Galaxy protocluster candidates at 1.6<z<2
  We present a study of protoclusters associated with high redshift radio
galaxies. We imaged MRC1017-220 (z=1.77) and MRC0156-252 (z=2.02) using the
near-infrared wide-field (7.5'x7.5') imager VLT/HAWK-I in the Y, H and Ks
bands. We present the first deep Y-band galaxy number counts within a large
area (200 arcmin2). We then develop a purely near-infrared colour selection
technique to isolate galaxies at 1.6<z<3 that may be associated with the two
targets, dividing them into (i) red passively evolving or dusty star-forming
galaxies or (ii) blue/star-formation dominated galaxies with little or no dust.
Both targeted fields show an excess of star-forming galaxies with respect to
control fields. No clear overdensity of red galaxies is detected in the
surroundings of MRC1017-220 although the spatial distribution of the red
galaxies resembles a filament-like structure within which the radio galaxy is
embedded. In contrast, a significant overdensity of red galaxies is detected in
the field of MRC0156-252, ranging from a factor of 2-3 times the field density
at large scales (2.5Mpc, angular distance) up to a factor of 3-4 times the
field density within a 1Mpc radius of the radio galaxy. Half of these red
galaxies have colours consistent with red sequence models at z~2, with a large
fraction being bright (Ks<21.5, i.e. massive). In addition, we also find a
small group of galaxies within 5" of MRC0156-252 suggesting that the radio
galaxy has multiple companions within ~50 kpc. We conclude that the field of
MRC0156-252 shows many remarkable similarities with the well-studied
protocluster surrounding PKS1138-262 (z=2.16) suggesting that MRC0156-252 is
associated with a galaxy protocluster at z~2.
