A search for galaxies in and around an HI overdense region at z=5
  We present the discovery of a large-scale structure of emission-line galaxies
at redshift z=4.86 behind a massive cluster of galaxies, A1689. Previous
spectroscopic observations of a galaxy, A1689-7.1 at z=4.87, near this
structure, revealed a possible overdense region of inter-galactic medium (IGM)
around the galaxy, which extends at least ~80 comoving Mpc along the line of
sight. In order to investigate whether this z~5 IGM overdense region contains a
galaxy overdensity, we undertook narrow- and broad-band imaging observations
around A1689-7.1 with Subaru/Suprime-Cam. We detected 51 candidate Ly-alpha
emitters at redshift z=4.86+-0.03 in the 32 x 24 arcmin^2 field of view. After
correction for lensing by the foreground cluster, we found a large-scale (~20 x
60 comoving Mpc) overdense region of galaxies around A1689-7.1 in the source
plane at z=4.86. The densest peak in this region has an overdensity of delta~4,
suggesting that this structure is probably a good candidate for a protocluster.
A1689-7.1 is located at the edge of this region, where the local galaxy density
is ~1.6 times the mean density and is close to the density contrast in the IGM
along the line of sight to A1689-7.1 estimated from the optical depth. The
overdensities of galaxies we have found may suggest that at least some parts of
the IGM overdense region have already started to form galaxies and moreover
they relate to the formation of a protocluster. Although we lack information on
the three dimensional distributions of both IGM and galaxy overdense regions,
the similarity of the scales of both regions may suggest that the two are parts
of a single large-scale structure, which would be an large edge-on sheet along
the line of sight with a size of ~20 x 60 x 80 comoving Mpc.
