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