Title: High star formation activity in the most distant X-ray
cluster at z=1.46

Author(s): Masao Hayashi, Tadayuki Kodama, Yusei Koyama,
Ichi Tanaka, Sadanori Okamura, Kazuhiro Shimasaku

Abstract:
We conduct an unbiased investigation of star formation activity in
a cluster XMMXCS J2215.9-1738 at z=1.46, which is the most distant
cluster with a detection of the X-ray emission to date, by [OII]
emission survey with NB912 narrow-band filter (lambda_c = 9139A,
FWHM=134A) through Subaru telescope.
We have obtained wide-field optical (B, z', NB912) and near-infrared
(J and Ks) data with Suprime-Cam and MOIRCS, respectively. We then
select 44 [OII] emitters in the central region of 6'x6' with all data
available for the cluster. The three sigma limiting flux corresponds
to dust-free star formation rate of 2.6 Msun/yr. [OII] identification
among possible lines is based on the BzK colors.
We find that many [OII] emitters reside in a central high density
region within a radius of 0.26 Mpc. Assuming that most [OII] emissions
are caused by their star formation, this suggests that XCS2215 cluster
is still actively forming stars at the centeral reigion, where the
local clusters have quenched the star formation. Interestingly, our
other narrow-band survey in a cluster RXJ1716.4+6708 at z=0.81 shows
a quite different situation that H_alpha emitters densely populate
in medium density regions away from the center.
Moreover, the color-magnitude diagram in XCS2215 cluster suggests a
deficit of red galaxies fainter than ~M*+0.5. However, robust red
sequenes are seen in lower redshifts down to fainter magnitude than
~M*+0.5.
These facts may indicate outward and down-sizing propagation of
star formation at the high redshift cluster.