Astron. Astrophys. 324, 490-504 (1997)

7. Conclusions

The comparison with Kennicutt's spectra has shown that the model nicely reproduces the emission lines of observed spectra, with the exceptions of the OIII line at , as previously discussed.

To a first approximation the relations between the intensities of the emission lines are similar both for normal galaxies and those showing a stronger astrogenetic activity: this is reasonable since all such quantities are only affected by young stars. However peculiar objects present a larger spread, consequence of the variety of the parameters characterizing the bursts.

The analysis performed with the models confirms and extends Kennicutt's conclusions (1992b): the and [OII] luminosities are good indicators of the present star formation. The equivalent widths of these lines, which usually are the only available data in the case of distant galaxies, can be also profitably used together with the continuum flux at the line wavelength. This last quantity can be estimated by means of a nearby photometric band: in the case of the [OII] line the nearest one is the U band.

The occurrence of current bursts can be ascertained by means of exceptionally large equivalent widths of , [OII] emission lines, by the presence of the H line in emission and by very small values of the D₄₀₀₀ index. A large equivalent width (EW ) of the H line in absorption is instead indicative of a burst already ended (by about 1-2 Gyr). It must be pointed out however that bursts cannot always be singled out. For instance models of galaxies with a burst with particular values of the parameters can fall in the EW(OII) - EW(H diagram just in the region filled by normal objects and this happens not only with the weak and short lasting bursts.

The information supported by the (U-B) e (B-V) colours is equivalent to that arising from the emission lines only in the case of normal galaxies. Very blue colour indices also witness current bursts. In the case of normal galaxies the history of star formation can be estimated from photometric data. The (U-B), or another broad band colour index, can be used to evaluate the e-folding time of the star formation process, while , for instance, gives then the initial value of the SFR. The D₄₀₀₀ index has been calibrated to yield the ratio of the SFR averaged over the last 5 billion years to the present SFR. If this last quantity is derived from the emission lines, the mean SFR can be estimated.

Attempts to give quantitative estimates of the SF histories in galaxies hosting a burst generally fail, essentially because of the large number of parameters involved.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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