Figure 5.
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.
The team’s strong lens modeling indicates
that SPT-0356 has a two-component mass
distribution, with one component centered
on the BCG and the other centered on a tight
clump of eight galaxies located about 22 arc-
seconds (170 kiloparsecs) west of the BCG.
The two components have similar masses,
with a 3:2 mass ratio being within the range
implied by the analysis, although the galaxy
distributions appear very different. More-
January 2020
over, the difference in their mean line-of-
sight velocities is only about 300 km/s, sug-
gesting that most of the relative motion is in
the plane of the sky. Thus, SPT-0356 appears
to be a face-on major merger at z > 1, remi-
niscent of the Bullet Cluster at much lower
redshift. However, additional data, including
deep X-ray observations and more galaxy
redshifts to supplement those supplied by
GMOS, are needed to fully characterize this
complex system.
Color-magnitude
diagram of galaxies
in the field of SPT-CL
J0356−5337 made from
Gemini GMOS-South and
Hubble/ACS data. Gal-
axies selected as being
on the red sequence are
marked with red stars;
filled symbols indicate
galaxies within 76 arc-
seconds (about 600
kiloparsecs) of the bright-
est cluster galaxy. Spec-
troscopically confirmed
members are indicated
by gold squares.
Credit: Mahler et al.,
arXiv:1910.14006
The study has been submitted to The Astro-
physical Journal, and a preprint is available
online.
John Blakeslee is the Chief Scientist at Gemini
Observatory and located at Gemini South in
Chile. He can be reached at:
[email protected]
GeminiFocus
13