metallicity cannot be fully resolved. For instance, recent results obtained by Gallagher et al. (1998) for the moderately star forming dwarf Pegasus show a large population of red giant stars bluer than their metallicity would suggest. Hence, the authors suggest that the bulk of the RGB stars were formed about 2 Gyrs ago. If we dismiss the Color-Metallicity diagram, the blue IR colors of some BCDGs indicate that the bulk of the underlying galaxy was formed only a few Gyrs ago, and is responsible for the observed metallicity. If we assume that the observed metals originate from an earlier burst then the present day metals are hidden in the hot phase (Tenorio-Tagle 1996).
However, in Sect. 3.3.2, we showed that the excess of IR was due to a non-negligible population of Red Giants stars of at least 5 Gyrs for a solar metallicity, hence much older at our lower metallicities. Studies are under way to determine the metallicities of the different stellar components and compare them with the gas metal content.
Copyright ESO 2001