by Clements et al (1996).

At the present epoch, the superluminous FIR galaxies that host the most luminous masers are relatively rare galaxies. This means that radio spectroscopic surveys of the sky would need to cover large solid angles before they would begin to detect OH masers at random from the local galaxy population. On the other hand, this paper will show that, since OH emission can be so very strong, it becomes probable that surveys with large spectral bandwidths and covering large depths are likely to identify cosmologically distant megamasers with moderate integration times.

The most luminous infrared sources known are the objects detected at high redshifts, $z\approx 0.4$ to 4.7, (Rowan-Robinson 1996, and references therein) with $L_{FIR} \sim 10^{12}$ to 10$^{14}h^{-2}L_{\odot}$. There is general consensus that the merging and interaction rate of galaxies was greater in the past with merging rate rising in proportion to $(1+z)^m$. Values for $m$ as high as 4 (Carlberg 1992, Lavery et al 1996) are mentioned in the literature, although there is some observational evidence pointing to $m\approx 1.2$ for $z<1$ in the HST Medium Deep Survey (Neuschaefer et al 1997) and $m=2.8\pm0.9$ from faint galaxy catalogs compiled from CFHT surveys (Patton et 