PG 1115: A Ghost of Lensing Past
Explanation:
In this tangle of quasars and galaxies lies a
clue to the expansion rate of the universe.
A diffuse glow evident in the picture
on the left reveals a normal
elliptical galaxy.
Directly behind this galaxy lies a normal
quasar. Because the
quasar is directly
behind the galaxy, however, the gravity of the
galaxy deflects quasar light like a
lens, creating
four bright images
of the same distant quasar.
When these images are all digitally subtracted,
a distorted image of the background galaxy that
hosts the quasar appears - here shown on the
right in ghostly white. Each
quasar image traces how the
quasar looked at different times in the past,
with the time between images influenced by the
expansion rate of the universe itself.
Assuming dark matter in the elliptical
lens
galaxy traces the visible matter, this
expansion rate
can be characterized by a
Hubble constant of H
o near 65 km/sec/Mpc,
a value close to that determined by
other methods.
Analysis of
this image by itself sheds little
light on whether the
global geometry of the universe
is affected by a
cosmological constant.
Authors & editors:
Robert Nemiroff
(MTU) &
Jerry Bonnell
(USRA)
NASA Web Site Statements, Warnings,
and Disclaimers
NASA Official: Jay Norris.
Specific
rights apply.
A service of:
LHEA at
NASA /
GSFC
& Michigan Tech. U.
