Our starting point is the desire to produce an IR survey which matches the Sloan Digital Sky Survey (SDSS) to produce a combined dataset of lasting importance. A large area shallow survey is the most efficient way (i) to provide large statistical samples, and (ii) to find rare objects. The result will be a greatly improved galaxy catalogue for the local (z~0.1) universe, and a more complete and less biased sample of quasars. But the IR perspective also allows us to look for particularly important rare objects - the very nearest and very smallest sub-stellar objects, and the most distant and luminous quasars.
The Sloan Digital Sky Survey (SDSS) aims to cover some 10,000 sq. degs principally around the north galactic pole. The survey 10-sigma limits vary from g'(~V)=22.6 to i'=21.7 ¹ and the survey is thus a major improvement on existing large area surveys such as those from the UK and Palomar Schmidts. The aims and initial results of the SDSS can be found at the SDSS home page.
Clearly the power of any survey to extend existing knowledge and produce surprises depends on its wavelength coverage. It seems scientifically desirable therefore to extend the SDSS into the near-IR by observing as large an area as possible to a depth matched to the existing optical dataset. Unfortunately, the 2MASS survey is too shallow (limits J=16, K=15) to provide near-IR photometry for most of the sources detected in SDSS - given a typical galaxy colour of I-K~3 a limit of K~18.5 is required. The UKIRT Wide Field Camera (WFCAM) is capable of mapping large areas of sky at a limit 4-6 magnitudes more sensitive than the 2MASS and DENIS surveys: reaching K~18.5 over 4000 sq. degs in a reasonable time. Thus the UKIDSS Large Area Survey (LAS) clearly provides a better match to Sloan than existing near-IR surveys and thus provides a unique opportunity to undertake joint optical/near-IR studies over very large regions of the sky.
Below we examine the likely scientific impact of the LAS, moving from the solar neighbourhood outwards. The three scientifically most exciting issues tackled by this survey are: (1) searching for the nearest, faintest objects; (2) identifying population II brown dwarfs and (3) surveys for QSOs at the highest redshifts possible.
¹ Note that quoted SDSS survey limits are in AB magnitudes, but we have converted these to equivalent Vega-based magnitudes for consistency with the rest of our case.