matter. Examples of such collisions
include the “Bullet Cluster” at redshift
z = 0.30 and “El Gordo” at z = 0.87.
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
scattered light disks and giant planets in the
association. The rich diversity of debris disks
seen around stars within a single young en-
vironment is remarkable, and we can expect
even more results to emerge from GPIES and
its follow-up programs in the near future.
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
12
GeminiFocus
Large numbers of distant clusters
have now been found via the Sun-
yaev-Zel’dovich (SZ) effect, the ap-
parent decrement in brightness of
the cosmic microwave background
(CMB) radiation resulting from the
scattering of CMB photons by high-
energy electrons in the intracluster
medium. In particular, hundreds of
cluster candidates have been iden-
tified in this way by the South Pole
Telescope (SPT), a 10-meter radio dish lo-
cated at the South Pole, designed for large-
area surveys at millimeter and submillimeter
wavelengths. Because the SZ signal does not
provide the redshift, additional observations
of the member galaxies are required.
The SPT-GMOS Survey, led by Matthew Bayl-
iss at Harvard (now at MIT), used the GMOS
instrument at Gemini South to measure the
redshifts of SZ-selected cluster candidates
identified by SPT. The survey measured red-
shifts 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 spectroscopically
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
January 2020