Forty seven new T dwarfs from the UKIDSS Large Area Survey
  We report the discovery of 47 new T dwarfs in the Fourth Data Release (DR4)
from the Large Area Survey (LAS) of the UKIRT Infrared Deep Sky Survey with
spectral types ranging from T0 to T8.5. These bring the total sample of LAS T
dwarfs to 80 as of DR4. In assigning spectral types to our objects we have
identified 8 new spectrally peculiar objects, and divide 7 of them into two
classes. H2O-H-early have a H2O-H index that differs with the H2O-J index by at
least 2 sub-types. CH4-J-early have a CH4-J index that disagrees with the H20-J
index by at least 2 subtypes. We have ruled out binarity as a sole explanation
for both types of peculiarity, and suggest that they may represent hitherto
unrecognised tracers of composition and/or gravity. Clear trends in z'(AB)-J
and Y-J are apparent for our sample, consistent with weakening absorption in
the red wing of the KI line at 0.77microns with decreasing effective
temperature. We have used our sample to estimate space densities for T6-T9
dwarfs. By comparing our sample to Monte-Carlo simulations of field T dwarfs
for various mass functions of the form phi(M) \propto M^-alpha, we have placed
weak constraints on the form of the field mass function. Our analysis suggests
that the substellar mass function is declining at lower masses, with negative
values of alpha preferred. This is at odds with results for young clusters that
have been generally found to have alpha > 0.
