The computed relative merger rate as a function of redshift is in agreement with the detected rate of -ray bursts up-to the limiting redshift of BATSE. For the models where elliptical galaxies experience an initial burst of star formation the occurrence rate of coalescence increases suddenly by more than a factor two at a redshift of 2.5, which is beyond our current detection limit. The synthetic distribution is compatible with the observations down to the completeness limit of -ray catalogues. The same is true for models based on observationally inferred cosmic star formation history (see e. g. Sahu et al. 1997) and for models based on galactic evolution (Totani 1997). They, however, predict a different behavior if star bursts occur at high z.
The absolute value of the merger rate is found to be 100 times larger than the GRBs frequency. Escape from this conundrum is obtained if the opening angle of the observed phenomenon is a few degrees, which is consistent with fire-ball models where leptons are converted into bulk barionic motion (Mészáros & Rees 1992).
If -ray bursts indeed originate from mergers and our model for star formation is correct, interesting implications follow. The progenitors of bright GRBs or bursts at low redshift () most likely belonged to early-type spiral galaxies whereas the progenitors of the dimmest bursts and those at high redshift () were located in elliptical galaxies. The majority of the bursts of gravitational waves in this model are expected to originate from early-type spiral galaxies. If most stars in the Universe formed in dwarf star-burst galaxies, a substantial fraction of the parental population may originate from them. Suggested by current observational data the decline in the star formation rate beyond (e. g. Connolly et al. 1997), if real, will show-up as a turn over in the distribution for GRBs.
As noticed by Tutukov & Yungelson (1994) and confirmed by PZY98 (their Figs. 6 and 8), space velocities of binaries may well exceed escape velocities of dwarf as well as giant galaxies and they may travel up to Mpc before coalescence. Thus, a significant fraction of the sites of GRBs may not be directly associated with star forming regions.
© European Southern Observatory (ESO) 1998
Online publication: March 30, 1998