Quasars
are galaxies with massive black holes at their cores around which vast
amounts of energy are being radiated. Indeed, so much light is emitted
that the nucleus of a quasar is much brighter than the rest of the
entire host galaxy, and their tremendous luminosities allow quasars to
be seen even when they are very far away. The quasar SDSSJ1029+2623,
for example, is so distant that its light has been traveling towards us
for 11.4 billion years, 83% of the age of the universe. This quasar is
particularly unusual because it happens to have five quasar neighbors in
the sky that look very similar to it and moreover are located at the
same cosmological distance.
SDSSJ1029+2623 is actually a gravitationally lensed quasar. Its light
is being magnified and distorted by the gravity of a cluster of
galaxies fortuitously lying between us, in accordance with Einstein's
prediction that light can be bent by gravity. Only a few other quasars
being gravitationally lensed into multiple images by clusters are known.
Over fifty years ago, astronomers predicted that in such cases, because
the light from each image travels along a different cosmological path,
any time delays between flaring events in the images can be used to
probe underlying cosmological parameters such as the age and rate of
expansion of the universe. Moreover, these delays can also probe the
surface density distribution of the lens. Such delays have now been
detected, with the longest delays being of the order of a few years. In
the case when individual galaxies (not clusters of galaxies) act as
lenses, the time delays are more often weeks or months.
CfA astronomer Matthew Bayliss and four of his colleagues undertook a
campaign to monitor the time delays in the images of SDSSJ1029+2623
using the 2.56-meter Nordic Optical Telescope in the Canary Islands,
Spain. Over three years of systematic observations they found a delay of
722 days between the image whose light is predicted to arrive first
("image C") and the component that is brightest, and a 47.7 day delay
between the two brightest components. Fortuitously, during this period
image C underwent a sharp flux increase, and models predict that this
event should be spotted in the other five images in the next few years.
The data are not quite good enough to refine any cosmological
parameters, at least not yet, but the team is continuing close
monitoring of the quasar and hopes to determine with precision the
timing delays in all six components over the next few observing seasons.
Refererence(s):
"Time Delay Measurements for the Cluster-Lensed Sextuple Quasar SDSS
J2222+2745," H. Dahle, M. D. Gladders, K. Sharon, M. B. Bayliss, and J.
R. Rigby, ApJ 813, 67, 2015.
