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 arcseconds (170 kilopar-
secs) west of the BCG. The two com-
ponents have similar masses, with a
3:2 mass ratio being within the range
implied by the analysis, although the
galaxy distributions appear very dif-
ferent. Moreover, the difference in
their mean line-of-sight velocities is
only about 300 km/s, suggesting 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, reminiscent of the Bul-
let Cluster at much lower redshift. However,
additional data, including deep X-ray obser-
vations and more galaxy redshifts to supple-
ment those supplied by GMOS, are needed
to fully characterize this complex system.
The study has been submitted to The Astro-
physical Journal, and a preprint is available
online.
OCTOBER 2019
Comet 2I/Borisov Breezes
Through Solar System,
Tail Streaming Behind
It was in October 2017, just days after this
writer joined Gemini, that the first interstel-
lar object, later designated 1I/‘Oumuamua,
was spotted making its expeditious escape
from our Solar System. Observations by
Gemini and many other observatories dem-
onstrated that ‘Oumuamua was surprisingly
asteroidal in nature, with no apparent coma
or tail. Moreover, judging from the dramatic
variations in its light curve, this first interstel-
lar visitor had an unusually large axis ratio,
perhaps 10:1, suggesting that it may be a
scattered shard from a violent collision that
January 2020 / 2019 Year in Review
ejected the object long ago from its home
planetary system.
Now, less than two years later, a second in-
terstellar emissary has arrived from the di-
rection of Cassiopeia, and it bears strikingly
little resemblance to the first. If the stars are
trying to tell us something, their message
is inconsistent. The new object was discov-
ered by the Crimean amateur astronomer
Gennady Borisov on August 30, 2019, using
a 65-centimeter telescope that he built him-
self. Subsequent observations have shown
that its orbital eccentricity with respect to
the Sun exceeds 3.3 (eccentricities above 1.0
correspond to unbound hyperbolic orbits;
‘Oumuamua had an eccentricity of 1.20).
Popularly known as “Comet Borisov” (even
though the amateur has discovered seven
other, more conventional, comets), the ob-
ject received the official interstellar designa-
tion 2I/Borisov from the International Astro-
nomical Union on September 24, 2019.
Figure 5.
Color-magnitude
diagram of galaxies
in the field of SPT-CL
J0356−5337 made
from Gemini GMOS-
South and Hubble/ACS
data. Galaxies selected
as being on the red
sequence are marked
with red stars; filled
symbols indicate galaxies
within 76 arcseconds
(about 600 kiloparsecs)
of the brightest cluster
galaxy. Spectroscopically
confirmed members
are indicated by gold
squares.
Credit: Mahler et al.,
arXiv:1910.14006
Gemini Observatory was first alerted to 2I/
Borisov by a Director’s Discretionary Time
(DDT) proposal received on the evening of
September 9th, when the object was in the
northern sky at a distance of 3.4 AU from
from the Earth and within 43 degrees of the
Sun. Following careful review, the proposal
was found to be compelling, with Gemini's
large aperture being well suited for inves-
tigating possible cometary activity during
GeminiFocus
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