Hubble's Cosmic Fairy Lights
Credit: ESA/NASA
This sparkling jumble is Messier 5 — a globular cluster consisting of
hundreds of thousands of stars bound together by their collective gravity.
But Messier 5 is no normal globular cluster. At 13 billion years old it dates
back to close to the beginning of the Universe, which is some 13.8 billion years
of age. It is also one of the biggest clusters known, and at only 24 500
light-years away, it is no wonder that Messier 5 is a popular site for
astronomers to train their telescopes on.
Messier 5 also presents a puzzle. Stars in globular clusters grow old and
wise together. So Messier 5 should, by now, consist of old, low-mass red giants
and other ancient stars. But it is actually teeming with young blue stars known
as blue stragglers. These stars spring to life when stars collide, or rip
material from one another.
European Space
Agency
Hubble Astronomers Use Supernovae to Gauge Power of
Cosmic Lenses
Distant exploding stars observed by NASA's Hubble Space Telescope are
providing astronomers with a powerful tool to determine the strength of
naturally-occurring "cosmic lenses" that are used to magnify objects in the
remote universe.
Two teams of astronomers, working independently, observed three such
exploding stars, called supernovae. Their light was amplified by the immense
gravity of massive galaxy clusters in the foreground -- a phenomenon called
gravitational lensing. Astronomers use the gravitational lensing effect to
search for distant objects that might otherwise be too faint to see, even with
today's largest telescopes.
"We have found supernovae that can be used like an eye chart for each lensing
cluster," explained Saurabh Jha of Rutgers University in Piscataway, N.J., a
member of the Cluster Lensing and Supernova survey with Hubble (CLASH) team.
"Because we can estimate the intrinsic brightness of the supernovae, we can
measure the magnification of the lens."
At least two of the supernovae appear to be a special type of exploding star
called Type Ia supernovae, prized by astronomers because they have a consistent
level of peak brightness that makes them a reliable tool for estimating
distances.
Astronomers from the CLASH team and the Supernova Cosmology Project are using
these supernovae in a new method for measuring the magnification, or
prescription, of the gravitational lenses. With these prescriptions, astronomers
are now equipped to make increasingly accurate observations of objects in the
distant, early universe and better understand the structure of galaxy
clusters,including its distribution of dark matter.
The power of a galaxy cluster as a gravitational lens depends on the total
amount of matter in the cluster, including dark matter, which is the source of
most of a cluster's gravity. Astronomers develop maps that estimate the location
and amount of dark matter in a cluster by looking at the amount of distortion
seen in more distant lensed galaxies. The maps provide the prescriptions -- how
much distant objects behind the cluster are magnified when their light passes
through the cluster.
The three supernovae in the Hubble study were each gravitationally lensed by
a different cluster of galaxies. The teams measured the brightness of each
supernova, with and without the effects of lensing. The difference between the
two measurements constitutes the amount of magnification because of
gravitational lensing. From the final measurements, one of the three supernovae
stood out, with an apparent magnification of about two times.
The supernovae were discovered in the CLASH survey, a Hubble census that
probed the distribution of dark matter in 25 galaxy clusters. The three
supernovae exploded between 7 billion and 9 billion years ago, when the universe
was slightly more than half its current age of 13.8 billion years old.
To perform their analyses, both teams used observations in visible light,
made by Hubble's Advanced Camera for Surveys, and in infrared light, made by the
telescope's Wide Field Camera 3. Each team then compared its results with
independent theoretical models of the clusters' dark-matter content, concluding
that the predictions fit the models.
Now that researchers have proven the effectiveness of this method of cosmic
magnification, they are searching for more Type Ia supernovae hiding behind
large galaxy clusters. Astronomers estimate they would need about 20 supernovae,
spread out behind a single cluster, to create a map of an entire cluster of
galaxies.
They are optimistic Hubble and future telescopes, such as NASA's James Webb
Space Telescope, will identify more of these unique exploding stars.
The CLASH team's results will appear in the May 1 issue of The Astrophysical
Journal and the Supernova Cosmology Project's findings will be published in the
May 1 edition of the Monthly Notices of the Royal Astronomical Society.
For images and more information about Hubble, visit:
NASA
Guillermo Gonzalo Sánchez Achutegui
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