From the stellar population analysis in Sect. 4 and the emission line spectrum in Sect. 5, combined with results from other studies, it is now possible to reveal some global properties of BCGs.
6.1. Age and star formation rate
There are two major competing theories for BCGs. The first one claims that BCGs are truly young systems undergoing the first star formation episode in the galaxy's lifetime. The second model suggests that BCGs are old galaxies, which are mainly composed of older stellar populations, with, however, a brief episode of violent star formation, in order to account for the observed spectroscopic features and spectral energy distributions.
Using the population synthesis method, we know the stellar population (different ages and metallicities) percentage contributions at Å for each of the 10 BCGs. These results show clearly that, for each, the old globular clusters and intermediate age components () make sizeable contributions to the galactic spectrum. The presence of large fractions of old or intermediate age components indicates that the star formation already happened at an early stage, and at a high rate. Our results support the second model that BCGs are old galaxies.
For IC1586, NGC4194 and MRK499, the contributions coming from the young and the old stellar components are large, but that from the intermediate age component is small. It suggests that the rate of star formation during the intermediate age period is smaller than in the other periods, and the star formation process is not continuous in these galaxies. For the BCDGs, the contribution from the intermediate age component is important, star formation was most vigorous in its intermediate age period, with relatively small contributions from the other periods. This also implies that star formation in these galaxies is also discontinuous.
The other result of our population synthesis is that while the observed properties of the bright BCGs (IC1586, NGC4194, MRK499) and the BCDGs are very similar, their stellar components and star formation regimes are generally different. There are many old and young stellar components in bright BCGs, and its recent star formation rate is very high. For BCDGs, the old and young stellar components are relatively small, but the contribution from intermediate age stellar populations is important.
The stellar population synthesis suggests that BCGs are old galaxies, in which the process of star formation is intermittent. Star formation has been violent in one of its evolution periods. These results are also supported by other observations (Papaderos et al. 1996, Sung et al. 1998, Aloisi et al. 1999). It illustrates that the present method is more than a simple population synthesis since it provides a direct estimate of the chemical evolution of the galaxy.
6.2. Internal reddening
When we investigate the internal energy source, physical condition and internal structure of galaxies, we must take into account the effect of internal reddening (Pizagno & Rix 1998). The effect of dust extinction on the emerging radiation is one of the least understood physical phenomena (Calzetti 1997, Ho et al. 1997). To study the internal reddening properties of BCGs, we quantify the discrepancy between the dust extinction measured from the emission line ratios and the optical continuum.
In the method of population synthesis used in this paper, the internal reddening is taken as an adjustable parameter, so that an estimate for the internal reddening is made at the same time as the stellar composition. We try various values of the internal reddening, make the appropriate correction in the continuum spectrum, then use this corrected continuum spectrum in the synthesis to find the best solution. This is an empirical way of determining the galactic reddening, its advantage is that it is assumption-free. The values of galactic internal reddening are listed in Table 6, is the result from the MMP method, is the result from the DCP method. We find that the values are small (), which is consistent with the BCGs being metal-poor and dust-poor. We find the reddening also clearly depends on the shape of the spectrum. The flattest spectrum (NGC4194) goes with the largest color excess. The steeper the spectrum, the lower the extinction.
The Balmer line ratio allows us to characterize the dust extinction in the regions where the nebular lines are produced. We measured the internal reddening value of the 10 BCGs using the observed emission lines and . The difference in the calculation of between the previous work and ours is that we can correct the underlying stellar absorption from the results of stellar population synthesis without making any hypotheses. The result of is listed in the last 3 columns of Table 6.
From this table, we can find that the internal reddening of the stellar continuum in BCGs is generally lower than that of ionized gas. A model of foreground dust clumps, with different covering factors for gas and stars, is a possible explanation for the difference. The covering factor by dusty clumps is greater for the gas region that generates the emission lines than for the stars that produce the continuum. That the continuum emission of stars is less obscured than are the emission lines of ionized gas, has been pointed out for other kinds of emission line galaxies (Calzetti et al. 1994).
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999