K.S. Dawson, W.L. Holzapfel, E.D. Reese

1
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, matter
density, Hubble expansion, and the equation of
state for dark energy. Galaxy clusters are the
largest 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. However, both methods of
observation are systematically biased against
finding high redshift clusters. A third method
of observing galaxy clusters, the
Sunyaev-ZelDovich 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 galaxies. The effect is
independent of redshift and can be used to
measure the size and mass of a cluster of
galaxies. 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.
K.S. Dawson, W.L. Holzapfel, E.D.
Reese University of California at Berkeley,
Berkeley, CA J.E. Carlstrom, S.J. LaRoque, D.
Nagai University of Chicago, Chicago, IL M.
Joy Marshall Space Flight Center, Huntsville, AL


SZ Effect in Clusters of Galaxies
Inverse scattering of CMB Photons by Hot
Intracluster Plasma
Icmb
Cluster
z1100 t300,000 years
Icmb DISZ
1 of CMB is scattered
e-
g

• Two Components of the Electron Velocities
• Thermal (Te100,000,000 K)
• Bulk Motion (Doppler Shift)
• Produce Two Components of the SZ effect

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.
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-ZelDovich (SZ) effect. The thermal SZ
effect is observed through distortions of the CMB
spectrum in the direction of a galaxy cluster.
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.

• Ten 6.1 Meter Dishes
• 6.3 Primary beam
• Close Pack 2D array

• 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
SZ Effect in Randomly Selected Fields
of 1 square degree of SZ sky
Cluster surveys probe (1)
, (2)
, (3)
volume-redshift relation
abundance evolution
structural evolution
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
l5500, corresponding to angular scales of 2
arcminutes, the other centered at l9500,
corresponding to an angular scale of 1 arcminute.
The third curve represents the sum of the two
bins.
Likelihood Functions of BIMA Analysis
l5500
l6500
l9500
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.
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 years data
should lower the uncertainty in the BIMA
measurements by a factor of two. This
improvement may be enough to constrain certain
cosmological models.
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.