An H-alpha search for over-dense regions at z=2.23
  We present the results of a narrow-band (H2S1, 2.121/0.021um) imaging search
with WFCAM/UKIRT for H-alpha emitters around several potential signposts of
rare (10^-7-10^-8 Mpc^-3) over-dense regions at z=2.23: an over-density of QSOs
(2QZ cluster), a powerful, high-redshift radio galaxy (HzRG), and a
concentration of submillimetre galaxies (SMGs) and optically faint radio
galaxies (OFRGs). In total, we detect 137 narrow-band emitter candidates down
to emission-line fluxes of 0.5-1 x 10^-16 erg s^-1 cm^-2, across a total area
of 0.56 sq. degrees (2.1 x 10^5 comoving Mpc at z=2.23) in these fields. The
BzK colours of the emitters suggest that at least 80% of our sample are likely
to be H-alpha emitters (HAEs) at z=2.23. This is one of the largest HAE samples
known at z>2. We find modest (~3-sigma) local over-densities of emitters
associated with all the three targets. In the 2QZ cluster field, the emitters
show a striking filamentary structure connecting four of the z=2.23 QSOs
extending over 30Mpc (comoving). In the HzRG and SMG/OFRG fields, the
structures appear to be smaller and seen only in the vicinities of the targets.
The K-band magnitudes and the H-alpha equivalent widths of the emitters are
weakly correlated with the over-density of the emitters: emitters in over-dense
region are more evolved systems compared to those in under-dense regions at
z=2.23. We find several examples of extended HAEs in our target fields,
including a striking example with a spatial extent of 7.5 arcsec (60 kpc at
z=2.23) in the 2QZ field, suggesting that these are relatively common in
high-density regions. We conclude that narrow-band H-alpha surveys are
efficient routes to map over-dense regions at high-z and thus to understand the
relation between the growth of galaxies and their surrounding large-scale
structures.
