ized it as a system of two clusters in the process of merging roughly in the plane of the
sky. It resembles the well-known Bullet Cluster. El Gordo is the hottest and most massive
cluster known at redshifts above 0.6.
The Spectroscopic Follow-up
Given the potential of this cluster sample as a
cosmological probe, we started a large spectroscopic follow-up campaign. We aimed to
secure the redshifts of the clusters and determine their masses from velocity dispersions
of member galaxies. These dynamical masses
provide a proxy we can use to calibrate the
SZE-mass scaling relation.
Over a total of seven nights at Gemini South
in 2009-2010 (programs GS-2009B-Q-2 and
GS-2010B-C-2, both joint Chile-U.S. programs), we observed some 1000 galaxies in
the direction of 11 clusters in the high-redshift ACT sample. These data, obtained with
GMOS in multi-object spectroscopy mode,
were augmented with an additional five clusters observed with the Very Large Telescope
during the same period (Sifón et al., 2013).
Our selection of target galaxies, based on
color cuts and further visual inspection, resulted in a high success rate. The
data allowed the robust identification
of cluster members. With an average of
60 members per cluster, we could determine precise redshifts for all of the
clusters and velocity dispersions with
typical uncertainties of ~10 percent. We
used a scaling relation calibrated with
numerical simulations to infer the total
masses of these 16 clusters. Typical uncertainties in the total masses of each
cluster are ~30 percent.
than 20 percent. Figure 2 shows the best-fit
scaling relation between dynamical mass and
the total SZE, integrated within a virial radius
r200 (the radius within which the average density is 200 times the critical density of the universe at the redshift of each cluster). The figure also shows several other determinations
of this scaling from different mass proxies.
Our results are consistent with results from
X-ray observations, weak lensing measurements, and numerical simulations.
A Second Sample:
More Gemini Data, Cosmological
Constraints, and Prospects
ACT also performed a second survey over the
equator, taking advantage of the rich archival
dataset available, largely thanks to the deep
optical observations of the Sloan Digital Sky
Survey Stripe 82. In this second survey we
detected 68 galaxy clusters up to z = ~1.4; of
them 19 are new discoveries (Hasselfield et al.,
2013). Menanteau et al. (2013) have presented
cluster properties from optical imaging and
X-ray archival data, including spectroscopic
redshifts from GMOS for many of them. We
are undertaking a large follow-up program ex-
Figure 2.
Best-fit scaling relation
between dynamical
mass and total SZE,
integrated within the
virial radius (described
in main text). Also
shown are previous
determinations of this
scaling relation from
different mass proxies.
We used these dynamical masses to obtain scaling relations between the SZE
and total mass. We were able to show
that these two quantities can be related
with low intrinsic scatter, probably less
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