1. Baryons at the Edge of the X-ray−Brightest Galaxy Cluster
Aurora Simionescu, et al.
Science 331, 1576 (2011);
DOI: 10.1126/science.1200331
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REPORTS
measure the characteristics of the faint emission
Baryons at the Edge of the from cluster outskirts more reliably. Even so,
few such observations have been published, and
X-ray–Brightest Galaxy Cluster the thermodynamic profiles at large radii are
not well resolved (6–11). The Perseus Cluster of
galaxies is the brightest extragalactic extended
Aurora Simionescu,1* Steven W. Allen,1 Adam Mantz,2 Norbert Werner,1 Yoh Takei,3 x-ray source. It is a relaxed system, both closer
R. Glenn Morris,1 Andrew C. Fabian,4 Jeremy S. Sanders,4 Paul E. J. Nulsen,5 (at a redshift of 0.0183) and substantially brighter
Matthew R. George,6 Gregory B. Taylor7,8 than any of the other clusters for which Suzaku
has previously been used to study the ICM prop-
Studies of the diffuse x-ray–emitting gas in galaxy clusters have provided powerful constraints erties at large radii. Its large angular size mitigates
on cosmological parameters and insights into plasma astrophysics. However, measurements the impact of residual systematic uncertainties
of the faint cluster outskirts have become possible only recently. Using data from the Suzaku x-ray in modeling the effects of Suzaku's complex point-
telescope, we determined an accurate, spatially resolved census of the gas, metals, and dark matter out spread function (PSF), making the Perseus Cluster
to the edge of the Perseus Cluster. Contrary to previous results, our measurements of the an ideal target in which to study cluster outskirts.
cluster baryon fraction are consistent with the expected universal value at half of the virial radius. A large mosaic of Suzaku observations of
The apparent baryon fraction exceeds the cosmic mean at larger radii, suggesting a clumpy the Perseus Cluster, with a total exposure time
distribution of the gas, which is important for understanding the ongoing growth of clusters
from the surrounding cosmic web. 1
Kavli Institute for Particle Astrophysics and Cosmology, Stan-
ford University, 452 Lomita Mall, Stanford, CA 94305, USA.
2
alaxy clusters provide critical constraints only the inner parts of clusters, where the emis- NASA Goddard Space Flight Center, Greenbelt, MD 20771,
G on cosmological parameters that are in-
dependent from those determined using
type Ia supernovae, galaxy surveys, and the pri-
sion is brightest, leaving a large fraction of their
volumes practically unexplored. Estimates of the
gas mass and total mass at large radii have relied
USA. 3Institute of Space and Astronautical Science, Japan Aero-
space Exploration Agency (JAXA), 3-1-1 Yoshinodai, Sagamihara,
Kanagawa 229-8510, Japan. 4Institute of Astronomy, Madingley
Road, Cambridge CB3 0HA, UK. 5Harvard-Smithsonian Center
mordial cosmic microwave background radia- on simple model extrapolations of the thermo- for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
6
tion (CMB) (1–3). In particular, knowledge of dynamic properties measured at smaller radii. Department of Astronomy, University of California, Berkeley, CA
their baryon content is a key ingredient in the X-ray spectroscopy of the outer regions of 94720, USA. 7Department of Physics and Astronomy, Univer-
sity of New Mexico, Alberquerque, NM 87131, USA. 8National
use of clusters as cosmological probes (4, 5). galaxy clusters was made possible only recently, Radio Astronomy Observatory, 1003 Lopezville Rd., Socorro,
Most baryons reside in the hot, diffuse, x-ray– with the use of the Suzaku satellite. Because NM 87801, USA.
emitting intracluster medium (ICM). Until re- of its much lower instrumental background than *To whom correspondence should be addressed. E-mail:
cently, x-ray observations have generally targeted that of other x-ray observatories, Suzaku can asimi@stanford.edu
1576 25 MARCH 2011 VOL 331 SCIENCE www.sciencemag.org

3. REPORTS
Fig. 1. X-ray surface
brightness image of the
NW (top) and E (bottom)
arm mosaics observed
with Suzaku, corrected
for vignetting and instru-
mental background. The
dashed white line marks
the virial radius; the red
circles mark excluded
point sources and instru-
mental artefacts. The im-
ages have been rotated
so that the cluster cen-
ter is toward the left.
Downloaded from www.sciencemag.org on April 5, 2011
of 260 ks, was obtained in August/September Fig. 2. Projected tempera-
2009. The pointings extend along two arms from ture (kT) and metallicity (Z)
near the cluster center toward the east (E) and profiles of the Perseus Clus-
northwest (NW), out to a radius of 2°, which cor- ter. Results from Suzaku ob-
responds to 2.8 Mpc for a Hubble constant H0 = servations of the NW arm are
70 km/s/Mpc. Here we focus on the data obtained shown in red and of the E
with the three available x-ray imaging spectrom- arm in blue. Earlier Chandra
eter (XIS) cameras [see supporting online material measurements of the clus-
(SOM) text and Fig. 1]. We extracted spectra from ter center (13) are shown in
annuli centered on the cluster center. After ac- black.
counting for background emission, we modeled
each spectral region as a single-temperature ther-
mal plasma in collisional ionization equilibrium,
with the temperature, metallicity, and spectrum
normalization as free parameters.
The best-fit radial profiles of temperature and
metallicity are presented in Fig. 2. Individual ele-
ments are assumed to be present with solar rela-
tive abundances (12). For comparison, we also
show results previously obtained from an ultra-
deep Chandra observation of the cluster center
(13). The Suzaku and Chandra data sets show
excellent agreement where they intersect, and
together they measure the temperature and met-
allicity structure of the intracluster gas with high near the compressed outskirts of two interacting with radius K º rb, with b ~ 1.1 to 1.2 (17, 18).
precision and spatial resolution out to the virial clusters (15). This previous result is in agree- Except for the E cold front region, the entropy
radius (defined here as r200, the radius within ment with our measurements when converted to profile in Perseus roughly follows this expected
which the mean enclosed mass density of the the solar abundance units (12) adopted here. trend until 2/3 r200. Beyond this radius and until
cluster is 200 times the critical density of the uni- From the Suzaku data, we have also deter- 0.95 r200, both the E and NW arms show a flat-
verse at the cluster redshift). In the narrow interval mined the electron density, entropy, and pres- tening from the power law shape, confirming
spanning 0.95 to 1.05 r200, the temperature is sure profiles, corrected for projection effects under hints from previous Suzaku results (7).
approximately a third of the peak temperature. the assumption of spherical symmetry (Fig. 3). The pressure profile is the most regular of the
Along the E arm, between 0.1 and 0.7 Mpc, the Outside the cold front at 0.7 Mpc, there is a good thermodynamic quantities plotted, and at most
temperature is systematically lower than toward match between the E and NW profiles, with the radii shows good agreement between the E and
the NW, and the x-ray emission is brighter. This electron density decreasing steadily with radius, NW. At large radii, the pressure profile appears
thermodynamic feature is known as a “cold front” approximately following a power law model ne º significantly shallower than would be expected
and typically arises after a merger between the r−a with slope a = 1.68 T 0.04. This is consistent by extrapolating the average profile of a sample
main cluster and a smaller subcluster (14). with previous results from ROSAT (Roentgen sat- of clusters studied previously with the XMM-
Our results show that the cluster outskirts are ellite) data extending out to ~1.4 Mpc (16). Newton satellite within ~ 0.5 r200 (19).
substantially metal-enriched, to a level amount- Standard large-scale structure formation mod- Invoking hydrostatic equilibrium of the ICM,
ing to approximately one-third of the solar met- els show that matter is shock-heated as it falls the gas pressure can be used to estimate the
allicity. Previously, the only measurement of into clusters under the pull of gravity. Simple underlying gravitational potential and total (dark
the metallicity close to the virial radius was ob- theoretical models of this process predict that matter plus luminous matter) mass profile of the
tained from a large region spanning 0.5 to 1 r200 the entropy K should behave as a power law cluster. Numerical simulations show that the lat-
www.sciencemag.org SCIENCE VOL 331 25 MARCH 2011 1577

4. REPORTS
ter typically follows a functional form described The best-fit mass model parameters are typical 1.79 T 0.04 Mpc, the corresponding enclosed
by Navarro, Frenk, White (20), also known as of those predicted from numerical simulations; total mass M200 = 6.65+0.43−0.46 × 1014 solar
the NFW profile. We used the data from the NW the NFW model provides a good description of masses, and the cumulative gas mass and gas
arm of the Perseus Cluster, which appears dy- the Suzaku data. mass-to-total mass fraction, fgas, as a function
namically relaxed, to determine the best-fit total Measuring the total mass profile allowed us of radius (Fig. 4). At relatively small radii of 0.2
mass profile, assuming an NFW form (SOM text). to calculate the virial radius of the cluster, r200 = to 0.3 r200, the measured fgas value is in good
agreement with direct measurements from the
Fig. 3. Deprojected electron den- Chandra X-ray Observatory (5) and measure-
sity (ne), entropy (K), and pressure ments of the Sunyaev-Zel'dovich (SZ) effect (21)
(P) profiles toward the NW (red data for two large samples of galaxy clusters. At about
points) and E (blue data points). half of r200, the integrated gas mass fraction
The red line shows the NW profiles reaches the cosmic mean value computed from
corrected for clumping. The expected the CMB (22), considering that on average 12%
entropy profile from simulations of of the baryons are in stars (23, 24) and the rest
gravitational collapse (17, 18) is a are in the hot x-ray–emitting gas phase. Outside
power law with index b ~ 1.1, over- 2/3 of the virial radius, where the entropy also
plotted as a black dotted line in the deviates from the expected power law behav-
entropy panel. The average profile ior, we find that the apparent fgas exceeds the
of a sample of clusters previously cosmic mean baryon fraction measured from
studied with the XMM-Newton sat-
Downloaded from www.sciencemag.org on April 5, 2011
the CMB (22).
ellite within ~0.5 r200 (19) is shown The most plausible explanation for this ap-
with a solid black curve in the pres-
parent excess of baryons at large radius is gas
sure panel; its extrapolation to r200
clumping. In the x-rays, the directly measurable
is shown with a dotted black line.
quantity from the intensity of the bremsstrah-
lung emission is the average of the square of
the electron density, < ne2 >, rather than < ne > .
If the density is not uniform (that is, the gas is
clumpy), which is expected to occur as matter
falls into the cluster, the average electron density
estimated from the bremsstrahlung intensity will
overestimate the true average, affecting the gas
density, gas mass fraction, entropy, and pressure
profiles.
Outside the central region, and inside the ra-
dius where clumping becomes important, the
measured fgas profile shows good agreement
with recent numerical simulations (25), where
a semianalytic model was used to calculate the
Fig. 4. The integrated, en- energy transferred to the intracluster gas by su-
closed gas mass fraction pernovae and active galactic nuclei during the
profile for the NW arm. galaxy formation process. This model did not
The cosmic baryon frac- include a realistic implementation of gas cool-
tion from WMAP7 (22) is ing and does not capture the complex processes
indicated by the horizon- in the central cool core of the cluster; the model
tal dashed black line; ac- is therefore not plotted in this region. Extrapolat-
counting for 12% of the ing this model into the outskirts where clump-
baryons being in stars ing is important, we used its predictions together
(23, 24) gives the expected with the measured fgas to determine by how
fraction of baryons in the much the electron density must be overestimated
hot gas phase, shown as a to produce the difference between the data and
solid black line. The val- the model. This factor (plotted in green in the
ues previously measured bottom panel of Fig. 4) reaches a value of up to
for a sample of relaxed clus- 4 in the last annulus centered around the virial
ters at smaller radii with radius. The dense clumps are likely to be infall-
Chandra (5) are shown
ing and may be confined by ram pressure.
with blue dots. Predictions
from numerical simulations Correcting the electron density using this fac-
(25) are shown in green. tor, and accordingly the entropy and pressure
The bottom panel shows profiles, we obtained the red lines shown in Fig.
by how much the electron 3. The clumping-corrected entropy profile along
density should be over- the NW arm is consistent with the expected power-
estimated in each annu- law profile. Moreover, the clumping-corrected
lus because of clumping, pressure is also consistent with that expected by
in order for the cumulative extrapolating the average profile of a sample of
fgas not to exceed the cor- clusters previously studied with XMM-Newton
respondingly colored curves in the plot above. (19). The corrected electron density decreases
1578 25 MARCH 2011 VOL 331 SCIENCE www.sciencemag.org

5. REPORTS
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An independent measurement of gas clumping References
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should exhibit a rough particle-hole symmetry,
From a Single-Band Metal to a or another form of (incipient) order (6–12), which
typically should induce particle-hole asymmetric
High-Temperature Superconductor spectral changes. Candidate orders include vari-
ous forms of density wave, nematic, or uncon-
via Two Thermal Phase Transitions ventional magnetic orders that break different
combinations of lattice translational (6–8, 13–19),
rotational (6, 9, 15, 17, 20–22), and time-reversal
Rui-Hua He,1,2,3* M. Hashimoto,1,2,3* H. Karapetyan,1,2 J. D. Koralek,3,4 J. P. Hinton,3,4
(7, 9, 23–26) symmetries.
J. P. Testaud,1,2,3 V. Nathan,1,2 Y. Yoshida,5 Hong Yao,1,3,4 K. Tanaka,1,2,3,6 W. Meevasana,1,2,7
We have focused on crystals of nearly op-
R. G. Moore,1,2 D. H. Lu,1,2 S.-K. Mo,3 M. Ishikado,8 H. Eisaki,5 Z. Hussain,3 T. P. Devereaux,1,2†
timally doped (OP) Pb0.55Bi1.5Sr1.6La0.4CuO6+d
S. A. Kivelson,1† J. Orenstein,3,4† A. Kapitulnik,1,2† Z.-X. Shen1,2†
(Pb-Bi2201, Tc = 38 K, T* = 132 T8 K) (27), and
The nature of the pseudogap phase of cuprate high-temperature superconductors is a major combined the ARPES measurement of the evo-
unsolved problem in condensed matter physics. We studied the commencement of the pseudogap lution of the band structure over a wide range of
state at temperature T* using three different techniques (angle-resolved photoemission spectroscopy,
polar Kerr effect, and time-resolved reflectivity) on the same optimally doped Bi2201 crystals.
We observed the coincident, abrupt onset at T* of a particle-hole asymmetric antinodal gap in 1
Geballe Laboratory for Advanced Materials, Departments of
the electronic spectrum, a Kerr rotation in the reflected light polarization, and a change in the Physics and Applied Physics, Stanford University, Stanford, CA
94305, USA. 2Stanford Institute for Materials and Energy
ultrafast relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA
signatures of superconductivity begin to grow close to the superconducting transition temperature 94025, USA. 3Advanced Light Source and Materials Sciences
(Tc), entangled in an energy-momentum–dependent manner with the preexisting pseudogap Division, Lawrence Berkeley National Laboratory, Berkeley, CA
features, ushering in a ground state with coexisting orders. 94720, USA. 4Department of Physics, University of California,
Berkeley, CA 94720, USA. 5Nanoelectronics Research Institute,
National Institute of Advanced Industrial Science and
s complex oxides, cuprate superconduc- cuprates and its relationship with superconduc-
A tors belong to a class of materials that
exhibit many broken-symmetry states;
unraveling the relationship between superconduc-
tivity. Angle-resolved photoemission spectroscopy
(ARPES) studies have shown that the pseudogap
develops below a temperature T* near the Brillouin
Technology, Ibaraki 305-8568, Japan. 6Department of Physics,
Osaka University, Toyonaka, Osaka 560-0043, Japan. 7School
of Physics, Suranaree University of Technology and Synchrotron
Light Research Institute, Nakhon Ratchasima, 30000 Thailand.
8
Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan.
tivity in the cuprates and other possible broken- zone boundary while preserving a gapless Fermi
*These authors contributed equally to this work.
symmetry states has been a major challenge of arc near the zone diagonal (1). A key issue is the †To whom correspondence should be addressed. E-mail:
condensed matter physics. A possibly related is- extent to which the pseudogap is a consequence zxshen@stanford.edu; aharonk@stanford.edu; jworenstein@lbl.
sue concerns the nature of the pseudogap in the of superconducting fluctuations (2–5), which gov; kivelson@stanford.edu; tpd@stanford.edu
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