Even though sensitivity is an key factor for spectral scans, the next step in molecular line surveys does not only applies to sensitivity but also to resolution. Fig. 2 shows the increase in both richness and complexity of the molecular structure in the central few hundred parsecs of NGC 253 as the resolution improves from single dish resolution to high resolution interferometric observations. The high resolution maps by Sakamoto et al. (2011) reached a similar spatial resolution to that achieved in large scale surveys towards the Galactic center with small telescopes (Ø = 60 cm, Sawada et al. 2001). The imaging capabilities of instruments like ALMA, together with the large bandwidth of the receivers will allow to carry out deep 3D spectral scans. It will be possible to study the chemical composition variations accross the center of nearby galaxies at a similar resolution to that currently achieved by large single dish telescopes towards our very Galactic center. Thus, we expect line surveys will move a step further from studying the averaged molecular composition over the central few hundred parsecs of nearby galaxies to spatially resolve the chemical composition of individual molecular complexes.
Figure 2. CO integrated emission maps of the starburst galaxy NGC 253 at different spatial resolutions. (left) CO 1-0 map at 12" resolution by Mauersberger et al. (1996) with the IRAM 30m telescope. (center) CO 2-1 map at 5" × 9" resolution by Canzian et al. (1988) with the OVRO interferometer. (right) CO 1-0 map at 1.1" resolution by Sakamoto et al. (2011).