Measurements of a_massive_galaxy_cluster

ApJ, in press
Preprint typeset using L TEX style emulateapj v. 5/2/11
A

IDCS J1426.5+3508: SUNYAEV–ZEL’DOVICH MEASUREMENT OF A MASSIVE IR-SELECTED CLUSTER
AT Z = 1.75
M. Brodwin,1,2 A. H. Gonzalez,3 S. A. Stanford,4,5 T. Plagge,6,7 D. P. Marrone,8 J. E. Carlstrom6,7,9 A. Dey,10
P. R. Eisenhardt,11 C. Fedeli,3 D. Gettings,3 B. T. Jannuzi,10 M. Joy,12 E. M. Leitch,6,7 C. Mancone,3
G. F. Snyder,2 D. Stern,11 and G. Zeimann4
ApJ, in press
arXiv:1205.3787v1 [astro-ph.CO] 16 May 2012

ABSTRACT
We report 31 GHz CARMA observations of IDCS J1426.5+3508, an infrared-selected galaxy cluster
at z = 1.75. A Sunyaev–Zel’dovich decrement is detected towards this cluster, indicating a total mass
of M200,m = (4.3 ± 1.1) × 1014 M⊙ in agreement with the approximate X-ray mass of ∼ 5 × 1014 M⊙ .
IDCS J1426.5+3508 is by far the most distant cluster yet detected via the Sunyaev–Zel’dovich effect,
and the most massive z ≥ 1.4 galaxy cluster found to date. Despite the mere ∼ 1% probability
of finding it in the 8.82 deg2 IRAC Distant Cluster Survey, IDCS J1426.5+3508 is not completely
unexpected in ΛCDM once the area of large, existing surveys is considered. IDCS J1426.5+3508
is, however, among the rarest, most extreme clusters ever discovered, and indeed is an evolutionary
precursor to the most massive known clusters at all redshifts. We discuss how imminent, highly
sensitive Sunyaev–Zel’dovich experiments will complement infrared techniques for statistical studies
of the formation of the most massive galaxy clusters in the z > 1.5 Universe, including potential
precursors to IDCS J1426.5+3508.
Subject headings: galaxies: clusters: individual (IDCS J1426.5+3508) — galaxies: clusters: intra-
cluster medium — galaxies: evolution — cosmology: observations — cosmology:
cosmic background radiation

1. INTRODUCTION 2010; Holz and Perlmutter 2010; Mortonson et al. 2011;
As the most massive collapsed objects, galaxy clusters Cayń et al. 2011; Hoyle et al. 2011; Enqvist et al. 2011;
o
provide a sensitive probe of the cosmological parameters Foley et al. 2011; Williamson et al. 2011; Hotchkiss 2011;
that govern the dynamics of the Universe. Traditional Paranjape et al. 2011). These rare, massive clusters were
cluster cosmology experiments constrain these parame- all identified from their hot intracluster medium (ICM),
ters by comparing predicted cluster mass functions with whether via X-ray emission or from prominent Sunyaev–
observed cluster counts (for a review, see Allen et al. Zel’dovich (SZ) decrements.
2011). Conversely, the most successful method for discov-
The recent discovery of a handful of very massive, ering large samples of high redshift galaxy clusters,
high redshift (z > 1) galaxy clusters (Rosati et al. 2009; out to at least z = 1.5, is via infrared selection
Brodwin et al. 2010; Foley et al. 2011) has prompted (e.g., Eisenhardt et al. 2008; Muzzin et al. 2008). For
several studies of the power of individual massive, rare instance, the Spitzer/IRAC Shallow Cluster Survey
clusters to probe the physics of inflation, in particular (ISCS, Eisenhardt et al. 2008), which employs a full
non-Gaussianity in the density fluctuations that seed three-dimensional wavelet search technique on a 4.5µm-
structure formation (Dalal et al. 2008; Brodwin et al. selected, stellar mass-limited galaxy sample, has dis-
covered 116 candidate galaxy clusters and groups at
1 Department of Physics and Astronomy, University of z > 1. More than 20 of these have now been spectro-
Missouri, 5110 Rockhill Road, Kansas City, MO 64110 scopically confirmed at 1 < z < 1.5 (Stanford et al. 2005;
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Elston et al. 2006; Brodwin et al. 2006; Eisenhardt et al.
Street, Cambridge, MA 2008; Brodwin et al. 2011).
3 Department of Astronomy, University of Florida,
Gainesville, FL 32611
The cosmological importance of individual galaxy clus-
4 Department of Physics, University of California, One Shields ters depends primarily on their rarity, with the most
Avenue, Davis, CA 95616 massive clusters at any redshift providing the most lever-
5 Institute of Geophysics and Planetary Physics, Lawrence
age. The maximum predicted cluster mass increases with
Livermore National Laboratory, Livermore, CA 94550
6 Kavli Institute for Cosmological Physics, University of time due to the growth of structure. Consequently, clus-
Chicago, Chicago, IL 60637 ters with moderate masses, by present-day standards,
7 Department of Astronomy and Astrophysics, University of
of a few ×1014 M⊙ are cosmologically interesting at
Chicago, Chicago, IL 60637 the highest redshifts (e.g., z > 1.5). Only one such
8 Steward Observatory, University of Arizona, 933 North
Cherry Avenue, Tucson, AZ 85721 cluster, XDCP J0044.0-2033 at z = 1.58 (Santos et al.
9 Dept. of Physics/Enrico Fermi Institute, University of 2011), has been found to date, although at such a high
Chicago, Chicago, IL 60637
10 NOAO, 950 North Cherry Avenue, Tucson, AZ 85719
redshift its LX –based mass is quite uncertain. The
11 Jet Propulsion Laboratory, California Institute of Technol- most sensitive, large angle, ICM-based cluster survey to
ogy, Pasadena, CA 91109 date, the 2500 deg2 South Pole Telescope Survey (SPT;
12 Department of Space Science, VP62, NASA Marshall Space Carlstrom et al. 2011), would only be able to detect such
Flight Center, Huntsville, AL 35812, USA high redshift clusters if they were more massive than

2 Brodwin et al.

≈ 3×1014 M⊙ (Vanderlinde et al. 2010; Andersson et al. nosity of L0.5-2keV = (5.5 ± 1.2) × 1044 erg/s. Using the
2011). A population of such high-redshift clusters, if M500 –LX scaling relation of Vikhlinin et al. (2009) and
massive enough, could provide a powerful opportunity the Duffy et al. (2008) mass–concentration relation, this
to study the physics of the early Universe and, in par- corresponds to a mass of M200,c ≃ 5.6×1014 M⊙ (S12)13 .
ticular, to either confirm or falsify the predictions of the The HST/ACS and WFC3 color image, along with the
ΛCDM paradigm. They would also be invaluable probes X-ray detection (yellow contours) are shown in the lower
of the early formation of massive galaxies in the richest panel of Figure 1.
environments. While IDCS J1426.5+3508 appears from the X-ray
To extend the ISCS infrared-selected cluster sample to data to be impressively massive, particularly at such an
the z > 1.5 regime, we carried out the Spitzer Deep, extreme redshift, the X-ray luminosity is based on only
Wide–Field Survey (SDWFS, Ashby et al. 2009), which 54 cluster X-ray photons. Further, it relies on a high-
quadrupled the Spitzer/IRAC exposure time, leading to scatter mass proxy, the M500,c -LX relation, which is only
a survey more than a factor of 2 deeper than the original calibrated at low (z < 1) redshift. The uncertainty of this
IRAC Shallow Survey (Eisenhardt et al. 2004). A new X-ray mass estimate is at least ∼ 40%.
cluster search, the Spitzer/IRAC Distant Cluster Survey
(IDCS), to be described in detail in forthcoming papers, 3. DATA
was carried out using new photometric redshifts calcu- 3.1. Sunyaev Zel’dovich Observations
lated for the larger, deeper SDWFS sample. Cluster
IDCS J1426.5+3508 at z = 1.75 (Stanford et al. 2012, IDCS J1426.5+3508 was observed with the SZA, an 8-
hereafter S12) is the first of the new high–redshift IDCS element radio interferometer optimized for measurements
clusters to be spectroscopically confirmed. of the SZ effect. During the time in which the data were
In this paper we report the robust detection of the obtained, the array was sited at the Cedar Flat location
SZ decrement associated with IDCS J1426.5+3508 in described in Culverhouse et al. (2010), and was in the
31 GHz interferometric data taken with the Sunyaev– “SL” configuration. In this configuration, six elements
Zel’dovich Array (SZA), a subarray of the Com- comprise a compact array sensitive to arcminute-scale SZ
bined Array for Research in Millimeter-wave Astron- signals, and two outrigger elements provide simultaneous
omy (CARMA). This is by far the most distant clus- discrimination for compact radio source emission. The
ter yet detected in the SZ effect, a consequence of the compact array and outrigger baselines provide baseline
fact that IDCS J1426.5+3508 is the most massive clus- lengths of 0.35-1.3 kλ and 2-7.5 kλ, respectively.
ter yet discovered at z > 1.4. In §2 we briefly describe The cluster was observed for four sidereal passes on
the discovery of cluster IDCS J1426.5+3508. In §3 we nights ranging over UT 2011 June 9–29, for a total
present the CARMA observations and reductions, and of 6.24 hours of unflagged, on-source integration time.
describe the measurement of the Comptonization and Data were obtained in an 8 GHz passband centered at
mass of IDCS J1426.5+3508. We also discuss various 31 GHz. The resulting noise level achieved by the short
systematic uncertainties present in our analysis. In §4, (SZ-sensitive) baselines was 0.29 mJy/beam, or 22 µKRJ
we calculate the likelihood of having found this clus- with a synthesized beam of 118′′ × 143′′ . For the long
ter, given our survey selection function, in a standard baselines, the noise level achieved was 0.29 mJy/beam
′′ ′′
ΛCDM cosmology. In §5, we place IDCS J1426.5+3508 in a 19. 9 × 14. 4 beam. The data were reduced us-
in an evolutionary context, finding that it is a pre- ing the Miriad software package (Sault et al. 1995), and
cursor to the most massive known clusters at all red- the absolute calibration was determined using periodic
shifts. In §6 we discuss our results in light of current measurements of Mars calibrated against the Rudy et al.
and upcoming SZ surveys. We present our conclusions (1987) model. A systematic uncertainty of 5% has been
in §7. In this paper, we adopt the WMAP7 cosmology applied to the SZ measurements due to the uncertainty
of (ΩΛ , ΩM , h) = (0.728, 0.272, 0.704) of Komatsu et al. in the absolute calibration. The SZA map is shown in
(2011). Except where otherwise specified (in §2 and the upper left panel of Figure 1.
§3.2), we use a cluster mass (M200,m ) defined as the mass
enclosed within a spherical region of mean overdensity 3.2. Comptonization and Mass of IDCS J1426.5+3508
200 × ρmean , where ρmean is the mean matter density on To determine the properties of IDCS J1426.5+3508
large scales at the redshift of the cluster. from the SZA data, we make use of the fact that the
integrated Compton y-parameter scales with total mass.
2. CLUSTER IDCS J1426.5+3508
Assuming a spherically symmetric pressure distribution
IDCS J1426.5+3508, first reported in S12, was discov- within the cluster, the integrated y-parameter is given
ered in the IDCS as a striking three-dimensional over- by
density of massive galaxies with photometric redshifts 2 σT
at zphot ≈ 1.8 (Figure 1, upper right panel). Follow- Y DA = P (r)dV, (1)
me c2
up spectroscopy, with HST/WFC3 in the infrared and
Keck/LRIS in the optical, identified seven robust spec- where DA is the angular diameter distance, σT is the
troscopic members at z = 1.75 and ten additional likely Thomson cross-section of an electron, me is the electron
members with less certain redshifts. The HST data also mass, c is the speed of light, P (r) is the pressure profile,
revealed the presence of a strong gravitational arc, the and the integral is over a spherical volume centered on
analysis and implications of which are discussed in a com- the cluster (see, e.g., Marrone et al. 2011). The pressure
panion paper (Gonzalez et al. 2012).
IDCS J1426.5+3508 is also detected in shallow Chan- 13 In S12 the mass is measured with respect to a critical over-

dra data from Murray et al. (2005), with an X-ray lumi- density, rather than mean overdensity we use in this paper.

IDCS J1426.5+3508: SZ Measurement of a Massive IR-selected Cluster at z = 1.75 3

Figure 1. Upper left: 12.8′ × 12.8′ SZA mm-wave map showing the SZ decrement of IDCS J1426.5+3508. The ﬂux scale in mJy/beam is
shown to the right of the map, and the FWHM of the CLEAN beam is shown in the lower left corner. Upper right: A 5′ × 5′ BW R[4.5]
pseudo-color image of IDCS J1426.5+3508, with the SZ contours overplotted. The optical and infrared data are from the NOAO Deep,
Wide–Field Survey (NDWFS; Jannuzi and Dey 1999) and SDWFS, respectively. Lower panel: HST F814W+F160W pseudo-color zoom–in
of IDCS J1426.5+3508. The SZ contours (green) from the present work and X-ray contours (yellow) from S12, corresponding to 0.131,
0.077 and 0.044 counts per 2′′ × 2′′ pixel in 8.3 ks in the 0.5-2 KeV band, are overplotted. Two sources are indicated by hash marks. The
northern source is an X-ray luminous QSO that is also a cluster member; the southern source is a radio-bright AGN, the emission from
which was removed from the SZA map. Both are discussed further in the text. In all panels North is up, East is left, and a scale bar is
given.

4 Brodwin et al.

profile P (r) is determined by fitting a model profile to and the total cluster mass. By convention, both the total
the SZA data. We adopt the generalized Navarro et al. cluster mass and the integrated y-parameter are reported
(1997) model proposed by Nagai et al. (2007): within a radius corresponding to a fixed overdensity
∆ = 500 relative to the critical density of the Universe
P0 at the redshift of the cluster. The integrated quantities
P (r) = , (2)
xγ (1 + xα )(β−γ)/α are determined by finding the overdensity radius r500
3
(equivalent to the mass, since M∆ = (4πr∆ /3)∆ρc (z))
where x = r/rs . We fix the power law exponents (α, β, γ) and integrated y-parameter Y500 that are mutually con-
to the “universal” values reported by Arnaud et al.
sistent with the Andersson et al. (2011) M500 –Y500 scal-
(2010), and leave P0 and rs free to vary14 . ing relation. We choose this relation since the clus-
One important source of systematic errors in SZ cluster ters with which it is measured have the highest mean
measurements is radio point sources. These synchrotron
redshift. The self-consistent values of r500 and Y500
sources are often variable, are known to be correlated are determined iteratively at each step in the MCMC,
with clusters, and can fill in the SZ decrement at low fre- and the scaling relation parameters are sampled at each
quencies (see, e.g., Carlstrom et al. 2002). Wide angu-
step using their reported means and variances. Using
lar scale surveys such as the SPT survey generally mask this method, we find that M500,c = (2.6 ± 0.7) × 1014
regions where bright point sources are detected, which M⊙ , where the error is statistical in nature. Assuming
changes the survey selection function but can be ac-
the Duffy et al. (2008) concentration relation, this cor-
counted for using simulations (Vanderlinde et al. 2010).
However, for targeted SZ follow-up of sources selected in responds to M200,c = (4.1 ± 1.1) × 1014 M⊙ . This mass
other bands, point source fluxes must be measured and agrees well with the X-ray mass of M200,c ≃ 5.6 × 1014
removed in order to accurately estimate the cluster mass. M⊙ , for which the error is at least 40%.
Due to the temporal variability of these sources, the point A significant uncertainty, inherent to both SZ and X-
source fluxes ideally should be determined concurrently ray measurements of the masses of high redshift clus-
with the SZ measurement. The SZA uses its longer ters, is due to the fact that no published mass-observable
outrigger baselines to perform such simultaneous point scaling relation has been estimated using a sample con-
source measurements. For spatially compact sources, the taining clusters at redshifts significantly greater than
emission measured on the outrigger baselines (smaller unity. While Andersson et al. (2011) found no com-
angular scales) can be assumed to be representative of pelling evidence for redshift evolution in the M500 –Y500
the emission measured on the short SZ-sensitive base- scaling relation out to z ∼ 1, their sample consisted
lines (larger angular scales), and can thereby be removed of lower-redshift and higher-mass objects whose scaling
from the SZ signal (see, e.g., Muchovej et al. 2007). properties may be systematically different from those of
Two such point sources were discussed in S12 and are IDCS J1426.5+3508.
indicated by hash marks in Figure 1. The northern To enable the SZ mass of IDCS J1426.5+3508 to be
source is an X-ray bright QSO that is also a spectro- easily calculated with future scaling relations, we also re-
scopically confirmed member of IDCS J1426.5+3508. As port the spherically-averaged dimensionless Comptoniza-
described in S12, the contribution to the cluster’s X-ray tion, Ysph,500 = (7.9 ± 3.2) × 10−12 . This corresponds to
flux from this QSO had to be removed to obtain a reliable a gas mass at r500 of Mgas = (2.9 ± 1.2) × 1013 M⊙ for a
cluster mass. This QSO is not detected in the radio, how- constant temperature of 5 keV. This temperature is con-
ever, with a formal 31 GHz flux density of −0.04 ± 0.31 sistent with the X-ray measurement (S12); for a cooler 4
mJy, and therefore it has no impact on the measured keV cluster the inferred gas mass would increase by 25%.
cluster mass. A second point source, bright enough in the To assess the SZ detection significance, we fit a clus-
radio to fill in the SZ decrement of IDCS J1426.5+3508, ter model to the visibility data and compare to a null
was identified near the cluster center at 14h 26m 32.1, s model. Our model consists of an elliptical Gaussian SZ
35◦ 08′ 15. 2, indicated by the southern hash mark in Fig-
′′ decrement located at the X-ray centroid along with the
ure 1. This source corresponds to a ∼ 95 mJy source nearby radio point source, and our null model consists
identified at 1.4 GHz in the NVSS (Condon et al. 1998) only of the point source. We find a ∆χ2 of 37.65 between
and FIRST (Becker et al. 1995) catalogs. The lower– the model and null model fits. Since the (fixed-position)
frequency radio survey data indicate that this source is elliptical Gaussian adds four degrees of freedom, this ∆χ2
unresolved at the angular scales to which the SZA is corresponds to a detection significance of 5.3 σ, or a false
sensitive, allowing it to be modeled as a point source detection probability of 1.32 × 10−7. Because the SZ sig-
and constrained using the long baseline data. The flux nal was sought only at the position of the X-ray emis-
density of this source is left as a free parameter in the sion, this detection is very secure. In contrast, a false
cluster model fit, and is found to be 5.3 ± 0.3 mJy. The 5.3 sigma detection is far more plausible in a wide–field
resulting spectral index of −0.93 ± 0.02 is consistent with survey like SPT because there are of order 106 indepen-
synchrotron radiation. dent pixels in which such a deviation can arise by chance.
A Markov Chain Monte Carlo (MCMC) fit is used to We therefore consider the pointed, follow-up detection of
jointly constrain P0 , rs , and the flux densities of the two IDCS J1426.5+3508 to be robust.
point sources in our fiducial cosmology. The results of If the cluster position is not fixed to the X-ray cen-
this fit are used to compute the integrated y-parameter troid, the best–fit centroid of the SZ signal is 14h 26m34.0,s

35◦ 08′ 02. 6, which is slightly offset (≈ 32′′ and 25′′ ) from
′′
14 Arnaud et al. (2010) suggest other power law exponents ap-
the X-ray and BCG positions, respectively. As the uncer-
propriate for clusters with a known morphology, but Marrone et al. tainty in the SZ centroid is ≈ 35′′ , this offset is not sta-
(2011) find that these alternative choices make a 5% difference
in the estimate of Y500 . tistically significant. Absent the X-ray positional prior,


the significance of the SZ detection is almost identical
(5.2 σ). 1016

4. PROBABILITY OF EXISTENCE IN A ΛCDM UNIVERSE Fu
ll S
ky
SP
While not as massive as the z ∼ 1 SPT clusters, TP
rev
IDCS J1426.5+3508, by virtue of its exceptional redshift SP
T(
iew
(25
17 00
and substantial mass, is arguably one of the more ex- 8d
eg 2
de
g2
)

M200,m [MO]
treme systems discovered to date. It is therefore inter- )

•
esting to calculate the probability of discovering it in the
1015
IDCS in a standard ΛCDM Universe. IDC
We use the exclusion curve formalism presented in S
Mortonson et al. (2011, hereafter M11), which tests
whether a single cluster is rare enough to falsify ΛCDM J1426.5

and quintessence at a given significance level, account-
ing for both sample and parameter variance. The M11
formalism uses cluster masses measured within a sphere
defined by an overdensity 200 times the mean, rather 1014
than critical, density of the Universe. In this convention,
0.0 0.5 1.0 1.5 2.0
adopting the Duffy et al. (2008) mass–concentration re- Redshift
lation, IDCS J1426.5+3508 has a mass of M200,m =
(4.3 ± 1.1) × 1014 M⊙ . Figure 2. M11-style plot showing the mass and redshift of
The M11 formalism requires that observed cluster IDCS J1426.5+3508 (large red circle), along with other z > 1
masses be corrected for Eddington bias caused by the ISCS clusters (Brodwin et al. 2011; Jee et al. 2011, small red cir-
steep slope of the cluster mass function at this mass cles) and SPT clusters (Williamson et al. 2011, small blue circles;
Brodwin et al. 2010, small cyan circle). The solid red curve is
scale and redshift. We follow the prescription given in the 95% exclusion curve for the IDCS area. The cyan, blue and
Appendix A of M11: black curves are the exclusion curves for the currently published
full depth SPT survey (178 deg2 , Vanderlinde et al. 2010), the
1 2 2500 deg2 SPT preview survey of Williamson et al. (2011) and
ln M = ln Mobs + γσln M , (3) the full sky, respectively. The square symbol represents clus-
2 ter SPT-CL J0102-4915, first reported as ACT-CL J0102-4915
where M is the expected value for the true mass given the (Menanteau et al. 2010). We plot this cluster at the spectroscopic
redshift of z = 0.870 reported in Menanteau et al. (2012). All
observed mass, γ is the local slope of the mass function, clusters are color-coded to the appropriate exclusion curve.
and σ is the uncertainty associated with the observations.
Here we use the fitting function for γ provided in M11, of the SPT survey does not contain a single cluster
which yields γ = −5.8. We thus derive that the expected comparable to IDCS J1426.5+3508 in mass and redshift
value for this bias-corrected mass is M200,m = (3.6 ± (Vanderlinde et al. 2010), which suggests that the true
0.9) × 1014 M⊙ . abundance of such clusters is far lower than our dis-
In Figure 2 we plot the appropriately bias-corrected covery would indicate. Neither the Atacama Cosmol-
mass for IDCS J1426.5+3508 along with 95% con- ogy Telescope (ACT) project nor the Planck project sur-
fidence exclusion curves using the M11 prescription. veys are sensitive enough to detect IDCS J1426.5+3508
For comparison we include rare, massive clusters from (Marriage et al. 2011; Planck Collaboration et al. 2011).
the SPT survey (Williamson et al. 2011; Brodwin et al. If we consider IDCS J1426.5+3508 to have been drawn
2010) and other massive z > 1 clusters from the ISCS from an area of at least ∼ 178 deg2 (shown as the cyan
for which weak lensing or X-ray masses are available curve in Fig. 2) rather than the IDCS volume, then the
(Brodwin et al. 2011; Jee et al. 2011). All masses are cluster is no longer in contradiction with ΛCDM; we were
deboosted in accordance with the M11 prescription. simply somewhat lucky to detect it in the comparatively
The lowest curve in the figure corresponds to the 95% tiny (8.82 deg2 ) IDCS area.
exclusion curve for clusters within the 8.82 deg2 IDCS
area. The central value of the deboosted SZ mass is 5. EVOLUTION TO THE PRESENT DAY
above the curve, formally falsifying ΛCDM at the 95% Although the mere existence of IDCS J1426.5+3508
level. We note, however, that the statistical error bar may not have significant cosmological implications, it
overlaps the exclusion curve. Furthermore, uncertainties is nevertheless among the rarest, most extreme clusters
in the scaling relation could easily lower the mass below ever discovered. As such, it is interesting to understand
the exclusion curve. its nature in the context of the growth of the largest
Rather than asking whether such a cluster should have structures.
been detected in the IDCS, perhaps a fairer question Using the Tinker et al. (2008) mass function, we
is to ask whether such a cluster should have been de- first identify the space density of clusters at z =
tected in the full area of sky covered by all cluster 1.75 that have masses greater to or equal to that of
surveys capable of detecting such a cluster. Numer- IDCS J1426.5+3508. At each redshift we then asso-
ous X-ray surveys of varying areas and depths could ciate the clusters with that space density with its de-
have detected IDCS J1426.5+3508, but for simplicity scendents. In Figure 3 we plot the mass evolution of
we consider the SPT survey, which has the sensitiv- IDCS J1426.5+3508 (large red circle) from z = 1.75 to
ity to detect clusters like IDCS J1426.5+3508, even at the present day (thick black line). For comparison we
z = 1.75 (Vanderlinde et al. 2010). The first 178 deg2 also plot a representative selection of the most extreme

6 Brodwin et al.

clusters found to date at all redshifts. This abundance– massive collapsed structures evolving in the Universe.
matching calculation is designed to predict the mean ex- Although it was discovered in a small (≈ 9 deg2 )
pected growth of IDCS J1426.5+3508. We note that infrared-selected cluster survey, the odds of finding such
there are intrinsic variations in accretion histories due a rare, massive cluster were quite low, 0.8% (0.2%) us-
to the stochastic nature of mergers (e.g., Wechsler et al. ing the Holz and Perlmutter (2010) formalism with the
2002), but consider a full treatment beyond the scope of deboosted (measured) mass. Finding the precursors of
the present work. IDCS J1426.5+3508 at even earlier times will require
As demonstrated in this figure, IDCS J1426.5+3508 is much larger surveys.
much more than just another massive, very high redshift In this section we examine why the current genera-
cluster. It is an evolutionary precursor to the most mas- tion of SZ surveys, despite covering areas ranging from
sive known clusters at all redshifts. Indeed, its descen- 102 –104 deg2 , have not yet seen a cluster as extreme as
dent population consists of the most massive virialized IDCS J1426.5+3508 at z > 1.5. We also discuss the po-
objects ever discovered. tential of the next generation of SZ surveys to discover
There are no other confirmed clusters at z > 1.75 its evolutionary precursors.
with masses greater than 1014 M⊙ . Between 1.5 < z <
1.75 only one cluster, XDCP J0044.0-2033 at z = 1.58 6.1. Current SZ surveys
(Santos et al. 2011; Fassbender et al. 2011), is compara-
ble to IDCS J1426.5+3508, though it is somewhat less The detectability of a cluster like IDCS J1426.5+3508
massive and below the growth trend. in an SZ cluster survey depends on three factors. The
The most massive z > 1 X-ray selected cluster, survey must have sufficient mass sensitivity at high red-
XMMU J2235.3-2557 at z = 1.39 (Mullis et al. 2005; shift, it must cover sufficient area so that the expectation
Rosati et al. 2009), falls along the evolutionary path of value of the number of clusters is greater than unity, and
IDCS J1426.5+3508, as does the most massive z > 1 the clusters in question must not contain bright radio
cluster in the SPT survey, SPT-CL 2106-5844 at z = 1.13 point sources nor have any along the line of sight.
(Foley et al. 2011; Williamson et al. 2011). Similarly, In terms of sensitivity, only the SPT survey is cur-
the only z ∼ 1 Planck cluster reported to date, PLCK rently capable of detecting IDCS J1426.5+3508 at this
G266.6-27.3 at z = 0.94 is also consistent. high redshift. Vanderlinde et al. (2010) report a for-
SPT-CL J0102-4915, first reported as ACT-CL J0102- mal 50% completeness of M200,m ≈ 3.5 × 1014 M⊙ at
4915 from the ACT survey (Menanteau et al. 2010; z = 1.5, and this improves to 90% at z = 1.75. That
Marriage et al. 2011), is among the most significant de- mass limit is below both the measured and deboosted
tections overall in both of these SZ surveys. Masses mass of IDCS J1426.5+3508. So the SPT should nom-
of this cluster from the two SZ surveys are plotted inally have detected such a cluster were it present at
as the square symbols at the spectroscopic redshift of z = 1.75 in the single-band analysis of their first 178 deg2
z = 0.870 (Menanteau et al. 2012). The lower SPT (Vanderlinde et al. 2010). However, since the SPT com-
mass is measured from the SZ decrement, whereas the pleteness limit at high redshift is uncertain at the 30%
higher ACT mass (slightly offset in redshift for clar- level in mass (Vanderlinde et al. 2010), the non-detection
ity) is combined from several methods. The masses of a single high–redshift cluster like IDCS J1426.5+3508
are consistent within the errors. IDCS J1426.5+3508 is unremarkable.
is consistent with growing into a cluster like this as A more interesting question, related to the second cri-
well. RX J1347-1145 (Allen et al. 2002; Lu et al. 2010) terion and to the rarity analysis of §4, is that of how
is the most luminous X-ray cluster in the sky, though many such clusters are actually expected in the cur-
it is not quite massive enough to be considered a de- rent SPT area? According to the Holz and Perlmutter
scendent of IDCS J1426.5+3508. In the local Universe (2010) formulation, we expect only 0.2 such clusters
the Coma cluster represents the kind of extreme cluster in 178 deg2 , and therefore none should have been
IDCS J1426.5+3508 will evolve into. found. In the complete, full-depth 2500 deg2 SPT sur-
Holz and Perlmutter (2010) recently predicted the vey, Holz and Perlmutter (2010) predict the presence of
properties of the single most massive cluster that can 2.4 such clusters, though shot noise will play a role in
form in a ΛCDM Universe with Gaussian initial con- whether any are found.
ditions and where the growth of structure follows the Finally, as discussed in §3.2, IDCS J1426.5+3508 has
predictions of general relativity. They predict this clus- a radio–bright AGN at z = 1.535 along the line-of-sight,
ter should be found at z = 0.22 and have a mass of indicated by the southern hash marks in Figure 1. At
M200,m = 3.8 × 1015 M⊙ . Their 68% contour in pre- a redshift separation of ∆z > 0.2, it is not physically
dicted mass and redshift is shown as a light blue contour associated with the cluster. Its radio emission was con-
and labeled HP10 in the figure. IDCS J1426.5+3508 is strained using the long SZA baselines, which are insen-
completely consistent with being an evolutionary precur- sitive to the arcminute angular scale SZ decrement. If
sor to the most massive halo predicted to exist in the the measured 1.4 to 31 GHz spectral index of this source
observable Universe. (−0.93) is valid to 150 GHz, the AGN flux would be 1.2
mJy at this frequency, well below the 5 mJy threshold
6. DISCUSSION for masking individual point sources in the SPT survey.
Given its extreme mass it is not surprising that However, it would partially fill in the ∼ 14 mJy SZ decre-
IDCS J1426.5+3508 was detected in shallow (5 ks) Chan- ment at 150 GHz, reducing the SPT detection signifi-
dra images (S12), despite its high redshift. The ro- cance and biasing the mass estimate low by ∼ 10 − 15%.
bust, high-significance SZA detection reported herein It is possible that the AGN content of clusters at the
confirms that IDCS J1426.5+3508 is one of the most highest redshifts may be substantially larger than at in-


1016
Coma

HP10
SPT-CL/ACT-CL
J0102-4915
RX J1347-1145

PLCK G266.6-27.3
M200,m [MO]
•

XMMU
15 J2235.3-2557
10
SPT SPT-CL IDCS
(Willamson et al. 2011) J2106-5844 J1426.5+3508

XDCP
J0044.0-2033

1014
0.0 0.5 1.0 1.5 2.0
Redshift
Figure 3. Predicted mass growth of IDCS J1426.5+3508 vs. redshift based on abundance matching. IDCS J1426.5+3508 is the large
red circle and the predicted growth in its mass is shown as a thick black line. The 1 σ errors, stemming from the mass measurement
errors, are shown as the shaded region. An assortment of the rarest, most massive clusters found at all redshift is shown for comparison
and discussed in the text. SPT-CL J0102-4915 was first reported as ACT-CL J0102-4915 by Menanteau et al. (2010) and Marriage et al.
(2011). The independent mass measurements from both surveys are plotted as the square symbols for this cluster at the spectroscopic
redshift (z = 0.8701) reported in Menanteau et al. (2012). IDCS J1426.5+3508 is consistent with being a member of the most extreme
population of virialized structures in the Universe.
termediate (z < 1) redshifts. Indeed, Galametz et al. simultaneously identify and remove contamination from
(2009) find evidence for a strong increase in the inci- such (generally variable) sources, as in this work, and
dence of AGN in clusters with redshift from 0.2 < z < thus recover accurate cluster masses.
1.4, particularly for X-ray and IR-selected AGN. The
evidence for rapid number density evolution in radio- 6.2. Next–Generation SZ surveys
selected AGN is not as conclusive. In some surveys (e.g., Several next–generation SZ experiments designed
Galametz et al. 2009) it increases as quickly in clusters as for CMB polarization measurements, such as SPTpol
in the field, whereas others (e.g., Gralla et al. 2011) find (Bleem et al. 2012) and ACTpol (Niemack et al. 2010),
it is roughly constant. One major advantage of CARMA are currently being deployed. These experiments will
for targeted SZ observations of clusters is the ability to possess typical SZ mass sensitivities a factor of sev-

8 Brodwin et al.

eral lower than present surveys. Here we calculate by far the most distant cluster with an SZ detection to
the expected yield of high–redshift clusters similar to date. Using the Andersson et al. (2011) M500 –Y500 scal-
IDCS J1426.5+3508 for a canonical 500 deg2 next– ing relation, we find M200,m = (4.3 ± 1.1) × 1014 M⊙ , in
generation survey with constant M200,m completeness agreement with the X-ray mass reported in S12.
limits of 1.2 × 1014 M⊙ and 1.5 × 1014 M⊙ at the 50% The chance of finding IDCS J1426.5+3508 in the ∼
and 90% levels, respectively. 9 deg2 IDCS was 1%, which, taken at face value,
The same methodology that explains the lack of a marginally rules out ΛCDM. Within the much larger to-
cluster like IDCS J1426.5+3508 in the current SPT tal area probed by other SZ and X-ray surveys, its exis-
survey area predicts that a significant number of very tence is not unexpected.
high redshift clusters, somewhat less massive than In a ΛCDM cosmology, with the growth of structure
IDCS J1426.5+3508, will be found with the polarization as predicted by general relativity, IDCS J1426.5+3508
experiments. Figure 4 shows the expected cumulative will grow to a mass of M200,m ≈ 4.7 × 1015 M⊙ by the
yield, based on the Holz and Perlmutter (2010) formula- present day. It therefore shares an evolutionary path
tion, at the two mass limits given above, accounting for with the most massive known clusters at every redshift.
the expected completenesses. Along with several of the rarest, high-redshift clusters
The combination of area and depth of the next genera- currently known, IDCS J1426.5+3508 is consistent with
tion SZ surveys will allow them to identify a large statis- evolving into the single most massive cluster expected to
tical sample of massive, hot clusters at 1.5 < z < 2.0. exist in the observable Universe.
As such they provide a natural extension of the in-
frared surveys, which have characterized galaxy clusters
to z = 1.5. A 100 deg2 region that will be covered by Support for CARMA construction was derived from
both SPTpol and ACTpol will also contain Spitzer in- the states of California, Illinois, and Maryland, the
frared data, from an ongoing Spitzer Exploration Class James S. McDonnell Foundation, the Gordon and Betty
program (PID 80096, PI Stanford). This will allow opti- Moore Foundation, the Kenneth T. and Eileen L. Norris
mal high-redshift cluster detection and study at both IR Foundation, the University of Chicago, the Associates
and mm wavelengths, the only two cluster probes that of the California Institute of Technology, and the Na-
are nearly redshift-independent at 1 < z < 2. tional Science Foundation. Ongoing CARMA develop-
Despite the wealth of new, high redshift clusters to ment and operations are supported by the National Sci-
be found by the next-generation SZ surveys, there are ence Foundation under a cooperative agreement, and by
only even odds (p ∼ 0.5) of finding a z = 2 precursor to the CARMA partner universities. The work at Chicago
IDCS J1426.5+3508. Unless, of course, we are fortunate is supported by NSF grant AST-0838187 and PHY-
once again. 0114422.
This work is based in part on observations made with
the Spitzer Space Telescope, which is operated by the
150 Jet Propulsion Laboratory, California Institute of Tech-
M200,m > 1.2 x 1014 MO
•
nology under a contract with NASA. Support for this
M200,m > 1.5 x 1014 MO
work was provided by NASA through an award issued by
•
JPL/Caltech. This work is based in part on observations
obtained with the Chandra X-ray Observatory (CXO),
100
under contract SV4-74018, A31 with the Smithsonian
Astrophysical Observatory which operates the CXO for
Number (>z)

NASA. Support for this research was provided by NASA
grant G09-0150A. Support for HST programs 11663 and
12203 were provided by NASA through a grant from the
Space Telescope Science Institute, which is operated by
50 the Association of Universities for Research in Astron-
omy, Inc., under NASA contract NAS 5-26555. This
work is based in part on data obtained at the W. M. Keck
Observatory, which is operated as a scientific partnership
among the California Institute of Technology, the Uni-
0 versity of California and the National Aeronautics and
1.5 1.6 1.7 1.8 1.9 2.0 Space Administration. The Observatory was made pos-
Redshift sible by the generous financial support of the W. M. Keck
Foundation. This work makes use of image data from
Figure 4. Predicted cumulative counts of massive 1.5 < z < the NOAO Deep Wide–Field Survey (NDWFS) as dis-
2 clusters at the 50% (M200,m = 1.20 × 1014 M⊙ ) and 90% tributed by the NOAO Science Archive. NOAO is op-
(M200,m = 1.5 × 1014 M⊙ ) completeness limits of a representa- erated by the Association of Universities for Research
tive 500 deg2 next–generation SZ survey. The curves account for in Astronomy (AURA), Inc., under a cooperative agree-
losses due to incompleteness. The arrow indicates the redshift of ment with the National Science Foundation.
IDCS J1426.5+3508.
We thank the anonymous referee for suggestions which
improved the manuscript, Bradford Benson for help-
7. CONCLUSIONS ful discussions, F. Will High and Keith Vanderlinde
We have presented a robust 5.3 σ SZ detection of the for providing published SPT masses in a digital form,
infrared-selected cluster IDCS J1426.5+3508 at z = 1.75, Matt Ashby for creating the IRAC catalogs for SDWFS,


Michael Brown for combining the NDWFS with SDWFS Gonzalez, A. H., et al. 2012, ApJ, submitted
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