The LAS will create a fundamental database of JHK photometry of galaxies in the local universe. A large fraction of the galaxies detected will have optical spectra from SDSS, and all will have photometric redshifts from combined optical and IR colours. Using a K-band selected sample of galaxies for large scale structure and galaxy population studies is superior because it provides a much clearer estimate of the stellar mass of galaxies regardless of the age and metallicity of their stellar population and the dust content of the galaxy (Bell & de Jong 2000).
The galaxy database will have many possible uses. However several principal science drivers stand out.
Measurement of the galaxy stellar mass function. Although the galaxy luminosity function can be constructed in any photometric band, theoretical models most readily predict the stellar mass of galaxies. The K band data thus test the key ingredients of these models with minimum dependence on the modelling of the metal abundance, age or dust content (Cole et al. 2001). The K band luminosity function is key to addressing issues such as the efficiency of galaxy formation in different environments.
A mass-selected census of galaxy properties. The K-band data will be used to select a sample of galaxies based on their mass. We can then use this to compare the colours, dust content and star formation rates of galaxies free from the biases generated by selecting the sample in optical bands. For example, most known low surface brightness giant galaxies are blue and gas rich (Malin 1 is a spectacular example). By selecting galaxies in the IR, we will be able to determine whether these galaxies are the prominent tip of a population which includes red, old LSB galaxies, or whether these galaxies represent a mode of star formation which has only become established at low redshift.
The quality of the Sloan spectra will also allow us to obtain velocity dispersions for many galaxies allowing detailed comparison of the stellar and dynamical masses of galaxies as a function of environment.
Local clusters of galaxies. The LAS will also greatly benefit the study of low redshift clusters of galaxies. For instance, the Virgo cluster which is in the LAS survey region provides a unique opportunity for studies of galaxy properties. Because it is so close, the galaxy population can be studied in unprecedented detail. This, combined with its richness, allows a representative volume-limited galaxy catalogue to be readily created. Near-IR photometry covering this region will create a galaxy catalogue that probes to fixed stellar mass (independent of star formation history) providing a unique measurement of the stellar mass, size and surface brightness distribution of galaxies.
The survey of the Virgo cluster region has six specific science goals that will maximise return from the wide-field optical survey currently being undertaken by the INT:
We therefore wish to prioritize observations of the Virgo cluster within the first year of UKIDSS operations to ensure rapid progress on this topic. Should deeper observations be warranted to study galaxies further down the luminosity function then this will be found from PATT or by diverting time within the LAS and/or DXS.