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OBSERVATIONS OF THE CORONA BOREALIS
SUPERCLUSTER WITH VSA: FURTHER
CONSTRAINTS ON THE NATURE OF THE NONGAUSSIAN COLD SPOT
Ricardo Génova-Santos
VSA collaboration
Astrophysics Group
Cavendish Laboratory
Cambridge
Instituto de Astrofísica
de Canarias (IAC)
43rd Rencontres de Moriond
Cosmology session
Statement of the problem
 The baryon problem:
•Total baryon budget at z=0
(Fukugita & Peebles 2004)
• Lyα forest at z>2 (Rauch et al. 1997)
CMB at z~1000 (Hinshaw et al. 2008)
BBN (Burles et al. 2001)
ΩB = (0.010±0.003)h-2
ΩB > 0.021h-2
ΩB = (0.023±0.001)h-2
ΩB = (0.020±0.002)h-2
Half of the baryons
in the present-day
Universe are yet to
be detected!
 Hydrodinamical simulations (Cen & Ostriker, 2006):
predict the formation, at low redshift (z<1), of the socalled Warm/Hot Intergalactic Medium (WHIM), a very
diffuse gas phase with Te ~ 105 – 107 K, δB ~ 10 – 30 and
L~10 Mpc, distributed in large-scale sheet-like structures
and filaments connecting clusters of galaxies
 Means for detecting it:
• Soft (0.1-2.4 keV) X-ray emission (Zappacosta et al. 2005)
• UV (Nicastro et al. 2005) or X-ray (Barcons et al. 2005)
absorption lines
• Sunyaev-Zel’dovich (SZ) effect. Correlations of CMB maps
with galaxy templates (Hernández-Monteagudo et al. 2004)
(Cen & Ostriker, 2006)
δB ~ 10 / 100 / 1000
Selection of the supercluster
 The WHIM is likely to be located in large regions with a high matter density, such
as supercluster of galaxies
 We selected the Corona Borealis supercluster for VSA observations in the wake of:
• Its appropiate angular size
• Observavility by the VSA
• Relatively high LX of its member clusters
• Lack of bright radio sources
 The CrB-SC is:
• in the northern hemisphere at Dec.~27º
• located at z≈0.07
• formed by 8 galaxy clusters. 4 of them
have reported X-ray emission, with
LX ~ 1-5x1044 h50-2 erg s-1
 We have focused on its core, which contains
6 clusters distributed in a region of 3x3 deg2
 Observations of this region has been carried out at microwaves with the VSA
(Génova-Santos et al. 2005, 2008) and AMI, millimeter wavelenghts with MITO (Battistelli et
al. 2006) and infrared range with the ING (Padilla-Torres et al. 2008)
The Very Small Array
 Interferometer with 14 antenna (91 baselines) located at the Teide Observatory at 2400m asl
 Frequency: 33 GHz, bandwidth: 1.5 GHz
 Belongs to the joint collaboration: Cambridge + Manchester + IAC
 Aimed for doing primordial CMB observations (Taylor et al. 2003, Grainge et al. 2003, Dickinson et al.
2004). Also succesfully used to do SZ observations on galaxy clusters (Lancaster et al. 2005).
Currently performing high-l primordial CMB observations
Compact (Oct.00-Sep.01)
Extended (Oct.01-Nov.04)
 Compact /
Extended /
Superextended:
• FOV: FWHMpb=
4.6º / 2.1º / 0.9º
Superextended (Dec.05-)
• Resolution: FWHMsb=
20’ / 11’ / 7’
• Multipole coverage: l~
150-900 / 300-1500 / 4502400
The Very Small Array
 Source subtractor, aimed for simultaneous observations of the radio sources in the
observed fields
 Sources were identified in the NVSS (1.4 GHz) and GB6 (4.85GHz) catalogues and their fluxes
extrapolated to 33 GHz
 Sources with extrapolated flux above 18 mJy were selected for SS observations, and finally
subtracted from the data
• 2-element interferometer
with a N-S baseline of 9.2m
• Parabolic dishes of 3.7m
• FWHMpb= 9’
• FWHMsb = 4’
Extended VSA observations
24 deg2 mosaic
in the CrB core
region, built up
from 9 pointings
with the
extended VSA
Very deep decrement
at ΔTRJ=-230±23μK
and coordinates
15 22 11.47 +28 54 06.2
(J2000)
(Genova-Santos et al. 2005)
Superextended VSA observations
3 deg2 mosaic in the region
of the decrement, built up
from 6 pointings with the
superextended VSA
(Genova-Santos et al. 2008)
Pointed observation on
the position of the
decrement
Pointed observation on
the position of the
decrement. Tapered map
Minimum decrement:
Minimum decrement:
ΔTRJ=-308±26μK
ΔTRJ=-190±9μK
WMAP-5th-yr on Corona Borealis
Q-band
V-band
 WMAP W-band (freq=33 GHz,
FWHMbeam=13.2’) temperature in the position of
the minimum: ΔTRJ=-120±53μK
 Disagrees at 1.3σ with the minimum in the
VSA tapered map (FWHMsb=13.2’)
 Agrees at 0.1σ with a SZ espectrum
W-band
Origin of the decrement
 Primary CMB anisotropy?
• 15000 simulations of Gaussian CMB in the
aperture plane adding: primordial CMB
(σCMB=41μK), thermal noise (σn=30μK) and residual
radio sources (σsour=15μK)
• The decrement is a 4.8σ deviation
• The probability of finding an at least as high
deviation is only 0.19%
• Computed the power
spectrum of the central
pointing
• 4.0σ deviation from
pure primordial CMB at
l≈500
Origin of the decrement
 Distant galaxy cluster?
• Number of collapsed halos above the minimum mass the cluster
needs to have to produce such SZ effect:
• Sheth-Tormen (1999) mass function,
predicts 0.3 clusters in the entire region initially surveyed which could
produce an at least as intense decrement
• The optical survey carried
out with the ING revealed a
high concentration of infrared
galaxies at z≈0.07 and ≈0.11,
but without a cluster-like
distribution (Padilla-Torres et
al. 2008)
(Padilla-Torres et al.,2008)
Origin of the decrement
 No X-ray excess emission is found in the position of the
decrement in the ROSAT-R6 (0.73-1.56 keV) data
 We may consider a large filament aligned along the l.o.s
• Central SZ decrement:
• Length < 40 Mpc:
• Lx < ROSAT-R6 map:
 Within the range of typical
WHIM Te a filament like this
could contribute up to ≈70%
of the total decrement
 A filament with <δb>~500
and Te~0.6 would give f=0.5,
and would have a total mass
comparable to the total baryon
content in the CrB clusters
MITO observations
 Observations with the Millimeter and
Infrared Testa Grigia Observatory (Italian Alps)
at 143, 214 and 272 GHz (Battistelli et al.
2006)
Thermodynamic temperature
maps derived from the MEM
reconstruction of the VSA and
the 3 MITO channels
(Battistelli et al. 2006)
MITO observations
Primary
anisotropy +
SZE maps
derived from a
ML analysis
VSA+MITO
detections spectral
dependency.
Overplotted SZ
spectrum + CMB
component
(Battistelli et al. 2006)
 These observations
support the hypothesis
of a combination of a
primary anisotropy
(ΔTCMB=-128±20μK)
with an SZ effect
(ΔTSZ=-42±26μK),
being 25% the
contribution from the
latter one
Conclusions
 We have confirmed, with the superextended VSA, the presence of a very deep
decrement on the CMB of -308±26 µK, in the Corona Borealis supercluster in a position
with no known galaxy clusters
 This feature is present in WMAP-5yr data. It’s minimum temperature disagrees with the
VSA value, but matches the SZ spectral dependency very well
 Our analyses revealed that it is a 4.8σ deviation from the Gaussian CMB, and that the
probability of finding a primordial CMB decrement at least as negative is only 0.19%
 The hypothesis of an SZ galaxy cluster is disfavoured by the fact that, due to the
absence of X-ray emission, a distant cluster may would be needed, and this is in conflict
with the large angular extension of the decrement
 A large-scale filamentary structure seems in some ways one of the most attractive
options. However, the absence of X-ray emission set constraints to its physical shape and
parameters that rule out such deep SZ effect within the typical WHIM temperatures and
length scales
 Therefore, the most reliable explanation would be a combination between a negative
primordial CMB feature and an SZ decrement