Star Formation Activities of Galaxies in the Large-Scale Structures at
  z=1.2
  Recent wide-field imaging observations of the X-ray luminous cluster
RDCSJ1252.9-2927 at z=1.24 uncovered several galaxy groups that appear to be
embedded in filamentary structure extending from the cluster core. We make a
spectroscopic study of the galaxies in these groups using GMOS on Gemini-South
and FORS2 on VLT with the aim of determining if these galaxies are physically
associated to the cluster. We find that three groups contain galaxies at the
cluster redshift and that they are probably bound to the cluster. This is the
first confirmation of filamentary structure as traced by galaxy groups at z>1.
We then use several spectral features in the FORS2 spectra to determine the
star formation histories of group galaxies. We find a population of relatively
red star-forming galaxies in the groups that are absent from the cluster core.
While similarly red star forming galaxies can also be found in the field, the
average strength of the hd line is systematically weaker in group galaxies.
Interestingly, these groups at z=1.2 are in an environment in which the
on-going build-up of red sequence is happening. The unusual line strengths can
be explained by star formation that is heavily obscured by dust. We hypothesize
that galaxy-galaxy interactions, which is more efficient in the group
environment, is the mechanism that drives these dust obscured star formation.
The hypothesis can be tested by obtaining spectral observations in the near-IR,
high resolution imaging observations and observations in the mid-IR.
