In the case of SDSS J2222 + 2745, six
photons leaving the quasar simultaneously at the speed of light encounter a massive galaxy cluster, whose
gravity sends them on different paths
to our telescope. The first may take a
few billion years to arrive; the second
may come a couple of years later, followed by the third a month or two
later, and so on. If we can measure
this time lag between the arrival of
light to the different image positions,
we can effectively measure the lightdistance along these different paths
and thus measure the geometry of
the Universe.
Promptly after the discovery of SDSS
J2222 + 2745 a follow-up campaign,
led by Håkon Dahle from the University
of Oslo, was initiated using the 2.5-meter
Nordic Optical Telescope (NOT) at the Canary Islands. The team observed the field of
SDSS J2222 + 2745 approximately every two
weeks, resulting in over 40 observations in
good conditions.
Dahle and his team used these observations
to construct a light curve for the brighter
three of the six images (Figure 2). They found
that the quasar’s brightness varies by as
much as 0.8 magnitude (a factor of two increase or decrease in flux) over four years.
Dahle then used computational techniques
to cross-correlate the light curves of images
A and B in order to identify similar patterns in
the fluctuation of light. The team found that
the time lag between image A and B is about
47.7 days, which means that if image A suddenly brightens, image B will do the same
thing 47.7 days later.
A robust measurement of the time lag of image C was harder to obtain. The team’s lens
model, computed by Keren Sharon (author
of this article) predicted Image C to lead images A and B by a few months to years; thus
January 2016
to measure it required a longer monitoring period. However, recent observations,
including imaging with NOT, and with the
Gemini North telescope awarded through a
Fast Turnaround proposal, helped pin down
the elusive measurement of a time delay of
image C. As predicted by the lens model, image C leads image A by about two years, at
about 722 days.
Figure 2.
Light curves of the quasar
images A (blue symbols), B
(green), and C (red). Figure
reproduced from Dahle et
al., 2015.
What’s Next?
Using spectroscopic observations with Gemini North (2015B; Principal Investigator Keren
Sharon), we were able to determine the redshifts of lensed galaxies recently identified
in our Hubble Space Telescope (HST) observations of this field. The new redshifts will
inform a more accurate and precise lensing
model of the cluster, which will improve our
theoretical understanding of this system.
Having measured the lags between three
of the six images of the quasar in SDSS
J2222 + 2745, this lens system provides us
with a rare tool: foresight. In particular, with
a measurement of a negative time lag of image C, we now have a two-year warning for
GeminiFocus
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