This course is designed for anyone who is interested in learning more about modern astronomy. We will help you get up to date on the most recent astronomical discoveries while also providing support at an introductory level for those who have no background in science.
8. Large Scale Structure
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Do curso por University of Arizona
Astronomia: Explorando oTempo e Espaço
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Galaxies
The architecture of the Milky Way galaxy is that of a disk, a bulge, and a halo, with the entire assemblage bound by enigmatic dark matter. Every galaxy contains a supermassive black hole, and entire population of galaxies is sculpted by gravity into subtle structures on large scales.
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Chris ImpeyDistinguished Professor
Astronomy
Mass in the universe is clumped on a variety of scales.
Starting with proximate scales, we see the mass of the Solar System mostly
concentrated in the sun with a few rocky objects around it, the planets.
In the solar neighborhood, there are stars, and
the density of stars increases towards the center of our galaxy.
Galaxies are almost isolated in space.
The vacuum of interstellar space contains, on average,
a tenth of an atom per cubic meter, a fantastically good vacuum, almost nothing.
The galaxies grouped by gravity into larger structures, groups and clusters.
And the largest bound structures in the universe involve thousands of galaxies
in regions of space roughly 100 million light years across, superclusters.
The story of the evolution of structure is the story of the operation of a single
force, gravity, operating over a vast amount of cosmic time.
The formation of structure in the universe takes place in competition
against the expansion of space which diffuses the material in the universe.
We can imagine that if the universe had expanded faster than it did, gravity might
not have been able to form structures against the fleeing matter in a big bang.
But in fact, gravity is able to form larger and larger structures,
even as the universe gets larger and larger over time.
The largest bound objects in astronomy are galaxy clusters, situations of hundreds or
thousands of galaxies contained within a region a few million light years across.
The largest coherent structures that we can see in surveys are actually
superclusters that are much larger.
But the dynamical evidence indicates that they're still collapsing or forming.
Galaxy clusters are what we say virialized, which is their motions
are governed by the virial theorem, and so they are an equilibrium.
The galaxy clusters provide our best laboratory for
the fact that galaxies can change their properties over cosmic time.
In the centers of galaxies, we see the effect of galaxy interactions and mergers.
Usually, a dense galaxy cluster has a giant CD galaxy at its center,
an object with a trillion times the stellar mass of the sun.
Which is maybe ten or
twenty times more stellar mass than the Milky Way itself at the center.
These galaxies also tend to have intense x-ray emission from the hot gas they
contain or attract to them.
These are the largest galaxies in the universe, and
they've got that way by creating other galaxies of similar size, and
much larger number of dwarfs over a ten billion years.
Galaxy clusters are important laboratories for studying and
understanding dark matter.
It turns out that there are three completely independent ways to measure
the mass of a galaxy cluster.
The first is the first method used historically to infer dark matter.
In the 1930s, Fritz Zwicky, a Caltech astronomer, measured radio velocities in
the Coma cluster, a large cluster of galaxies in the northern hemisphere,
at a distance of about 100 million light years.
He saw phenomenally fast motions, and
back in the mid 1930s, speculated that that cluster would be
flying apart unless it was held together by large amounts of unseen matter.
It was a case of a speculation that was too far ahead of its time.
Other astronomers were not prepared to accept Zwicky's supposition, and so
the observation sat in the literature unnoticed.
By the 1970s and 1980s, Zwicky's observation being replicated for
other clusters.
It turned out that every cluster of galaxies had velocities of its individual
galaxies too large to be explained by the visible matter.
The radio velocities and cluster become evidence for dark matter.
The second method involves the hot million degree gas
that tends to concentrate at the center of a galaxy cluster.
Application of the virial theorem to this gas
allows you to measure the mass of the center of the cluster.
Once again, dark matter is part of this calculation.
The third method involves the deviation of the light of background galaxies
by the cluster itself.
Background galaxies have light that's amplified, distorted and
bent by the intervening cluster.
The striking thing is that each of these three methods applied to the same set of
clusters give essentially the same number for the mass of the cluster.
And in all cases,
these three methods indicate that most of the mass of the clusters is dark matter.
We can see an example of the lensing geometry
that leads to this kind of mass determination.
The lensing technique is very mature in terms of giving us the mass of hundreds
of clusters in the universe out to distances of nearly a billion light years.
One survey has done enormous service to astronomy by mapping out
the distribution not only of normal galaxies, but of active galaxies or
quasars out to very high red sheets.
It's the Sloan Digital Sky Survey.
A pioneering project using a two and a half meter telescope on Apache Point,
this survey was based on phenomenally high quality and
large imagers and spectrographs.
Remember, this telescope is considered small by modern standards.
It would not break the top 50 of the world's largest telescopes.
But its customized hardware and phenomenal detection systems,
and its dedicated purpose on one survey,
has meant that it's transformed the extra-galactic landscape.
The Sloan Digital Sky Survey allows us to place galaxies in three dimensional space
up to distances of a billion or two billion light years.
And see the large scale structure of the universe in exquisite detail.
What's the best description of the large scale structure of the universe that is
the way matter is distributed on the largest scales beyond the galaxies.
Sometimes phenomena in nature are strikingly similar,
even though they are fundamentally different in their regime.
One example of this is the scale free structure we'd see in the brain in terms
of network of neuronal connections, versus the distribution of mass in the universe.
In this case, a dark matter map.
Both distributions have what's called a fractal quality,
where there are almost equal amounts of power on different scales.
It even turns out that the nature of the structure you see and
the light reflections on a pool, which are caustic patterns, are very similar
to the structure seen in the larger scale distribution of matter in the universe.
In the mathematics, that describes them as similar.
We see structure in the universe on all scales beyond galaxies
ranging up to a few hundred million light years aclo, across.
Huge superclusters of galaxies with tens of thousands of galaxies in them.
The largest bound structures in the universe are galaxy clusters.
And three different ways of measuring their mass affirmed that most of
the universe on large scales is dark matter.
The large scale structure of the universe has a delicate, filigree pattern.
And it has a fractal dimension of one point seven.
If we imagine one as strings, two as sheets, and three as a filled volume,
one point seven corresponds to halfway between strings and sheets.
That's the pattern we see in the three dimensional distribution of galaxies on
the largest scales.
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