A New Kind of Galaxy
Cluster Sample
The Sunyaev-Zel’dovich Effect (SZE; Sunyaev and Zel’dovich, 1972) corresponds to
the scattering of photons coming from the
Cosmic Microwave Background (CMB) by the
electrons in the ICM. This typically boosts the
energy of individual photons, resulting in a
distinct frequency dependence of the effect.
Thus, the SZE is observed as a change in temperature in the direction of clusters with respect to the average CMB line-of-sight.
This effect is observed as a decrease in temperature relative to the undistorted CMB at
frequencies below approximately 218 Gigahertz (GHz), peaking around 130 GHz, where it
is of the order of a few hundreds of microkelvins (µK) for the most massive clusters. At approximately 218 GHz the net change in temperature is null, and the SZE is observed as a
temperature increment at higher frequencies.
Importantly, since the SZE is a scattering process, the surface brightness of the SZE is strictly independent of the distance to the cluster. It
is moreover, to first order, dependent only on
the line-of-sight integral of the gas pressure
through the ICM. Of course, more massive
clusters tend to be hotter and have a denser
ICM. This implies that they host ICM atmospheres with the highest pressures, making
the SZE particularly sensitive to cluster mass.
The bottom line is that the most massive clusters produce the strongest SZE signals.
The selection function for a sample of galaxy
clusters detected with the SZE is essentially
distance independent. Thus, an accurate calibration of the SZE brightness of clusters to
their mass gives exciting prospects for the
use of galaxy clusters as cosmological probes.
This is especially true for two reasons: 1) Being almost independent of redshift, the SZE
is best suited to detect the most massive galaxy clusters at high redshift (as mentioned
before, it provides the strongest leverage to
cosmological constraints); and 2) The SZE
brightness is expected from numerical simulations to be cleanly related to the total mass
of clusters, with an intrinsic scatter as low as
10 percent.
ACT Weighs In
The 6-meter Atacama Cosmology Telescope (ACT) in northern Chile was designed
to scan the sky at millimeter wavelengths
with a resolution approaching 1 arcminute.
It is one of only two ground-based millimeter-band telescopes sensitive enough to
conduct large area surveys (covering thousands of square degrees of sky) to study the
CMB and other temperature fluctuations
from astrophysical sources (i.e., SZE clusters
and distant galaxies).
The first results — based on a 450-squaredegree survey of the southern sky at 148
GHz with a sensitivity of 36 µK — revealed 23
clusters in the redshift range 0.12 < z < 1.07,
of which 10 at z > 0.28 were newly discovered
(Marriage et al., 2011). The initial characterization of the sample, using optical imaging
and archival X-ray data, confirmed that this
was a sample of massive clusters spanning
all redshifts, as expected from mass function
predictions (Menanteau et al., 2010).
However, a detailed analysis, including mass
measurements and cosmological implications, had to wait for further observations.
Figure 1 shows four sample clusters detected
by ACT, showing the temperature decrements observed in 148 GHz (top) and the
corresponding optical images (bottom), with
the photometric redshift given in each image. Of these, the three highest redshift clusters were followed up with the Gemini MultiObject Spectrograph (GMOS).
One of the highlights of this cluster survey
was the discovery of ACT-CL J0102 4915,
-
dubbed “El Gordo,” which is at a redshift of
0.87 (Menanteau et al., 2012). We character-
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