North, Gladders’ team was able to
confirm these lensing clusters, and
most importantly, measure the
spectroscopic redshifts of the lensed
background sources.
Of the hundreds of lensing clusters found in the SGAS, SDSS
J2222 + 2745 is the only one that
lenses a background quasar into at
least six images of the same background source (Dahle et al., 2013).
This lensing configuration is so
unique, that, in fact, this case is only
the third one known of a quasar that
is strongly lensed by a galaxy cluster.
The other ones, SDSS J1004 + 4112
and SDSS J1029 + 2623, (Inada et
al., 2003; Inada et al., 2006) split the
light from a background quasar into
five and three images, respectively.
Figure 1.
A 30’’ x 30’’ field around
the center of the galaxy
cluster in the SDSS
J2222+2745 lensing
system. The composite
color image combines
Nordic Optical
Telescope and Gemini
North Fast Turnaround
data. The slight color
difference in the three
images of the quasar (A,
B, and C) results from
the different optical
filters used during the
observations that were
made at different times.
4
ing equation: light is allowed to travel from
point A to point B on more than one path,
resulting in multiple images of the same
background source.
Such is the case of the SDSS J2222 + 2745
system — a galaxy cluster at z = 0.5, that
strongly lenses a few background sources,
including a quasar at z = 2.82 and a galaxy at
z = 2.3 (Figure 1).
This lensing cluster was discovered by an
international team of researchers, led by
Michael Gladders from the University of Chicago, who mined data from the Sloan Digital Sky Survey to find strong lensing clusters
that stretch and distort background galaxies
into giant arcs. As part of this Sloan Giant
Arcs Survey (SGAS), the team examined images of ~ 30,000 galaxy clusters, and systematically identified evidence of gravitational
lensing in hundreds of galaxy clusters, many
of which were discovered for the first time.
Using follow-up imaging and spectroscopy
with several telescopes, including Gemini
GeminiFocus
Time-Stamping Light
Quasars are among the most luminous objects known in the Universe. They are powered by supermassive black holes accreting
matter in an active galaxy’s nucleus. What
makes lensed quasars uniquely interesting
is that their luminosity changes in time. A
quasar’s brightness can vary on time scales
from days to months, owing to random physical changes in the accretion disk, and to the
small region from which the light is emitted
(the supermassive black hole). This variability allows us to put a time stamp on the arrival
of light from a lensed source.
Whenever multiple lensed images of the
same source are formed, each one is a snapshot of the source taken at a different cosmic time. This is because the light from each
source has taken a different path from the
quasar to us; and the time it takes light to arrive depends on the length of each path and
the gravitational potential along this path.
January 2016