Observations of neutral gas in absorption against the continuum emission of AGNs are a powerful way to study the circumnuclear gas. Limited studies of the gas kinematics on the lines of sight toward compact sources (typically milli-arcsec) have provided some evidence for infall and an estimate of the accretion rate of gas, that might eventually fuel the black hole. If the nuclear radio source is resolved then the rotation velocity of the circumnuclear disk can be measured, providing an estimate of the black hole's mass.
Examples of both kinds of studies exist, but suffer from two major problems. The rotation velocities of the circumnuclear gas are very high, resulting in broad ( km/s), extremely shallow (peak optical depths ) profiles, requiring a better spectral dynamic range than is generally achievable. Total bandwidth restrictions make it impossible to search at the higher frequencies for molecular absorption covering such large velocities. Another problem (and intriguing possibility) is that large amounts of gas may be locked up in tiny, very cold clouds, which have intrinsic velocity widths of a fraction of a km/s. Extremely narrow channels are needed to detect those clouds.
The proposed bandwidth and number of channels in the new correlator will alleviate both the velocity coverage problem for molecular studies and the velocity resolution problem for the very cold clouds. A high spectral dynamic range (:1) is also required. The A+ configuration will significantly increase the number of objects against which rotation velocities of the circumnuclear disk can be imaged. The 1.4 GHz extension to lower frequencies increase the number of sources strong enough to be targets for such work.