luminosity. The vast majority of the line profiles fall in the range 30 to 300 km s$^{-1}$. Only the most luminous source known shows a much broader profile (Baan et al, 1992a).

The sensitivity of a radio telescope is generally specified (cf. Crane & Napier 1989) in terms of its receiver noise level as parameterized by a system temperature, $T_{sys}$, and the antenna sensitivity, $K_a = 10^{-26}\epsilon_{ap}A/2k$, which is a relation between flux density, $S_{\nu}$ in Jy and antenna temperature $T_a=K_a S_{\nu}$ K. The quantities $\epsilon_{ap}$, $A$, and $k$ are the aperture efficiency, the antenna collecting area and the Boltzmann constant in mks units, respectively. The noise level attained by observing a single polarization with a spectral resolution of $\Delta\nu$ and an integration time $\tau$ is $\sigma_{Jy}=T_{sys}K_c/(K_a\sqrt{\Delta\nu\,\tau})$ where $K_c$ is a factor with values typically between 1 and 1.4 depending on the details of the digital correlation spectrometer used to obtain the spectra.

As an example of sensitivities that can be attained at present or in the near future, consider the specifications and noise levels for typical observations with the Westerbork Synthesis Radio Telescope. For this system, receivers at 600 and 1400 MHz will have $T_{sys}$ of 45 and 25 