Issue |
A&A
Volume 461, Number 3, January III 2007
|
|
---|---|---|
Page(s) | 823 - 845 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20053941 | |
Published online | 16 October 2006 |
Protoclusters associated with z > 2 radio galaxies
I. Characteristics of high redshift protoclusters
1
Sterrewacht Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands
2
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK e-mail: venemans@ast.cam.ac.uk
3
Lawrence Livermore National Laboratory, PO Box 808, Livermore CA, 94550, USA
4
University of California, Merced, PO Box 2039, Merced, CA 95344, USA
5
European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany
6
INAF, Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125, Firenze, Italy
7
Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Italy
8
University of California, Davis, 1 Shields Ave, Davis, CA 95616, USA
9
Dept. of Physics & Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore MD, 21218-2686, USA
Received:
29
July
2005
Accepted:
5
October
2006
We present the results of a large program conducted with
the Very Large Telescope and augmented by observations with the
Keck telescope to search for forming clusters of galaxies near
powerful radio galaxies at . Besides MRC 1138-262
at
, the radio galaxy observed in our pilot program, we
obtained narrow- and broad-band images of eight radio galaxies
and their surroundings. The imaging was used to select candidate
Lyα emitting galaxies in ~
Mpc2 areas near the
radio galaxies. A total of 300 candidate emitters were found with
a rest-frame Lyα equivalent width of EW
Å and
significance
. Follow-up spectroscopy was performed on 152 candidates in
seven of the radio galaxy fields. Of these, 139 were confirmed to be
Lyα emitters, four were low redshift interlopers and nine were
non-detections. With the adopted criteria the success rate is
%. In addition, 14 objects with EW
and/or
were confirmed to be Lyα emitters. Combined
with the 15 Lyα emitters near MRC 1138-262, we have determined
Lyα redshifts for 168 objects near eight radio galaxies.
At least six of our eight fields are overdense in Lyα emitters
by a factor 3-5 as compared to the field density of Lyα
emitters at similar redshifts, although the statistics in our
highest redshift field (
) are poor. Also, the emitters
show significant clustering in velocity space. In the overdense
fields, the width of the velocity distributions of the emitters
is a factor 2-5 smaller than the width of the narrow-band
filters. Taken together, we conclude that we have discovered six
forming clusters of galaxies (protoclusters). We estimate that
roughly 75% of powerful (
erg s-1 Hz-1 sr-1) high redshift radio galaxies
reside in a protocluster. The protoclusters have sizes of at least
1.75 Mpc, which is consistent with the structure
sizes found by other groups. By using the volume occupied by the
overdensities and assuming a bias parameter of
, we
estimate that the protoclusters have masses in the range
. These protoclusters are likely to be
progenitors of present-day (massive) clusters of galaxies. For
the first time, we have been able to estimate the velocity
dispersion of cluster progenitors from
to ~2. The
velocity dispersion of the emitters increases with cosmic time,
in agreement with the dark matter velocity dispersion in
numerical simulations of forming massive clusters.
Key words: galaxies: active / galaxies: clusters: general / cosmology: observations / cosmology: early Universe / cosmology: large scale structure of Universe
© ESO, 2007
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