SPECTRUM OF THE SUPERNOVA RELIC NEUTRINO BACKGROUND AND EVOLUTION OF GALAXIES

The spectrum of the supernova relic neutrino background (SRN) from col lapse-driven supernovae that have ever occurred in the universe is cal culated by using a realistic, time-dependent supernova rate derived fr om a standard model of galaxy evolution based on the population synthe sis method. The SRN spectrum we show here is the most realistic at pre sent, because the largest uncertainty in previous theoretical predicti ons has come from unrealistic assumptions of the supernova rate so far made. The SRN is one of the targets of the Superkamiokande (SK) detec tor, which will be constructed in a year, and the SRN, if at all detec ted, would provide a new tool to probe the history of supernova explos ions in the universe. The expected event rate at the SK is therefore c alculated in this paper. Our major results include the following: (1) the supernova rate is much higher in the early phase of evolution of g alaxies, and there appears a hump in the SRN spectrum in the low-energ y region of less than or similar to 5 MeV, (2) the SRN flux depends on the Hubble constant (H-0) in a way approximately proportional to H-0( 2) and only weakly on the density parameter of the universe (Ohm(0)) a nd a cosmological constant (lambda(0)), (3) the uncertainty in the sta r formation history of spiral galaxies affects the resulting SRN flux by about a factor of 3, and (4) the plausible event rate at the SK is 1.2 yr(-1) in the observable energy range of 15-40 MeV. Such a low eve nt rate is due mainly to a quite low supernova rate at present, which is averaged over the morphological types of galaxies. The most optimis tic rate in our model is found to be 4.7 yr(-1) in the same energy ran ge, and if more events are detected, we will have to reconsider our cu rrent understanding of collapse-driven supernovae and evolution of gal axies.