Transcript Berkeley Physics Poster Session, 2002.

The SZ Effect as a Probe for Galaxy Clusters
Formation of large scale structure in the universe is very sensitive to cosmological parameters such as baryon density,
K.S. Dawson, W.L. Holzapfel, E.D. Reese matter density, Hubble expansion, and the equation of state for dark energy. Galaxy clusters are the largest
University of California at Berkeley, Berkeley, CA
gravitationally bound structure in the universe and are considered an excellent target for observing the formation of
large scale structure. To date, all galaxy clusters have been discovered in optical observations or X-Ray observations.
J.E. Carlstrom, S.J. LaRoque, D. Nagai However, both methods of observation are systematically biased against finding high redshift clusters. A third method
University of Chicago, Chicago, IL
of observing galaxy clusters, the Sunyaev-Zel’Dovich effect, was postulated soon after the discovery of the Cosmic
Microwave Background (CMB). The (SZ) effect is a distortion of the CMB spectrum in the direction of a cluster of
M. Joy
galaxies. The effect is independent of redshift and can be used to measure the size and mass of a cluster of galaxies.
Marshall Space Flight Center, Huntsville, AL
The BIMA array in Hat Creek, CA has been adapted to observe galaxy clusters through the SZ effect. The results
from the BIMA array are the most sensitive SZ survey to date.
SZ Effect in Clusters of Galaxies
SZ surface brightness independent of z
t=300,000 years
SZ Thermal Effect
Icmb +ISZ
~1% of CMB is scattered
g
e-
Two Components of the Electron Velocities
•Thermal (Te~100,000,000 K)
•Bulk Motion (Doppler Shift)
2
Cluster
I n t e n s it y ( e r g /(s rcH
mz )
Icmb
•Due to Thermal Velocity of Electrons
•Mean Energy of Photons Increased
z~1100
Inverse scattering of CMB Photons by
Hot Intracluster Plasma
4 .5 E - 1 5
4 E -1 5
3 .5 E - 1 5
3 E -1 5
2 .5 E - 1 5
2 E -1 5
1 .5 E - 1 5
1 E -1 5
5 E -1 6
0
10
100
1000
X-ray surface brightness:
F re q u e n c y [G H z ]
 IT  I
fffff
cmb
yg
x 
I cmb
2h 3
 2 x
c e 1


y 
Produce Two Components of the SZ effect
Non-Relativistic Limit: kTe
/mc2 <<
1
g (x) 
e
xe
x
x
 1


kT e
n e
2
m ec

 x coth

T
dl
h
 x 
 x 

4



kT cmb
 2 

Free electrons and ionized nuclei fall into the gravitational well of a cluster of galaxies. The charged particles gain large amounts of energy (several keV) in
the process. This energy is transferred from the hot intracluster plasma to the cold CMB photons though Compton scattering. The process is referred to as the
Sunyaev-Zel’Dovich (SZ) effect. The thermal SZ effect is observed through distortions of the CMB spectrum in the direction of a galaxy cluster.
Unlike X-ray and optical observations, the SZ effect is independent of
redshift. This property should allow an SZ survey to identify galaxy
clusters that are too distant to be discovered in an X-Ray survey,
producing a more complete catalog of clusters.
•Ten 6.1 Meter Dishes
•6.3’ Primary beam
•Close Pack 2D array
To date only the biggest and brightest
clusters have been mapped through the
SZ effect with the BIMA and OVRO
arrays. These have been follow up
observations to clusters discovered in
X-Ray and optical surveys. Observing
clusters over a wide range of redshift
is useful in calculating the rate of
expansion of the universe.
Calculations of Hubble constant have
been consistent with other methods.
Data from follow-up observations can
also be combined with X-Ray data to
derive cluster masses and baryon
density. Data is currently being
analyzed to compare masses derived
from SZ and X-Ray observation to
masses derived from optical lensing.
•28.5GHz HEMT Receivers
•Tsys(summer) ~40K
•800 MHz Digital Correlator
The Berkeley-Illinois-Maryland Association (BIMA) array is designed for mm-wave observations. During
the summer, these telescopes are outfitted with sensitive cm-wave receivers. The resulting synthesized
beam of the BIMA telescope is approximately 2 arcminutes using nearest neighbor telescopes. Sensitivity
to arcminute scale structure is essential for imaging clusters of galaxies.
Hydrodynamical Simulation
of 1 square degree of SZ sky
Carlstrom et al.
Clusters should be visible out to the redshift of
their formation. For CDM, zf ~1/m-1 ~2
Comptonization
Measure of Integrated Pressure:
S X  (1  z )
4
SZ Effect in Randomly Selected Fields
Cluster surveys probe (1) volume-redshift relation , (2) abundance evolution, (3) structural evolution
BIMA Blank Field Survey
Time = 77.6 hours
u-v >2.4 k,
Beam=21”X22”
RMS~130Jy/beam
u-v = 0.63-1.2 k,
Beam=98”X116”
RMS~110Jy/beam ~ 12K
Now have 10 fields
Of comparable depth
Springel, White, Hernquist 2001
Hydrodynamical simulations predict a rate of structure formation that has a strong dependence on cosmological parameters.
Observations in the next few years are expected to reveal number counts in the thousands, creating a catalog of clusters with
the potential to constrain cosmological models.
Window Functions for BIMA
Analysis
We have performed the most sensitive SZ survey to date with the BIMA telescope.
The survey covers 0.1 square degrees. Data collected in the summer of 2002
doubles the sky coverage and is being analyzed now. One only expects several low
signal to noise clusters to lie in the BIMA survey. It is therefore useful to determine
a power spectrum from the raw data. Analysis of a power spectrum will include
contributions from clusters that lie near the noise level.
The window function describes the angular scales on which the experiment
is sensitive. The BIMA data is divided into two bins, one centered at
l=5500, corresponding to angular scales of 2 arcminutes, the other centered
at l=9500, corresponding to an angular scale of 1 arcminute. The third
curve represents the sum of the two bins.
Likelihood Functions of BIMA
Analysis
l=5500
l=6500
Radio point sources, such as active galactic nuclei (AGN), contaminate CMB
observations by contributing to excess power. To account for point source
contamination, each BIMA field is observed at 5 GHz with the VLA. Point
sources are identified with the VLA observations and removed from the
analysis of the BIMA data. Since point sources are expected to be much
brighter at the low frequency, we expect to remove all point sources which lie
near the noise levels of the BIMA images. It would not be possible to remove
these point sources with the BIMA data alone.
l=9500
The likelihood function describes the probability that
the data is described by a given level of excess power.
The function is normalized with respect to the
scenario described by zero excess power.
Only the CBI experiment run by Cal Tech and the BIMA experiment measure the
power spectrum on scales at which the SZ effect is expected to dominate. The
addition of this year’s data should lower the uncertainty in the BIMA
measurements by a factor of two. This improvement may be enough to constrain
certain cosmological models.