= \phi^*\left(M_{HI}/M_{HI}^*\right)^{-\gamma}
   e^{- M_{HI}/M_{HI}^*}\,dM_{HI}
  \label{himassfnct.eqn}$$ with $\phi^*=0.014$ Mpc$^{-3}$, $\gamma=1.2$ and $M_{HI}^* = 10^{9.55}M_{\odot}$. The luminosity in the 21cm line from an optically thin cloud of neutral hydrogen with HI mass $M_{HI}$ is $L_{HI}/L_{\odot}=6.2{\times}10^{-9}M_{HI}/M_{\odot}$.

Curves for detection rates in the hydrogen line are plotted in Figs. \[cosmcompare.fig\] and \[ratecombo.fig\] for comparison with the OH line strengths. The exponential cutoff to the HI luminosity function causes the HI detection rate to cutoff very hard around $z=0.2$ for 1 mJy sensitivity. The power law function for the OH masers permits the bright end of the OH maser population to dominate the detection rate at frequencies below ${\sim}$1200 MHz for reasonable integration times with existing telescopes.

Not only are the typical velocity widths of the OH profiles comparable to velocity spreads measured in the 21 cm line for low mass and face-on galaxies, but also the pair of strong OH lines at 1665.4 and 1667.4 MHz could produce a spectrum that, at low signal to noise ratio, could imitate a double horned galaxy profile with a splitting of ${\sim}$360 km s$^{-1}$, although the horns are likely to be asymmetric due to the usual weakness of the 1665 line relative to the 1667 line.

Sifting 