Issue |
A&A
Volume 516, June-July 2010
|
|
---|---|---|
Article Number | A101 | |
Number of page(s) | 8 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361/201014356 | |
Published online | 21 July 2010 |
Spectroscopic confirmation of a galaxy cluster associated with 7C 1756+6520 at z=1.416
A. Galametz1,2 - D. Stern3 - S. A. Stanford4 - C. De Breuck1 - J. Vernet1 - R. L. Griffith3 - F. A. Harrison5
1 - European Southern Observatory, Karl-Schwarzschild-Strae 2, 85748 Garching, Germany
2 - Observatoire Astronomique de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France
3 - Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
4 - Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
5 - Space Radiation Laboratory, MS 220-47, California Institute of Technology, Pasadena, CA 91125, USA
Received 4 March 2009 / Accepted 17 April 2010
Abstract
We present spectroscopic follow-up of an overdensity of galaxies photometrically selected to be at
1.4 < z < 2.5 found in the vicinity of the radio galaxy 7C 1756+6520 at
z =
1.4156. Using the DEIMOS optical multi-object spectrograph on the Keck 2 telescope, we observed a total of 129 BzK-selected sources, comprising 82 blue, star-forming galaxy candidates (sBzK) and 47 red, passively-evolving galaxy candidates (pBzK*),
as well as 11 mid-infrared selected AGN candidates. We obtain robust
spectroscopic redshifts for 36 blue galaxies, 7 red galaxies and 9 AGN
candidates. Assuming all foreground interlopers were identified, we
find that only 16% (9%) of the sBzK (pBzK*) galaxies are at z<1.4. Therefore, the BzK
criteria are shown to be relatively robust at identifying galaxies at
moderate redshifts. Twenty-one galaxies, including the radio galaxy,
four additional AGN candidates and three red galaxy candidates are
found with
,
forming a large scale structure at the redshift of the radio galaxy. Of
these, eight have projected offsets <2 Mpc relative to the
radio galaxy position and have
velocity offsets <1000 km s-1 relative to the
radio galaxy redshift. This confirms that 7C 1756+6520 is
associated with a high-redshift galaxy cluster. A second compact group
of
four galaxies is found at
,
forming a sub-group offset by
km s-1 and approximately
east of the radio galaxy.
Key words: large-scale structure of Universe - galaxies: clusters: general - Galaxy: evolution - galaxies: individual: 7C 1756+6520 - galaxies: clusters: individual: 7C1756+6520
1 Introduction
Galaxy clusters provide an important tool for studying both the formation of galaxies and for deriving cosmological parameters. Out to the furthest redshifts studied thus far, the oldest, most massive galaxies reside within clusters, and thus clusters provide ideal probes of the formation and evolution of galaxies. As the largest collapsed structures in the universe, the cosmic history of galaxy clusters is sensitive to key cosmological parameters (e.g., Stern et al. 2010; Vikhlinin et al. 2009). Significant work has been done to discover more distant galaxy clusters. However, the number of clusters currently confirmed at z>1 remains low. Two of the most distant confirmed galaxy clusters were identified from the extended X-ray emission of the intracluster medium: XMMU J2235.3-2557 at z=1.39 (Rosati et al. 2009; Mullis et al. 2005; Lidman et al. 2008) and XMMXCS J2215.9-1738 at z=1.46 (Stanford et al. 2006; Hilton et al. 2007). However, the X-ray identification of candidate clusters is very difficult at z>1 since the surface brightness of the extended X-ray emission fades as (1+z)4.
Another method to find galaxy clusters has been to detect overdensities of red sources in optical imaging data using the so-called ``red sequence method''. Gladders & Yee (2000) showed that two filter imaging is sufficient to perform a cluster search through the detection of the red sequence of early-type galaxies. The colours of such galaxies are quite distinct due to the strong 4000 Å break (D4000) in their spectra. However, this break shifts into the near-infrared at z>1.5, and the colors can become degenerate. Using sensitive mid-infrared data obtained with the Spitzer Space Telescope, the Spitzer Adaptation of the Red-Sequence Cluster Survey (SpARCS; Wilson et al. 2009) team has recently pushed the technique to higher redshift and confirmed one galaxy cluster at z = 1.34. Using full photometric redshifts with effectively a stellar mass-selected galaxy sample, Eisenhardt et al. (2008) have identified 106 galaxy cluster candidates at z > 1, 13 of which have been spectroscopically confirmed to date. This technique, which does not depend on the presence of a red sequence, has confirmed three galaxy clusters at z > 1.3, with the most distant at z = 1.41 (Stanford et al. 2005).
An alternative method to find high-redshift galaxy clusters is to
look in the surroundings of powerful, high-redshift radio galaxies
(HzRGs). HzRGs are among the most massive galaxies in the Universe
(
;
Seymour et al. 2007) and therefore
are likely to inhabit dense regions. Narrow-band imaging surveys have been
intensively conducted in the surroundings of HzRGs, mostly at z>2to search for overdensities of Ly
and/or H
emitters.
Numerous overdensities have been detected and spectroscopically
confirmed around radio galaxies at z>2, reaching even to z=5.2(TN J0924-2201; Venemans et al. 2007). However, Ly
emitters
are small, faint, young star-forming galaxies with masses of a few
(Overzier et al. 2008), and probably
represent only a small fraction of the total stellar mass of these clusters.
Furthermore, at such high redshifts, these overdensities are suspected
to still be forming and not yet bound. The term ``protoclusters'' is
commonly used to describe such systems. A complementary approach is
to isolate the evolved, massive galaxy population near HzRGs using
purely near-infrared colour selection (Kajisawa et al. 2006; Kodama et al. 2007).
Though this method has successfully found overdensities of red
galaxies at
,
it has been challenging to spectroscopically
confirm their association with the HzRGs (e.g., Doherty et al. 2010).
A few studies have also applied related methods to slightly lower
redshift HzRGs - e.g., Stern et al. (2003) and Best et al. (2003)
found overdensities of extremely red galaxies in the environments
of radio-loud active galactic nuclei (AGN) at
.
Recently, Galametz et al. (2009a) (G09 hereafter) presented an
overdensity of galaxy candidates at z>1.4 in the field of the
radio galaxy 7C 1756+6520. The radio galaxy was initially reported
to be at z = 1.48 by Lacy et al. (1999) based on the tentative
identification of a single, uncertain emission feature. Based on
deeper Keck spectroscopy, we find that the radio galaxy is, in fact,
at z=1.4156 (see Sect. 3.1). G09 made use of a revised version of the
so called ``BzK criteria'', a two-colour selection technique based
on BzK photometry (Daddi et al. 2004) to isolate galaxies at
and classify them as either red, passively evolving
(pBzK) or blue, star-forming (sBzK) systems. The star-forming
candidates are selected by
.
The original Daddi et al. (2004) criterion selected passive (pBzK)
systems by
,
but was empirically
shown in G09 to have a low success rate at
.
In G09,
we therefore extended the selection criteria to reliably identify
galaxies at the low-redshift end of the BzK criteria by defining
pBzK* galaxies to have
.
G09 selected sBzK and pBzK* galaxies in the field around 7C 1756+6520 using deep, multiwavelength data: B-band and z-band images from the Large Format Camera (LFC; Simcoe et al. 2000) on the Palomar 5 m Hale telescope and a Ks-band (hereafter K) image from the Wide-field Infrared Camera (WIRCAM; Puget et al. 2004) on the Canada-France-Hawaii telescope (see G09, Table 1 for details).
Relative to the four deep MUSYC blank fields (Quadri et al. 2007; Gawiser et al. 2006), G09 found an overdensity of cluster member candidates around 7C 1756+6520 for both sBzK and pBzK* galaxies - by a factor of 2 and 4.7, respectively. Using the Spitzer/IRAC colour-colour selection of Stern et al. (2005), we also isolated 12mid-infrared selected AGN candidates around 7C 1756+6520, which represents an overdensity by a factor of two compared to the IRAC Shallow Survey (Eisenhardt et al. 2004).
We describe in this paper the results of our spectroscopic follow-up
of the overdensity found in the surroundings of 7C 1756+6520. The
next section presents the selected targets, observations and data
reduction. Section 3 describes the results of our spectroscopy and
presents the redshift distribution of the observed sources. Section
4 reports the discovery of a concentration of 20 galaxies whose
redshifts are close to that of the radio galaxy. We assume a
CDM cosmology with H0 = 70 km s-1 Mpc-1,
and
.
The magnitudes are
expressed in the AB photometric system unless otherwise stated.
2 Keck/DEIMOS Spectroscopy
In order to examine whether the detected overdensities were indeed associated with 7C 1756+6520, we started a spectroscopic follow up campaign at the Keck 2 telescope using the Deep Imaging Multi-Object Spectrograph (DEIMOS; Faber et al. 2003).
We observed one slit mask on UT 2008 August 31 using the 600ZD
grating which is blazed at 7500 Å, covers a typical wavelength
range of
5000-10 000 Å, and has a spectral resolution of 3.7 Å (FWHM). This slitmask targeted 47 sources, including one AGN
candidate, 22 sBzK galaxies and 24 pBzK* galaxies. Eight
1800 s exposures were obtained over the course of three nights using
the same mask; conditions were clear with 0
8 seeing on average.
Two additional slit masks were observed in September 2009, again
with DEIMOS and the 600ZD grating. The first mask, observed for a
total of 2.5 h on UT 2009 September 16, contained 54 objects:
the radio galaxy itself, five AGN candidates, 34 sBzK galaxies
and 14 pBzK* galaxies. This slit mask was oriented so that
a pBzK* galaxy
south-west of 7C 1756+6520 was on the
same slitlet as the radio galaxy itself (e.g., PA
).
The second slit mask, observed for a total of 1.75 h on UT 2009
September 17, contained 56 sources: seven AGN candidates, 36sBzK galaxies and 13 pBzK* galaxies. Both nights were
exceptional, with photometric conditions and 0
5 seeing.
Fourteen BzK galaxies and two AGN were observed multiple times
across these three slit masks. Thus, the total number of distinct
targets was 129 BzK-selected targets - including 82 (47)
sBzK (pBzK*) galaxies - as well as 11 AGN candidates and
the radio galaxy itself. Figure 1 shows the distribution
of the targets in the two colour-colour diagrams used in G09 to
select candidate cluster members: the ``BzK'' diagram for the
BzK-selected galaxies and the IRAC
[3.6]-[4.5] vs.
[5.8]-[8.0]diagram for the mid-infrared selected AGN. We refer to G09 for details
on the selection techniques. The last panel of Fig. 1
shows the distribution of our targets in a J-K vs. Kcolour-magnitude diagram. Figure 2 shows the spatial
distribution of these targets around the radio galaxy. The targeted
sources have optical magnitudes in the range
20.8<z<24.5 with
and near-infrared magnitudes in the
range
20.2<K<23.3 with
.
All data were processed using a slightly modified version of the pipeline developed by the DEEP2 team at UC-Berkeley, and data were flux calibrated using archival sensitivity functions for the same instrument configuration derived using standard stars from Massey & Gronwall (1990). The uncertainty on the wavelength calibration, derived from sky lines, was found to be 0.4Å and consistent from mask to mask.
![]() |
Figure 1:
Keck/DEIMOS spectroscopic targets. Top: Distribution of targeted BzK galaxies in the modified Daddi et al. (2004) ``BzK''
diagram. Middle: Distribution of targeted AGN candidates and 7C 1756+6520 (red star) in the IRAC colour-colour diagram from Stern et al. (2005). Bottom: Colour-magnitude diagram (J-Kvs. K) of spectroscopic targets with reliable J and/or K photometry. We also plot the expected locations of the red sequence at z=1.42 (dashed line) for a formation redshifts of zf=4(lower curve) and zf=5
(upper curve). For the three panels, small dots indicate all sources
observed but not yielding a redshift. Spectroscopically confirmed
members of the structure around
7C 1756+6520 (e.g., with
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![]() |
Figure 2:
J-band image of 7C1756+6520 (red star) illustrating the spatial distribution of our targets. See Fig. 1
for description of symbols. Left and bottom axes are in J2000
coordinate system; right and top axes indicate distances relative to
the radio
galaxy. Image is
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3 Results
3.1 Spectrum of 7C 1756+6520
![]() |
Figure 3: Keck/DEIMOS spectrum of 7C 1756+6520 obtained on UT 2009 September 16 (black, with a smoothing box of 13 Å applied for clarity in the large panel; the inset, detailing the [OII] doublet, was not smoothed). The sky spectrum, in grey, has been scaled down by a factor of 10. The locations of prominent lines are labeled (MgII is not detected in these data). |
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Lacy et al. (1999) tentatively assigned z=1.48 to the radio galaxy
based on an uncertain emission line at 6005 Å assumed to be
[NeIV]
Å. This redshift was assigned a quality
``
'', indicating an ``uncertain'' redshift. We obtained a quick
(10 min) spectrum of the radio galaxy during twilight on UT 2009
April 27 using the Low Resolution Imaging Spectrometer
(LRIS; Oke et al. 1995) on the Keck 1 telescope. These data
detected only a single, faint emission line at 6759 Å from
which no reliable redshift could be assigned. The
deeper Keck/DEIMOS spectrum obtained in September 2009 (Fig. 3)
yielded a redshift of 1.4156 based on three clearly detected
emission lines: [NeV]
Å, [OII]
Å and
[NeIII]
Å. The [OII] line (Fig. 3, inset)
is clearly split in these data. In retrospect, the faint emission line in the LRIS
spectrum in fact corresponds to MgII
Å. We note that
the emission line in Lacy et al. (1999), from which their redshift
identification was based, is not detected in either of our deeper
Keck spectra. It would correspond to restframe 2486 Å at the
redshift of the radio galaxy and was therefore most likely a spurious
detection.
3.2 Spectroscopy of candidate cluster members
The redshift range 1.4<z<2.5 is often described as the ``redshift
desert'' because optical spectroscopic confirmation of targets in
that redshift range is challenging, with most of the strongest
classical spectral features (e.g.,
[OII]
Å, D4000, H
Å,
[OIII]
Å, H
Å) redshifted
longward of the wavelength range where CCDs are most sensitive
(e.g.,
4000-9000 Å). DEIMOS uses a modern, red-sensitive CCD
detector which, combined with the grating we had installed for these
observations, provides data out to
m and would, in
principle, detect the [OII] emission line to
.
However,
the sensitivity of the spectrograph detectors rapidly decreases at
the longest wavelengths and telluric OH emission becomes progressively
problematic, especially at
Å (corresponding to
z>1.5 for the [OII] line).
A visual inspection of the reduced spectra permitted us to assign
spectroscopic redshifts (
)
to 43 BzK galaxies,
9 AGN candidates and the radio galaxy (see Sect. 3.1). Due to the
design of the masks and the length of the slits, we also obtained
additional spectroscopic redshifts of several serendipitous sources.
We determined redshifts by fitting emission lines by Gaussian
profiles. When clearly split, the [OII] doublet was fit by a double
Gaussian profile. We derived uncertainties on the spectroscopic redshifts
by adding in quadrature the fitting uncertainties and the small uncertainty in
the wavelength calibration (see Sect. 2; 0.4 Å corresponds to
).
We assigned a quality flag ``A'' or ``B'' to all measured redshifts: ``A''
indicates a highly certain
based on at least two
spectral features or [OII]
Å being clearly identified
as a doublet; ``B'' indicates high-level confidence in the
based on one well-detected spectral feature. Lower confidence
quality flags were recorded during our data analysis, but are not
reported here.
Figure 4 presents the distribution of our spectroscopic
redshifts. Coordinates,
,
redshift, quality flags
(Q), magnitudes and the target selection criteria of all confirmed
sources are provided in Table 1 (for members of the
HzRG large-scale structure) and Table 2 (for the
foreground/background sources). The final column of these tables
lists the spectral features used to determine redshifts.
Table 1: Spectroscopic members of the structure around 7C 1756+6520.
Table 2: Other spectroscopic redshifts.
![]() |
Figure 4:
Histogram of the 52 spectroscopic redshifts, including the 43 BzK-selected
objects (black), the nine mid-infrared selected AGNs and the radio
galaxy (the AGN are all shown in grey). We also include the
spectroscopic redshifts of the three serendipitous objects detected in
our slits with
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3.2.1 BzK-selected sources
We obtained spectroscopic redshifts for 43 (33%) of the targeted
BzK-selected sources, including 36 (44%) of the sBzK targets
and seven (15%) of the pBzK* targets. The majority (79%)
of the redshifts were calculated from the [OII]
Å doublet,
which is resolved at the spectral resolution of the DEIMOS data.
Other features, such as MgII
Å (either in absorption
or emission) or the D4000 break, were also present in several of
the spectra, permitting us to unambiguously confirm our redshift
identifications.
Of the sources for which we derived successful spectroscopic
redshifts, 64% (43%) of the sBzK (pBzK*) galaxies are at
.
Based on these spectroscopic results, it is
challenging to derive definitive conclusions regarding the reliability
of the BzK criterion to select high-redshift galaxies. At z>1.3,
bright and numerous sky lines (see Fig. 4, sub-panel)
coupled with decreasing detector sensitivity at longer wavelengths
limits our ability to measure redshifts based on the [OII] doublet.
Furthermore, our redshift coverage does not permit detection of
[OII] at z>1.7 (and at slightly lower redshifts for many sources
since the spectral coverage of any given source will depend on where
it is located on the slitmask). With these caveats in mind, we now
briefly discuss the efficiency of the two BzK criteria to identify
galaxies at z > 1.4.
sBzK galaxies: Assuming that the sBzK criterion successfully
identifies sources with strong signatures of star formation -
e.g., strong [OII], [OIII] and H
emission lines - then
our Keck/DEIMOS data should have yielded robust spectroscopic
redshifts for the vast majority of sBzK galaxies at z<1.4.
Indeed, all but four of the successfully identified sBzK sources
show these emission lines. The last four are at
1.6 < z <
2.4 where the [OII] line had shifted beyond the spectroscopic
coverage of our instrument; their redshifts instead are based on
absorption lines such as CIV
1549 and
AlII
1670 which are commonly seen in the UV spectra of
star-forming galaxies. If we thus assume
that our data identified all of the interlopers and that the failed
sources are all at z > 1.4, then we find that the sBzK criterion
is quite robust, with only 16% of such targets at z<1.4.
pBzK* galaxies: As expected, the pBzK* galaxies often
presented very red optical continuum with no clear features (e.g.,
spectral breaks) - indeed, all seven of the
pBzK* galaxies for which we did derive spectroscopic redshifts
had [OII] emission features. For evolved galaxies, the D4000 break
is generally the strongest spectral signature. However, assuming
that a spectrum must cover out to restframe 4100 Å to robustly
identify this feature, then the m cut-off of the DEIMOS data
implies that we can only identify D4000 out to z = 1.44; we are
thus forced to rely on weaker spectroscopic features such as B2900 or B2640
to identify evolved galaxies at
and beyond. This makes
redshift determinations challenging, and is presumably the
reason for our low success rate for the pBzK* sources. Assuming
that our data identified all of the foreground interlopers and that
the failed sources are all at z > 1.4, we find that the
pBzK* criteria are also robust, with only 9% of such targets
at z < 1.4. We note, however, that this assumption is less robust
than the similar assumption for the sBzK galaxies, given the low signal-to-noise
ratio of the pBzK spectra.
![]() |
Figure 5: Two-dimensional spectra of the spectroscopically confirmed members of the structure, centered near the [OII] emission line. The position of the [OII] doublet at z=1.4156 is indicated by the vertical red dashed line. The redshift and ID number of each source is provided on the right end side. The top axis indicates the corresponding velocity distribution relative to the redshift of the radio galaxy. |
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In the colour-magnitude diagram in Fig. 1 (bottom panel),
we show the expected location of the red sequence at z=1.42 for
a formation redshift zf=4-5 (T. Kodama, private communication;
see also Kodama et al. 1998). We targeted numerous pBzK* galaxies
with colours consistent with the red sequence for which we unfortunately
could not determine a redshift. However, the three
pBzK* galaxies that were confirmed to be at
all have
2.2<z-K<2.5 would have been missed by the standard
Daddi et al. (2004) criterion for pBzK galaxies (see Sect. 1). Our
modified pBzK* criterion presented in G09 increases the completeness
of galaxies at
.
On the other hand, our selection is also
contaminated by lower redshift (albeit still relatively distant)
objects. The other four pBzK* with
are foreground
interlopers with
1.18<z<1.26.
3.2.2 Mid-infrared selected AGN candidates
Eleven of the 12 AGN candidates presented in G09 were observed
and we were able to assign spectroscopic redshifts for 9 of these 11.
The AGN selection criterion of Stern et al. (2005) does not preferentially
select AGN in any specific redshift range (other than an inefficiency
at selecting sources at
where H
shifts into the
IRAC channel 1 and produces blue [3.6]-[4.5] colours; see Assef
et al. 2010). The mid-infrared selection criteria
are also sensitive to both obscured and unobscured AGN
(e.g., Eckart et al. 2010). Unsurpringly, therefore, we identified
AGN at a range of redshifts (
0.37 < z < 3.70) and with a range
of properties.
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Figure 6:
The 20 spectroscopically confirmed members of the structure around 7C 1756+6520 (plotted on our J-band data in filled symbols). The four colours (blue, yellow, green and orange,
see redshift scale on the right) indicate different ranges of redshifts: z<1.415,
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4 A structure of galaxies associated with 7C 1756+6520
4.1 Companions close to the radio galaxy
In G09 we stated that an sBzK galaxy, a pBzK* galaxy and an AGN
candidate are all found within
of 7C 1756+6520. For
z
= 1.4156, this corresponds to a projected separation of 50 kpc.
The probability of finding three candidates in such a small area
is less than 0.2%, suggesting that these sources are all associated
with the radio galaxy. Both the pBzK* and the AGN candidates
were observed with DEIMOS. The pBzK* source shows a clear, resolved
[OII] doublet at z=1.4244, and is thus associated with the radio
galaxy (Table 1, ID 12; quality A). The AGN candidate
shows a single, broad emission line which we interpret to be MgII
at z=1.4153 (Table 1, ID 7; quality B). The sBzK galaxy was
not observed. We therefore believe that the radio galaxy has
at least one, most probably two, and perhaps three close-by companions.
4.2 Two compact sub-groups in a large scale structure
A clear peak is seen in the spectroscopic redshift distribution
near the radio galaxy redshift (Fig. 4). Not including
the HzRG, 20 galaxies are found with
,
corresponding to peculiar velocities
3000 km s-1 with respect to the radio galaxy. Table 1
reports the coordinates and BzK magnitudes of these 20 galaxies
(21 including the HzRG), comprising four mid-infrared AGN candidates,
three pBzK* galaxies, 10 sBzK galaxies and three serendipitous
galaxies. The first serendipitous galaxy, Cl1756.6 (or serendip.1),
is blended in all of our imaging bands with an object at z=0.76.
The foreground object, whose photometry is contaminated in all three
bands by its projected neighbor, was one of our targeted sBzKgalaxies. The second (Cl1756.8, or serendip.2) was observed in the
same slit as Cl1756.9, one of the sBzK targets. The two sources
are close both on the plane of the sky and in redshift, and are
therefore close-by companions. Cl1756.8 is very faint in Ks (detected
at
)
and was not considered by our BzK criteria. The
last serendipitous object (Cl1756.20, or serendip.3) was not selected
by any of our criteria. The coordinates and magnitudes of the three
pBzK* (Cl1756.5, Cl1756.12 and Cl1756.15 in Table 1)
were previously reported in G09 as galaxies 31, 43 and 63,
respectively (see Table 5 of G09). G09 did not tabulate the sBzKcandidates.
The two dimensional spectra of the 20 galaxies associated with 7C 1756+6520 are shown in Fig. 5, centered near the [OII] line used to determine their redshifts. As expected, the pBzK* galaxies (sources 5, 12 and 15) show fainter [OII] emission lines than most of the AGN or star-forming (sBzK) candidates.
There is still no clear and agreed upon definition of
a galaxy cluster. Eisenhardt et al. (2008) consider a z>1 cluster
spectroscopically confirmed if five galaxies are robustly identified
within a radius of 2 Mpc and
km s-1. According to this definition, 14 objects
(including the radio galaxy) would therefore be part of our galaxy
cluster. However, the spectroscopically-confirmed, high-redshift
galaxy clusters in Eisenhardt et al. (2008) and elsewhere usually show much narrower
velocity distributions. The spectroscopically confirmed members
of XMMXCS J2215.9-1738 at z=1.457 all have peculiar velocities
offset by less than 1000 km s-1 with respect to the
cluster redshift (Hilton et al. 2007). Recently,
Hayashi et al. (2009) presented an [OII] emission survey of this same
cluster. They identified 44 [OII] emitters over a larger area centered on the cluster with
km s-1 with respect to the cluster redshift.
The spatial distribution of the 20 objects is shown in Fig. 6
(the 17 red symbols in Fig. 2 and the three additional
serendipitous objects). Seven galaxies (IDs 6-12) have velocities
within 1000 km s-1 of the radio galaxy redshift (i.e., the
assumed cluster redshift) and are found within 2 Mpc of the HzRG
(yellow circles;
). The lowest redshift sources
(IDs 1-5; blue circles; z<1.415) are found with projected
separations more than 1 Mpc with respect to the radio galaxy
(3/5 beyond 2 Mpc). Objects with
1.424<z<1.436 (IDs 13-16;
green circles) are all more than 2 Mpc distant from the radio galaxy.
In contrast, the highest redshift sources (IDs 17-20; orange
circles;
)
are found within
from each other
and appear to form a compact sub-structure. We therefore
confirm that 7C 1756+6520 is part of a large scale structure
of galaxies composed of (at least) two main galaxy groups - the
galaxy cluster centered on the radio galaxy redshift (z=1.4156)
and a compact galaxy sub-group at
.
Following the methodology of Beers et al. (1990) for a small number of galaxy cluster members,
we estimated velocity dispersions using the `jackknife of the gapper' estimator. We found that the
velocity dispersion of sources within 1.5 Mpc from the radio galaxy (i.e., IDs 6-10
+ ID 12 + IDs 17-20 + the radio galaxy) is
km s-1. A
velocity dispersion of
km s-1 was derived for the galaxy cluster associated with the radio galaxy (yellow circles in Fig. 6). Given the small number of
galaxies used in the calculation, the velocity dispersions given here are only indicative.
4.3 A high fraction of AGN cluster members
Four mid-infrared AGN candidates have been confirmed to be associated with 7C1756+6520 (IDs
1, 7, 10 and 18). Galaxies 4 and 13, both targeted as
sBzK galaxies, also show AGN signatures in their spectra with
strong, broad MgII emission lines. Assuming that the mid-infrared selected sources are indeed
all active, six AGN (seven with the radio galaxy) have
therefore been spectroscopically confirmed in close proximity both
spatially and in redshift space. Three of them are found within
of the radio galaxy. Studying the surface density of
luminous AGN associated with a sample of 330 galaxy clusters at
0<z<1.5, Galametz et al. (2009b) found an excess of AGN within
0.5 Mpc of the center of clusters at z>0.5. They identify AGN
using three different selection methods including the Stern et al. (2005)
mid-infrared selection. Our spectroscopy brings additional evidence
that galaxy clusters at z>1 have a significant fraction of members
being AGN and that they lie preferentially near the cluster center.
5 Conclusions
The observations presented here confirm the existence of a large
scale structure of galaxies associated with 7C 1756+6520. We assign
a new reliable redshift of z=1.4156 to the radio galaxy.
Our optical spectroscopy demonstrates the efficiency of the BzKselection technique of Daddi et al. (2004) at finding galaxies at
z>1.4. It also shows that the modified pBzK* criterion presented
in G09 increases the completeness of galaxies at z>1.4. We find twenty
galaxies with spectroscopic redshifts consistent with the
redshift of 7C 1756+6520. Seven of these galaxieshave velocity offsets
km s-1 relative to the redshift of the radio
galaxy (assumed here to be the galaxy structure mean redshift) and
are within 2 Mpc of the radio galaxy.
A second compact group of four galaxies (all within
of each other)
lies at
and forms an associated sub-group
of galaxies, offset by about
to the east of the radio
galaxy. 7C 1756+6520 is therefore part of a large scale galaxy structure composed
of (at least) two main groups - a galaxy cluster centered on the radio
galaxy at z=1.4156 (with 8 spectroscopically confirmed members including the radio galaxy)
and a compact group at
,
east of the radio galaxy (with 4 confirmed members so far).
This work is based on a spectroscopic campaign at the W. M. Keck Observatory, a scientific partnership between the University of California and the California Institute of Technology, made possible by a generous gift of the W. M. Keck Foundation. We are very grateful to Tadayuki Kodama for having provided the models of red sequence presented in this paper. We thank the anonymous referee for his/her careful reading of the manuscript and constructive comments. The work of DS and RLG was carried out at Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. S.A.S.'s work was performed under the auspices of the US Department of Energy, National Nuclear Security Administration by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.
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All Tables
Table 1: Spectroscopic members of the structure around 7C 1756+6520.
Table 2: Other spectroscopic redshifts.
All Figures
![]() |
Figure 1:
Keck/DEIMOS spectroscopic targets. Top: Distribution of targeted BzK galaxies in the modified Daddi et al. (2004) ``BzK''
diagram. Middle: Distribution of targeted AGN candidates and 7C 1756+6520 (red star) in the IRAC colour-colour diagram from Stern et al. (2005). Bottom: Colour-magnitude diagram (J-Kvs. K) of spectroscopic targets with reliable J and/or K photometry. We also plot the expected locations of the red sequence at z=1.42 (dashed line) for a formation redshifts of zf=4(lower curve) and zf=5
(upper curve). For the three panels, small dots indicate all sources
observed but not yielding a redshift. Spectroscopically confirmed
members of the structure around
7C 1756+6520 (e.g., with
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
J-band image of 7C1756+6520 (red star) illustrating the spatial distribution of our targets. See Fig. 1
for description of symbols. Left and bottom axes are in J2000
coordinate system; right and top axes indicate distances relative to
the radio
galaxy. Image is
|
Open with DEXTER | |
In the text |
![]() |
Figure 3: Keck/DEIMOS spectrum of 7C 1756+6520 obtained on UT 2009 September 16 (black, with a smoothing box of 13 Å applied for clarity in the large panel; the inset, detailing the [OII] doublet, was not smoothed). The sky spectrum, in grey, has been scaled down by a factor of 10. The locations of prominent lines are labeled (MgII is not detected in these data). |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Histogram of the 52 spectroscopic redshifts, including the 43 BzK-selected
objects (black), the nine mid-infrared selected AGNs and the radio
galaxy (the AGN are all shown in grey). We also include the
spectroscopic redshifts of the three serendipitous objects detected in
our slits with
|
Open with DEXTER | |
In the text |
![]() |
Figure 5: Two-dimensional spectra of the spectroscopically confirmed members of the structure, centered near the [OII] emission line. The position of the [OII] doublet at z=1.4156 is indicated by the vertical red dashed line. The redshift and ID number of each source is provided on the right end side. The top axis indicates the corresponding velocity distribution relative to the redshift of the radio galaxy. |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
The 20 spectroscopically confirmed members of the structure around 7C 1756+6520 (plotted on our J-band data in filled symbols). The four colours (blue, yellow, green and orange,
see redshift scale on the right) indicate different ranges of redshifts: z<1.415,
|
Open with DEXTER | |
In the text |
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