An international team of astronomers using
the NASA/ESA Hubble Space Telescope has discovered
a ghostly ring of dark matter formed long
ago during a colossal collision between two
galaxy clusters.
This is the first time that dark matter has
been found with a distribution that differs
substantially from the distribution of ordinary
matter.
Scientists reckon that most of the matter
in our Universe is something called “dark
matter”, an unknown type of matter that
neither emits nor reflects light.
But does dark matter really exist?
Can scientists prove it?
The NASA/ESA Hubble Space Telescope is helping
to answer these questions.
In 2004 an international team of astronomers
pointed Hubble towards the constellation of
Pisces, the Fish, to observe a galaxy cluster
that goes by the telephone number of CL0024+17
and which is located 5 billion light-years
away from Earth.
Hubble’s Advanced Camera for Surveys produced
a stunning image of the cluster.
The galaxies in the cluster are seen here
in yellow.
Analysing the image over the last couple of
years the team discovered a ring of dark matter
– seen here in blue –
and realised that its position of this ring
did not match at all the position of the hot
gas and the galaxies in the galaxy cluster.
The ring itself is 2.6 million light-years
across.
This is the first time that dark matter has
been found with a distribution that is so
radically different from the distribution
of the ordinary matter.
This remarkable finding is attributed to the
collision of the cluster with another cluster
1 to 2 billion years ago.
The team's computer simulations show – here
seen from the side - that when the two clusters
smashed together, the dark matter fell to
the centre of the combined cluster and bounced
back out.
In reality the collision occurred along our
line of sight, so that we have a head-on view
of it.
From this perspective, the dark-matter structure
looks like a ring, just like the new observations
show.
So how did astronomers spot the ring of dark
matter?
Tracing dark matter is not an easy task, the
reason is of course that dark matter does
not emit or reflect any light.
The most direct way to detect its influence
is to study the way its gravity deflects light.
To do this, astronomers study the faint light
from galaxies that lie behind the cluster,
and whose light gets distorted and smeared
into arcs and streaks by the gravity of the
dark matter in the foreground cluster.
This powerful trick called gravitational lensing.
To illustrate this, imagine that I am a background
galaxy being lensed by a massive foreground
cluster.
So by mapping the distorted light, astronomers
can deduce the mass of the cluster and they
can trace the distribution of dark matter
within the cluster.
This amazing image shows us some spectacular
examples of faint background galaxies that
had their light bent by the cluster’s strong
gravitational field.
One of them, located about two times farther
away than the yellow cluster galaxies in the
foreground, has been multiple-imaged into
five separate arc-shaped pieces.
Hubble’s high resolution can even show the
details of this background galaxy.
The ring's discovery is among the strongest
evidence that dark matter actually exists
and it increases confidence in our current
theory of gravity .
The NASA/ESA Hubble Space Telescope has captured
a new image of the galaxy NGC 1132 which is
most likely to be a “cosmic fossil” –
the aftermath of an enormous multi-galactic
pile-up, where the carnage of collision after
collision has built up a brilliant but fuzzy
giant elliptical galaxy far outshining typical
galaxies.
In this episode we will take a closer look
on the latest image from Hubble Space Telescope.
It shows the giant elliptical galaxy NGC 1132.
Now you may ask: “what is so special about
this fuzzy, seemingly bland object?”
Well, the interesting thing about it is not
so much what it looks like today, but rather
what happened in its past.
Let’s try to trace its history by taking
a very close look at its present features.
NGC 1132 is located about 320 million light-years
away from Earth, in the constellation of Eridanus,
the River.
At first glance NGC 1132 looks like any other
ordinary elliptical galaxy – it is smooth,
featureless and contains hundreds of millions
of stars whose yellowish colour is a telltale
sign of their great age.
But closer up, we see that NGC 1132 is rather
special.
It is humongous!
Many times larger than the average elliptical
galaxy.
It belongs to a category of galaxies called
giant ellipticals.
Seen in visible light, NGC 1132 appears as
a single, almost isolated, giant galaxy.
But this is only the tip of the iceberg.
Scientists have found that NGC 1132 resides
in an enormous halo of dark matter,
comparable to the amount usually found in
an entire group of tens to hundreds of galaxies.
It also has a strong X-ray glow from an abundance
of hot gas – an amount normally only found
in galaxy groups.
In fact its X-ray glow extends over a region
of space ten times larger than the 120,000
light-years radius seen in visible light.
This is a glow equal in size to that of an
entire group of galaxies.
So there’s enough dark matter and hot gas
for an entire group of galaxies and yet we
see only a single, although gargantuan galaxy.
Well, actually, not quite.
If we look closely at the image, we can see
that NGC 1132 is associated with a whole bunch
of small dwarf galaxies –
which look a little bit as a huge wads of
cotton - but there are definitely no medium-sized
galaxies.
So what’s going on?
The most likely explanation is that NGC 1132
is the result of galactic cannibalism.
It is probably a so-called “fossil group”.
In other words what we are looking at here
are the remains of an entire group of galaxies
that have all merged together into a single
galaxy at some point in its past.
If we examine the image closely we can also
see that NGC 1132 is surrounded by thousands
of ancient globular clusters, swarming around
the galaxy like bees around the hive.
These globular clusters are most likely survivors
of the disruption of their parent galaxies
that have been swallowed by NGC 1132.
And because of that, they can tell us a lot
about the merging history of the whole group.
There is a stunning tapestry of numerous galaxies
that are much further away and have nothing
to do with the fossil group in the foreground.
Many galaxies, including our own Milky Way,
reside in groups that are gravitationally
bound together.
Now, there is plenty of evidence that the
Milky Way is also a cannibal and has snacked
on numerous smaller galaxies throughout its
lifetime, inheriting their stars in the process.
So, what will happen to the Milky Way and
its neighbours over the next few billions
of years?
Well, this is precisely one of the questions
that astronomers are trying to answer when
they study the structure and the evolution
of other galaxies such as NGC 1132.
By analysing their properties, it is possible
to trace back the history and to better understand
what will happen in our own neighbourhood
in the future.
An international team of astronomers using
ESO’s Very Large Telescope has measured
the distance to the most remote galaxy so
far.
This is the first time that astronomers have
been able to confirm that they are observing
a galaxy as it was in the era of reionisation
when the first generation of brilliant stars
was making the young Universe transparent
and ending the cosmic Dark Ages.
Studying these first galaxies is extremely
difficult; they are very faint and small and
by the time their dim light gets to Earth
it falls mostly in the infrared part of the
spectrum because it has been stretched by
the expansion of the Universe.
To make matters worse, at this very early
time, less than a billion years after the
Big Bang, the Universe was not completely
transparent.
It was filled with hydrogen, which acted kind
of like a fog and absorbed the ultraviolet
radiation from the young galaxies.
So, holding the record for having measured
the redshift of the most distant object in
the Universe is not just a trophy to hang
on the wall, it does have important astrophysical
implications.
This is the first time that we’ve managed
to obtain spectroscopic observations of a
galaxy from the era of reionisation
in other words from the time when the Universe
was still clearing out the hydrogen fog.
Despite the difficulties of finding these
early galaxies, the new Wide Field Camera
3 on the NASA/ESA Hubble Space Telescope discovered
several very good candidate objects earlier
in 2010.
They were thought to be galaxies shining in
the early Universe at redshifts greater than
eight, but confirming the distances to such
faint and remote objects
is an enormous challenge and can only reliably
be done using spectroscopy from very large
ground-based telescopes.
A 16 hour exposure with the VLT and SINFONI
of the galaxy UDFy-38135539 did indeed show
the very faint glow from hydrogen at a redshift
of 8.6,
which means that this light left the galaxy
when the Universe was only about 600 million
years old.
This is the most distant galaxy ever reliably
confirmed.
One of the puzzling things about this discovery
is that the ultraviolet radiation emitted
by the galaxy does not actually seem to be
strong enough to be able to clear out the
hydrogen fog around the galaxy.
So one possible explanation is that there
must be other galaxies, probably fainter and
less massive neighbours,
that helped ionise the hydrogen in the region
of space around the galaxy, thus making it
transparent.
Without this additional help the brilliant
light from the main galaxy would have been
trapped in the surrounding hydrogen fog and
it could not have even started its 13 billion-year
journey towards Earth.
Studying the era of reionisation and the formation
of the first galaxies is really pushing the
capability of current telescopes and instruments
to the limit.
But, this will be exactly the type of science
that ESO’s European Extremely Large Telescope
will excel at.
Once operational, this will be the largest
optical and infrared telescope in the world.
This is Dr J signing off for the ESOcast.
Join me again next time for another cosmic
adventure.
