Heavily Reddened Quasars at z~2 in the UKIDSS Large Area Survey: A
  Transitional Phase in AGN Evolution
  We present a new sample of purely near infra-red selected KVega<16.5
extremely red ((J-K)>2.5) quasar candidates at z~2 from 900 sq deg of data in
the UKIDSS Large Area Survey (LAS). Five of these are spectroscopically
confirmed to be heavily reddened Type 1 AGN with broad emission lines bringing
our total sample of reddened quasars from the UKIDSS-LAS to 12 at z=1.4-2.7. At
these redshifts, Ha (6563A) is in the K-band. However, the mean Ha equivalent
width of the reddened quasars is only ten per cent larger than that of the
optically selected population and cannot explain the extreme colours. Instead,
dust extinction of Av~2-6 mags is required to reproduce the continuum colours
of our sources. This is comparable to the dust extinctions seen in
submillimetre galaxies at similar redshifts. We argue that the AGN are likely
being observed in a relatively short-lived breakout phase when they are
expelling gas and dust following a massive starburst, subsequently turning into
UV-luminous quasars. Some of our quasars show direct evidence for strong
outflows (v~800-1000 km/s) affecting the Ha line consistent with this scenario.
We predict that a larger fraction of reddened quasar hosts are likely to be
submillimetre bright compared to the UV-luminous quasar population. We use our
sample to place new constraints on the fraction of obscured Type 1 AGN likely
to be missed in optical surveys. Taken at face-value our findings suggest that
the obscured fraction depends on quasar luminosity. The space density of
obscured quasars is ~5x that inferred for UV-bright quasars from the SDSS
luminosity function at Mi<-30 but seems to drop at lower luminosities even
accounting for various sources of incompleteness in our sample (abridged).
