the intracluster medium. In particu-
lar, hundreds of cluster candidates
have been identified in this way by
the South Pole Telescope (SPT), a
10-meter radio dish located at the
South Pole, designed for large-area
surveys at millimeter and submilli-
meter wavelengths. Because the SZ
signal does not provide the redshift,
additional observations of the mem-
ber galaxies are required.
Figure 4.
Color composite image
of the merging cluster
SPT-CL J0356−5337 at z
= 1.036, made by com-
bining Gemini/GMOS-
South g and i images
with Hubble/ACS F606W.
The yellow ellipses mark
cluster members; several
strongly lensed arcs are
visible near the center of
the field.
Credit: Mahler et al.,
arXiv:1910.14006
The study is led by Justin Hom of Arizona
State University, and a preprint is available
online.
Strong Lensing by Colliding
Clusters at High Redshift
Clusters of galaxies, the largest self-grav-
itating structures in the Universe, form
via hierarchical assembly, increasing their
masses through the accretion of individual
galaxies and small groups, often funneled
inward along cosmic filaments. Occasional-
ly, two massive clusters coalesce, providing
an opportunity to study high-speed galaxy
interactions and shock physics within the
colliding intercluster media, the dominant
baryonic component in such clusters. If the
timing and geometry are favorable, and if
each cluster is massive enough to produce
detectable gravitational lensing of back-
ground sources, then the event also affords
a rare opportunity to constrain the physical
properties of the nonbaryonic cluster dark
matter. Examples of such collisions include
the “Bullet Cluster” at redshift z = 0.30 and “El
Gordo” at z = 0.87.
Large numbers of distant clusters have now
been found via the Sunyaev-Zel’dovich (SZ)
effect, the apparent decrement in bright-
ness of the cosmic microwave background
(CMB) radiation resulting from the scattering
of CMB photons by high-energy electrons in
30
GeminiFocus
The SPT-GMOS Survey, led by Mat-
thew Bayliss at Harvard (now at MIT),
used the GMOS instrument at Gemini South
to measure the redshifts of SZ-selected clus-
ter candidates identified by SPT. The survey
measured redshifts for nearly 1,600 member
galaxies in 62 SPT clusters, including several
with strong lensing features. The cluster SPT-
CL J0356–5337 (or SPT-0356) at z = 1.036,
for which Bayliss and collaborators spectro-
scopically confirmed eight members, was
among the highest-redshift strong lensing
clusters in the sample.
In a new study, Guillaume Mahler of the Uni-
versity of Michigan and collaborators pres-
ent a strong lensing analysis of SPT-0356 and
expand the sample of likely cluster members
using single-band F606W Hubble Advanced
Camera for Surveys (ACS) imaging combined
with Gemini/GMOS-South g- and i-band
imaging. Figure 4 shows a color composite
made from the Gemini and Hubble data, with
yellow ellipses enclosing galaxies lying on
the cluster red sequence; the largest ellipse
marks the brightest cluster galaxy (BCG). The
red sequence selection is based on the color-
magnitude diagram shown in Figure 5, made
from a combination of Gemini and Hubble
photometry. To enable the lensing analysis,
the team used Magellan Observatory to ob-
tain redshifts of three multiply-imaged back-
ground galaxies, lensed into the arcs visible
near the center of Figure 4, about 9 to 15 arc-
seconds west of the BCG.
January 2020 / 2019 Year in Review