We have analyzed the properties of the star formation episodes taking place in a sample of blue compact and irregular galaxies by comparing their multiwavelength observational properties with the predictions of evolutionary population synthesis models .
This method has allowed us to constrain the age , star formation regime ( instantaneous or extended ) and Initial Mass Function ( IMF ) slope , as well as the shape and strength of the interstellar extinction in these regions .
We find that star formation episodes are essentially short with a mean age of 3.5 Myrs .
Some galaxies may be undergoing their first global episode of star formation while for the rest of the sample older stars contribute to at most half the optical emission .
The Wolf-Rayet star population ( WR ) is well reproduced by the models and provides the strongest argument in favor of a short duration of the star formation episode .
Supernova rates are relatively large .
The accumulation of supernova explosions within few Myr has contributed to a quick metal enrichment of the ISM and to its disruption by the release of huge amounts of mechanical energy . V - K colors agree well with the prediction that red supergiant stars are rare in low metallicity regions .
A general agreement is found between the predicted and observed far infrared emissions suggesting that the fraction of hidden stars contributing to the ionisation is minimum , except in some specific objects .
A saillant result of this study is that the IMF slope appear to be very universal , on average very close to that of the solar neighborhood and with no dependence on the metallicity , contrary to previous claims .
We have also found no dependence whatsoever between the shape of the extinction law and the metallicity .
It is likely that the strong radiation associated to the bursts destroys the dust component responsible for the 2175 Å bump .
Finally we confirm that extinction affecting the stellar continuum is in some cases significantly weaker than that derived from the Balmer emission lines .
Such a discrepancy can lead to underestimations in the value of the H \beta equivalent width by a factor as large as 2 , leading to an overestimation of the age of the burst .
Similarly , the Wolf-Rayet bump to the H \beta luminosities ratio can also be affected by this differential reddening leading to an overestimation of the WR star population .
As bursts get older they appear dustier , possibly as a result of dust ejection during the evolution of their most massive stars .
Finally , we have found a serious general discrepancy between the predicted and the measured radio luminosities .
While part of this discrepancy might be attributed to aperture mismatching in some cases , it points to the presence of additional radio sources not included in present evolutionary models .