Astron. Astrophys. 336, 786-790 (1998)

4. Conclusions

Our results show the importance of taking into account effects of the global star formation rate evolution. It is seen that in some realistic cosmological models the stochastic background produced by extragalactic merging binary WD can be about 0.1 of the mean galactic value. For a 1-year LISA observation the mean (i.e. angle-averaged) Galactic background becomes "transparent" at Hz, so at higher frequencies the search for other GW backgrounds becomes possible. The level of the extragalactic binary WD background (as well as any noise produced by astrophysical sources) is proportional to the fraction of baryonic matter , which lies in the range (Fukugita et al. 1997). Although most baryons are still in the form of ionized gas, we can substitute, as an upper limit, the value into Eq. (10), thus increasing the extragalactic GW background by four times. Such an extreme situation is feasible, for instance, if all baryons had passed a stellar stage during an early star formation burst at higher redshifts , where spheroidal systems formed rapidly (the so-called "third population stars"; e.g. the model of Eggen, Lynden-Bell & Sandage 1962). The evolutionary history of the oldest ellipticals and low-surface brightness galaxy may also differ significantly from the global average star formation discussed above. The traces of the early star formation is difficult to obtain by direct studies of the UV luminosity density evolution because of a strong dust extinction. Perhaps, the very detection of an isotropic GW background, together with independent studies of far-IR background, would help revealing the true star formation history at high redshifts.

At present, the lack of observational data on the SFR behaviour at high redshift does not allow us to make more robust estimates. We conclude that unless the global star formation rate goes on increasing with a redshift at , the extragalactic GW background energy density is ten times smaller than the mean galactic value in the LISA frequency range Hz.

The galactic binary GW noise will be modulated by the LISA orbital motion, whereas the extragalactic one will not. If the latter is comparable with galactic values at some galactic latitudes, it can impede detection by LISA of some interesting relic cosmological GW backgrounds (e.g. Grishchuk 1997), and specific statistical features of the relic GW should be used to separate them against the noise from astrophysical sources.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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