>> So, we have seen how large-scale
structure is suppose to form theoretically
but now, let us take a look at what
observations have to say.
The basic paradigm, of course, is the
density fluctuations from the early
earth's collapse.
Into structures that we see today and that
formation continues through hierarchical
co-merging of smaller pieces into ever
larger ones.
So, clearly, we distinguish between
galaxies which are obviously seen as unit
things in , in, in a space and there are
[unknown] formed structure achieving
density contrast of the order of million
relative to the average density, which
clearly puts them apart from large-scale
structure itself.
Consisting of assemblies of really dark
matter with galaxies used as tracers.
So by large-scale structure, we, we talk
about galaxy groups, filaments, clusters,
sheaths, superclusters, and that is what
we'll be addressing now.
Note that the basic assumption here is
that we do know that most of the mass is
in form of non-bionic dark matter and that
most of the bionic matters, we don't see
directly.
So we hope, and that turns out to be
actually a pretty good assumption, that
galaxies, visible ones, trace the
large-scale structure that's actually
there in invisible material.
The basic way of quantifying the
large-scale structure is to first map it
up and find out where all the galaxies are
and see what their density field is like.
To do this, we obviously measure two
coordinates in the sky and we need the
third radial coordinate.
This is obtained through redshifts,
assuming of course that Hubble expansion
works.
So, we need to do large redshift surveys,
measuring redshifts for as many galaxies
as possible in order to trace the
large-scale structure, structure.
It was recognized already in the early
20th century that galaxies are not
randomly distributed in space, but rather,
they seem to congregate.
However, it took until about 1970s to
really start quantifying this, so in
1930s, Shapley and Zwicky, who started
first systematic sky surveying noted the
galaxies do seem to form large structures,
shown here on the left is one of the
contour maps of projected galaxy densities
on sky from Zwicky's atlases.
The quantity theory now is this, or galaxy
clustering, started in 1950s by Shane and
Wirtanen astronomers who counted galaxies
per unit area on photographic sky place
taken at Lick Observatory.
Those so called Lick galaxy counts turn
out to be a basic material for a lot of
the early theoretical studies of
large-scale structure.
From 50s to 1970s, more and more attention
was paid to it.
Gerard de Vaucouleur in particular,
noticed what we now call the local
supercluster and more and more galaxy
redshift surveys were conducted.
The redshift surveys really came into
prominence in the 1980s and beyond, and
today, we have redshifts for maybe 2
million galaxies or so.
Here is a simple projection on sky of the
6,000 brightest galaxies.
Three things are immediately apparent.
There is the belt where galaxies are
absent and that is called the zone of
avoidance.
We now understand that this is due to the
interstellar dust absorption in our own
galaxy that we simply cannot see very much
through the dusty interstellar medium in
the Milky Way's disk.
Outside of that, you see that there is
some structure.
Now, if you were to make plot of 6,000
brightest stars in the sky, it would be
exactly the opposite.
They are all concentrate in what is the
zone of avoidance of the galactic world,
the angular disk of the Milky Way, and
otherwise, they'll be fairly randomly
distributed, on the rest of the sky.
So here is the 3D picture of our immediate
extragalactic neighborhood, the so-call,
so-called Local Group of galaxies.
Milky Way and Andromeda are the two
dominate galaxies in the group and there
is a large number of their dwarf galaxy
satellites.
Groups of galaxies are indeed where most
of the galaxies are.
The, they are the most common building
block of larger scale structures.
We mentioned Local Supercluster earlier.
This was noted as an elegant structure in
the sky and that we are kind of in its
plane.
The center is on Virgo cluster, the
nearest cluster of galaxies to us and it's
roughly 60 megaparsecs in, in scale.
As superclusters go, this is not a very
large supercluster, but it is the one in
which we belong.
In addition to the commonly used
equatorial or ecliptic or galactic
coordinates from astronomy, we now define
so, so-called supergalactic coordinates.
The xy plane of the supergalactic
coordinates is the plane of the local
supercluster And we are at its origin, and
vertical z direction is orthogonal to it.
Later on, we will see plots of some of the
distributions projected onto this xy plane
of the supergalactic coordinates.
So here is the rough 3D sketch of what
local supercluster might look like.
This plot is centered on a local group of
galaxies, but really, the center of the
Local Supercluster is at Virgo.
The rest of the structure are various
galaxy groups or individual galaxies.
Actually, there are very few, isolated
galaxies, most of them are in groups.
It is not the perfectly flat system like
the disk of the Milky Way, but it is flat.
Two other clusters that are sort of a
referral to the Local Supercluster are
Fornax and Eridanus.
There are [inaudible] from other side of
the sky from Virgo.
Now those become little more apparent if
we plot not 6,000 but 15,000 brightest
galaxies in the sky.
This is typical of galaxy catalogs until
say 1990, where those are galaxies
selected by eye or on photographic sky
survey plates.
Here, we can see that there is a great
circle shown here through these arches
that does go through a lot of galaxies,
and that is indeed, the plain of the Local
Supercluster.
To the upper left, sort of, you see a
structure called Centaurus or
Hydra-Centaurus, this is another
supercluster of galaxies, and in fact,
it's bigger than Local Supercluster and
we're falling to it.
We'll talk more about that when we talk
about peculiar velocities.
On the other side is Perseus-Pisces
Supercluster, roughly the same distance.
So, cluster, superclusters themselves,
they're to form slightly more coherent
structures, and this is not surprising
from everything that we know about
large-scale structures.
So the mapping of large-scale structure
begins with catalogs of galaxies on the
sky and those were counted by eye, well,
originally by Messier and Herschel and so
on.
But, really, the more modern ones,
beginning with Shapley and Ames in 1930s
and then Lick galaxy counts in 1950s, and,
their famous catalogs like Uppsala General
Catalog of Galaxies and the Southern
Extension.
Still, the Shane-Wirtanen Lick counts were
the largest up until 1980s they counted
close to 1 million galaxies in the sky,
which is why this was a good statistical
sample for early studies.
The next step came with digitization of
sky survey plates and that was done for
both northern and southern skies, starting
in, sometime in late 1980s.
And APM is for Automated Plate Measuring
machine in Cambridge, England and people
who then do this work, produced the
catalog of roughly 2 million galaxies.
The northern version of it, from the
Second Palomar Sky Survey, the so-called
Digital Palomar Observatory Sky Survey or
DPOSS counted about 50 million galaxies.
By mid 2000s, Sloan Digital Sky Survey
obtained much higher quality data for a
couple hundred million galaxies at first.
Nowadays, they all have, have, have almost
a billion, both stars and galaxies.
In the future, we'll look towards even
larger sky surveys and Large Synoptic
Survey Telescope or LSST, which certainly
will not be completed in 2015.
This is an old slide, more like 2020, will
probably map the billion of galaxies, but
that's just pictures in the sky.
In order to get the third dimension,
redshifts are needed.
And the first modern redshift survey was
probably the one done at the
Harvard-Smithsonian Center for
Astrophysics in mid 80s.
They obtained redshift of the order of
thousand nearby galaxies and started to
quantify large-scale structure near us.
Along with its Southern Extension, they
covered in the order 2500 galaxies, this
was already good enough to begin first
modern quantitative structures of
Blackwell galaxy clustering.
Now you may recall the zone of avoidance
was placing this artificial belt of
missing galaxies in the sky, simply due to
the extinction in our own Milk Way.
The way to bypass this was to look at
infrared, as infrared light goes through
the interstellar dust relatively
unobscured.
By mid 1990s there was a good catalog of
infrared selected galaxies from the all
sky survey by IRAS satellite.
That produced fairly unbiased set of
galaxies that enabled astronomers to go
deep into the zone of avoidance.
The resulting redshift surveys, Now went
up to 9,000 galaxies.
When going by their initial success, the
Center for Astrophysics conducted a second
generation of redshift survey which
compiled the redshifts of about 25,000
galaxies.
John Huchra was the leading astronomer in
that enterprise, and also, his
collaborators including Margaret Geller
and others.
Roughly, at the same time, Carnegie
Observatories did [inaudible] redshift
survey in the southern hemisphere using
one of the first modern multi-object
spectrographs, and they obtained redshift
for comparable number for about 23,000
galaxies.
These surveys of 1990s started revealing
more interesting features in the galaxy
distribution, but really, the state of the
art was achieved by two large surveys at
one in Australia, to two-degree g survey,
and a Sloan Digital Sky Survey.
We'll talk a little more about them later,
qhich together, now mapped well over a
million galaxies.
Let's now look at the sky in successive
redshift shells as we step out.
The closest one would be shown here.
This is galaxies with recession velocities
due to the Hubble Expansion up to 3,000
kilometers per second, and the biggest
feature you see, is the Local
Supercluster.
Most of the galaxies being centered on
Virgo, but with some pieces in Fornax an.
Stepping further out between 3,000 and
6,000 kilometers per second, new
structures come in,the most notable one is
the Perseus-Pisces super cluster opposite
from Virgo.
This was largely done by Giovanelli and
Haynes at Cornell and their collaborators,
who conducted their work in parallel with
the CFA group.
And again, sort of opposite to
Perseus-Pisces and almost hidden by the
galactic plane, is the Hydra-Centarus
Supercluster, which later made an
appearance under the name of Great
Attractor.
But we'll talk about that later, and now,
even further out between six and out,
9,000 kilometers per second, the biggest
feature is the Coma Abell 1367
supercluster, composed largely of these
two rich clusters.
But there are also some vestiges of
others, especially the Perseus Pisces
side.
And so, here is a 3D sketch of what our
local super galactic neighbourhood might
look like, with Local Supercluster and
nearby superclusters like Perseus-Pisces
and Hydra-Centaurus.
You have probably noticed that
superclusters, at least, those near us are
given names by the constellations in which
most of the galaxies are placed, Those
constellations of course have no physical
meaning.
They're still used as a traditional
convenience to designate where in the sky
some things might be.