Long-term NIR Variability in the UKIDSS Ultra Deep Survey: a new probe
  of AGN activity at high redshift
  We present the first attempt to select AGN using long-term NIR variability.
By analysing the K-band light curves of all the galaxies in the UKIDSS Ultra
Deep Survey, the deepest NIR survey over ~1 sq degree, we have isolated 393
variable AGN candidates. A comparison to other selection techniques shows that
only half of the variable sources are also selected using either deep Chandra
X-ray imaging or IRAC colour selection, suggesting that using NIR variability
can locate AGN that are missed by more standard selection techniques. In
particular, we find that long-term NIR variability identifies AGN at low
luminosities and in host galaxies with low stellar masses, many of which appear
relatively X-ray quiet.
