Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994eso..pres....5.&link_type=abstract
ESO Press Release, 04/1994
Astronomy and Astrophysics
Astronomy
Scientific paper
Very difficult and time-consuming observations
performed with the ESO 3.5-metre New Technology
Telescope (NTT) in November 1993 by an international
team of astronomers [1], indicate that up to 90
percent of the matter in a distant giant galaxy may
be of a kind that cannot be seen by normal
telescopes.
The astronomers were able to observe the individual
motions of 37 extremely faint Planetary Nebulae [2] in
the outskirts of the giant elliptical galaxy NGC 1399
that is located at the centre of the southern Fornax
cluster of galaxies, at a distance of about 50 million
light-years. The mass of the galaxy can be inferred
from these motions: the faster they are, the more
massive is the galaxy. Surprisingly, the total mass of
NGC 1399 found from these new measurements is about
ten times as large as the combined mass of the stars
and nebulae seen in this galaxy.
These new results also have important implications for
the current ideas about the formation of giant
galaxies.
GIANT GALAXIES
Galaxies are the basic building blocks of the
Universe. Some look like spinning spirals, like our
own Milky Way galaxy, with its several hundreds of
billions of stars in a flat, rotating disk. Some
galaxies lead a comparatively quiet life, others are
violent and explosive. But perhaps the most enigmatic
of them all are the largest ones, the giant elliptical
galaxies. They are huge collections of stars and hot
gas, 100 times brighter than the Milky Way and in many
of them, the hot gas is a powerful emitter of radio
waves and X-rays. The giant galaxies are mostly found
at the centres of vast clusters of hundreds or
thousands of smaller galaxies, like swarms of bees
about the central hive.
How did these great galaxies form at the centres of
their clusters? Astronomers who make computer
simulations of the early Universe believe they know
the answer. In their simulations, they see these giant
galaxies forming by gradual aggregation of small
clumps of matter falling towards the centre, thereby
making larger and larger bodies as time
progresses. But how sure can we be that this theory is
correct ? It turns out that a crucial test is to
measure how the matter now moves in the outskirts of
these huge galaxies, at distances of 100,000
light-years or more from their centres.
MOTIONS IN GIANT GALAXIES
Swirling motion, or rotation, in galaxies comes
originally from clumps of matter raising tides on each
other through their gravitational pull, just as the
Moon raises tides on the Earth. The tug of these
tides makes the clumps spin. When the swirling clumps
come together in computer simulations of what is going
on in a newborn galaxy, they keep interacting, and the
amount of swirling motion (``angular momentum'') is
gradually shifted outward into the far outer regions
of the new galaxy.
If this theory is correct, we should therefore now see
slow swirling motion or rotation in the inner parts of
the giant galaxies, but quite rapid motion in their
far outer regions. The first part is not so difficult
to check observationally: the inner parts of giant
galaxies are relatively bright and we can easily
measure their rotation from the observed Doppler shift
of the light from the stars and nebulae which are
located here. However, to measure the rotation in the
outer parts has, until now, proved impossible, because
out there the light from the galaxy is just too faint
to be observed, even with large astronomical
telescopes.
PLANETARY NEBULAE AS BEACONS
Fortunately, a few years ago it was realised that
there are some excellent beacons that we can use to
measure the swirling motion far out in giant
galaxies. These are the planetary nebulae that are
created during the last dying act of stars like the
Sun. Such objects are rare, because the planetary
nebula phase does not last long in astronomical terms,
but in these huge galaxies a few hundred of them may
still be present in the outer regions at any time.
The shining gas in a planetary nebula emits most of
its light at one particular wavelength in the green
part of the spectrum [3]. This fortunate concentration
of the light energy makes it possible to see them and
to measure the velocities of individual planetary
nebulae in galaxies, even at relatively large
distances.
The present team of astronomers had earlier used
planetary nebulae to study the motions in several
nearby galaxies (closer than 20 million light-years),
but never before had they attempted to investigate a
giant elliptical galaxy. This is because even the
nearest of these rare objects is so far away (50
million light-years) that the light from its planetary
nebulae is extremely faint and therefore in principle
out of range for existing astronomical telescopes.
ATTEMPTING OBSERVATIONS OF NGC 1399
Still, the team decided to try. As Magda Arnaboldi,
the leader of the team puts it: ``We thought that such
an observation may just be possible with one of the
best optical telescopes in the world, the ESO
3.5-metre New Technology Telescope (NTT) at La Silla
in Chile. So we applied for observing time and were
pleased to obtain three nights in November 1993.''
The object of their investigation was the giant
elliptical galaxy NGC 1399, the supposedly nearest
galaxy of its type and located at the very centre of
one of the largest clusters of galaxies in the
southern sky, the Fornax cluster (referring to the
constellation towards which it is seen). The visual
magnitudes of the planetary nebulae in NGC 1399 are
around 27, i.e., they are 250,000,000 times fainter
than what can be seen with the unaided eye. It is not
too difficult to record direct point-like images of
each of them with the NTT. However, the measurement of
their motions implies that this sparse light must be
dispersed and spectrally analysed, an almost
impossible feat for such faint objects.
For this daunting task, the astronomers used the ESO
Multi-Mode Instrument (EMMI), which incorporates a
multi-object spectrograph that allows to measure the
velocities of many planetary nebulae at once. In view
of the very long exposure times needed, this is an
absolute must in order to perform these observations
within the available telescope time.
Before the observations can begin, the exact positions
of the planetary nebulae are measured. A metal mask is
then prepared with holes that permit the light from
these objects to pass into EMMI, but at the same time
blocks most of the much brighter, disturbing light
emitted the by Earth's atmosphere. With an additional
optical filter, all but the green light is effectively
filtered out; this further ``removes'' unwanted light
and improves the chances of effective registration of
the faint light from the planetary nebulae in NGC
1399.
VELOCITIES OF PLANETARY NEBULAE IN NGC 1399
The careful preparations paid off and this
observational strategy was successful. During two of
the allocated nights (the third was lost due to bad
weather), the Australian observers (Magda Arnaboldi
and Ken Freeman) were able for the first time to
measure individual velocities for 37 planetary nebulae
in NGC 1399. Some of these are indicated on the
picture that accompanies this Press Release. The
difficulty of this observation is illustrated by the
fact that in order to catch enough light from these
faint objects, the total exposure time was no less
than 5 hours and only one field on either side of the
galaxy could be observed per night.
Already at the telescope the astronomers realised that
the new results are very exciting; this was fully
confirmed by the following long and complicated
process of data reduction. In fact, although the inner
parts of this galaxy rotate quite slowly, the
planetary nebulae in the outer regions are in rapid
motion and clearly indicate a fast rotation of these
parts of the galaxy.
This new observation is just as expected from the
above described theory for the formation of giant
galaxies and therefore provides very strong support
for this theory.
LOTS OF DARK MATTER IN
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